MetOc products for maritime operations · data using ENVI ZOOM blending tools. The project observed remarkable improvement in thermal feature detection by using latest-pixel composite

MetOc products for maritime operationsThermal fronts, eddies and water mass boundaries with RADARSAT

B. G. Whitehouse

The scientific or technical validity of this Contract Report is entirely the responsibility of the contractor and the contents do not necessarily have the approval or endorsement of Defence R&D Canada.

Defence R&D Canada --- Ottawa CONTRACT REPORT

DRDC Ottawa CR 2007-101 May 2007

MetOc products for maritime operations Thermal fronts, eddies and water mass boundaries with RADARSAT

B. G. Whitehouse OEA Technologies Incorporated Prepared by: OEA Technologies Incorporated 3650 Hammonds Plains Road Unit 14, Suite 393 Upper Tantallon, NS, B3Z 4R3 Project Manager: B.G. Whitehouse, Tel: (902) 240-0230 Contract number: W7714-050947/001/SV Contract Scientific Authority: P.W. Vachon, Tel: (613) 991-2584, Defence Scientist

The scientific or technical validity of this Contract Report is entirely the responsibility of the contractor and the contents do not necessarily have the approval or endorsement of Defence R&D Canada.

Defence R&D Canada – Ottawa Contract Report DRDC Ottawa CR 2007-101 May 2007

DRDC Ottawa CR 2007-101

Scientific Authority

Original signed by Paris W. Vachon

Paris W. Vachon

Defence Scientist

Approved by

Original signed by Gary W. Geling

Gary W. Geling

Head, Radar Applications and Space Technologies Section

Approved for release by

Original signed by Cam Boulet

Cam Boulet

Chair, Document Review Panel

This work was supported by a Canadian Space Agency (CSA) Government Related Initiatives Program (GRIP) Project entitled: "Impacts of the Marine Environment on Ship Detection by Commercial Spaceborne SAR". The project, which was initiated in April 2005 and was completed in March 2007, was lead by Defence R&D Canada - Ottawa. This document represents the final report of the GRIP project.

© Her Majesty the Queen as represented by the Minister of National Defence, 2007

© Sa Majesté la Reine, représentée par le ministre de la Défense nationale, 2007

DRDC Ottawa CR 2007-101 i

Abstract

Thirty-five RADARSAT images collected over eastern Canadian waters in 2006 were analyzed to determine this sensor's potential application to MetOc (meteorology and oceanography) operations. Analyses focused on blending RADARSAT, sea-surface temperature and bathymetric data as a means of detecting oceanographic features that have surface thermal signatures. Results demonstrate remarkable improvement in thermal feature detection by using latest-pixel composite sea-surface temperature products instead of the mean composite products presently used by federal maritime operations. RADARSAT consistently detected certain oceanographic features having surface thermal signatures, but not others. Conclusive explanation of this difference requires analysis of associated surface currents and atmospheric boundary conditions, however, results suggest there is a threshold in the magnitude of the surface thermal gradient associated with these frontal features, of about 0.4 to 0.5 oC/km, below which RADARSAT does not detect the feature. Although preliminary, these results suggest that under conditions of low to moderate wind speeds, the surface thermal gradient can be used as an indicator of ocean features detected by RADARSAT. Results also suggest a possible feature orientation bias in RADARSAT's ability to detect surface thermal features which stems from the sensor's directional bias in detecting the surface current gradient. RADARSAT consistently detected moderate to strong wind fronts to seaward of Georges Bank and these wind fronts co-located with thermal features attributable to either the shelf-slope front or aspects of the Gulf Stream. This observation suggests that detection of persistent wind fronts associated with certain thermal fronts, such as the northern wall of the Gulf Stream, could be used as an indicator of the thermal front itself.

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Résumé

En 2006, 35 images ont été prises par RADARSAT au-dessus des eaux canadiennes orientales et analysées pour évaluer la possibilité d’utiliser ce satellite dans le cadre d’opérations MetOc (météorologiques et océanographiques). Les analyses étaient axées sur la fusion de données de RADARSAT, de données sur la température à la surface de l’océan et de données bathymétriques pour détecter des entités océanographiques présentant des signatures thermiques superficielles. Les résultats obtenus montrent une remarquable amélioration de la détection thermique d’entités, lorsque cette dernière repose sur les plus récentes données composites en pixels sur la température à la surface de l’océan plutôt que sur les données composites moyennes actuellement utilisées lors des opérations maritimes fédérales. RADARSAT a détecté de manière constante certaines entités océanographiques présentant une signature thermique superficielle, mais aucun autre type d’entité. Pour trouver une explication concluante à ce phénomène, une analyse des courants de surface et des conditions dans les couches limites atmosphériques qui s’y rattachent doit être effectuée. Toutefois, les résultats laissent supposer que le gradient thermique superficiel de ces entités frontales présente un seuil de magnitude d’environ 0,4 à 0,5 ºC/km, sous lequel RADARSAT ne peut plus les détecter. Bien qu’ils demeurent provisoires, ces résultats laissent aussi croire que par vents faibles à modérés, le gradient thermique superficiel peut servir d’indicateur des entités océaniques détectées par RADARSAT. Les résultats semblent également indiquer des distorsions possibles quant à l’orientation des entités thermiques superficielles détectées par RADARSAT, lesquelles seraient attribuables à des distorsions relatives à la direction des gradients de courant de surface détectés par le capteur du satellite. RADARSAT a détecté de manière constante des fronts de vent fort au large du banc Georges, fronts qui coïncidaient avec des entités thermiques rattachées au front du plateau et du talus ou à des éléments du Gulf Stream. Cela porte à croire que la détection de fronts de vent persistants liés à certains fronts thermiques, comme le mur nord du Gulf Stream, pourrait permettre celle de fronts thermiques.

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Executive summary

MetOc products for maritime operations: Thermal fronts, eddies and water mass boundaries with RADARSAT

Whitehouse, B.G.; DRDC Ottawa CR 2007-101; Defence R&D Canada – Ottawa; May 2007.

Introduction

In maritime waters, the state of the atmosphere is influenced by the state of the ocean and vice versa, thus maritime defence and security operations are supported by integrated MetOc (meteorology and oceanography) facilities. Traditionally, the Canadian Forces has focused its MetOc operations on forecasting atmospheric conditions and monitoring water temperature.

The temperature of seawater influences the state of the atmosphere and the performance of sonar technologies, and therefore the rapid environmental assessment component of Canadian Forces antisubmarine warfare, mine countermeasures and submarine operations. It also influences joint Canadian Coast Guard / Canadian Forces search and rescue operations, from a hypothermia perspective, and it can be used as a aid to navigation.

The water temperature aspect of MetOc operations does not simply determine temperature. It also locates oceanographic features that have a thermal signature, specifically, fronts, eddies and water mass boundaries, all of which can influence sonar performance. On the East coast, for example, MetOc's thermal ocean feature product provides deployed forces with information pertaining to the location of the northern wall of the Gulf Stream.

The primary method used by both civilian and military operations to detect meso-scale thermal oceanographic features is Earth observation, specifically, spaceborne thermal IR sensors. Federal agencies rely on NOAA's constellation of AVHRR thermal IR sensors, however, with the launch of NASA's Terra and Aqua environmental satellites this approach has expanded to include the thermal channels on the MODIS sensors.

Unfortunately, thermal IR sensors are impeded or rendered useless by cloud cover, and thermal image compositing techniques used by Canadian federal operations are suspected of being inferior to those used by the U.S. Navy.

In recognition of these observations, DRDC Ottawa, in co-operation with the Canadian Space Agency and MetOc Halifax, contracted OEA Technologies Incorporated to investigate (i) RADARSAT's ocean feature detection potential, (ii) thermal image compositing techniques used by the U.S. Navy and (iii) inter-relationships between the two.

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Results

Thirty-five RADARSAT images collected over eastern Canadian waters in 2006 were analyzed to determine this sensor's potential application to MetOc operations. Thermal features were detected in RADARSAT imagery by blending it with sea-surface temperature and bathymetric data. Without this auxiliary information it is not possible to unequivocally identify such oceanographic features in RADARSAT imagery. This suggests that the role of RADARSAT in MetOc maritime sonar-related operations is complimentary – it should be used in combination with other spaceborne and in situ data. For example, the project observed a potential ability to estimate surface current vectors associated with fine-scale thermal features by blending RADARSAT imagery with grey-scale latest-pixel composite sea-surface temperature imagery and bathymetric data using ENVI ZOOM blending tools.

The project observed remarkable improvement in thermal feature detection by using latest-pixel composite sea-surface temperature products provided by the U.S. Naval Research Laboratory instead of the mean composite sea-surface temperature products presently used by Canadian federal maritime operations.

But water temperature per se is not the determining factor when detecting thermal oceanographic features with RADARSAT. When RADARSAT detects these type of oceanographic features it is detecting other physical parameters associated with the thermal feature. In this case, either surface wind, current or natural surfactant patterns or some combination thereof.

As a result, RADARSAT consistently detected certain oceanographic features having surface thermal signatures, but not others. For example, it consistently detected thermal features located off southwest Nova Scotia in August and September and Georges Bank in October, but it never detected a large-scale thermal feature off the northern coast of Maine, which is attributed to the Eastern Maine Coastal Current.

Conclusive explanation of these differences requires extensive analysis of associated surface currents and atmospheric boundary conditions, which is beyond the scope of this investigation, however, available information suggests there is a threshold in the surface thermal gradient associated with these frontal features, of about 0.4 to 0.5 oC/km, below which RADARSAT does not detect the feature.

Results also suggest a possible feature orientation bias in RADARSAT's ability to detect such features which stems from the sensor's directional bias in detecting the surface current gradient.

A hypothesis investigated by this study is that DRDC Ottawa's RADARSAT-derived surface wind product can be used to create a detection mask – i.e. threshold, below or above which there is no point searching for the target of interest because wind conditions are not conducive to detection. Results indicate that this hypothesis is not valid when the target of interest is oceanographic features that have a surface thermal signature. Such features were detected when the RADARSAT-derived wind data indicated zero wind speed. This was observed almost over the entire range of sensor incidence angles used in this study (18 to 40 degrees).

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It was generally observed that the RADARSAT images were dominated or consumed by atmospheric features above wind speeds of about 9 to 10 m/s, however, RADARSAT consistently detected moderate to strong wind fronts in the vicinity of and to seaward of Georges Bank and these wind fronts co-located with thermal features attributable to either the shelf-slope front or warm core features of Gulf Stream origin. This observation suggests that detection of persistent wind fronts associated with certain thermal fronts, such as the northern wall of the Gulf Stream, could be used as an indicator of the front itself. The point here is not to focus on wind speed per se but on the fact that the gradient of the wind speed can be used as an indicator for ocean features that have a meteorological signal, such as strong, synoptic-scale thermal fronts. An additional incentive for developing this technique is that it involves conditions under which traditional synoptic surface thermal techniques are rendered useless or of greatly diminished value due to spatially extensive and temporally persistent cloud cover.

Significance

Since the launch of NASA's SeaSat satellite in 1978, and every civilian spaceborne synthetic aperture radar sensor since, it has been recognized that such sensors are capable of detecting marine fronts, eddies, water mass boundaries, upwelling zones and freshwater plumes, at spatial scales of tens of metres. And synthetic aperture radar techniques have been developed to detect surface and internal waves, bathymetry, surface current gradients and natural slicks. Yet, to the best of our knowledge, the only facility world wide that is using these civilian sensors to derive ocean feature products operationally is the European Centre for Medium-range Weather Forecasting, and it is only using a surface wave mapping product.

Results of this investigation address technical barriers to overcoming this situation. Although the scope of this research and development initiative did not permit development of operational products, results define and provide direction for work that will lead to such products, as summarized in the report's recommendations.

Future plans

Results will be incorporated into a proposed spaceborne ocean intelligence network designed to provide additional MetOc support to federal maritime operations.

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DRDC Ottawa CR 2007-101 vii

Sommaire

MetOc products for maritime operations: Thermal fronts, eddies and water mass boundaries with RADARSAT

Whitehouse, B.G.; DRDC Ottawa CR 2007-101; R & D pour la défense Canada – Ottawa; mai 2007.

Introduction

Dans les eaux maritimes, les conditions atmosphériques et celles de l’océan ont une influence réciproque. C’est pourquoi les opérations de défense et de sécurité maritimes bénéficient de l’appui d’installations MetOc (météorologiques et océanographiques) intégrées. Généralement, les Forces canadiennes axaient leurs opérations MetOc sur la prévision des conditions atmosphériques et la surveillance de la température de l’eau.

La température de l’eau de mer influe sur les conditions atmosphériques et sur le rendement des sonars et, du même coup, sur la capacité des Forces canadiennes d’évaluer rapidement les conditions environnementales lors d’opérations sous-marines et de lutte contre les sous-marins et les mines. Elle a également une incidence lors d’opérations de recherche et de sauvetage réalisées conjointement par la Garde côtière canadienne et les Forces canadiennes, en raison des zones hyperthermiques qu’elle peut produire et qui peuvent faciliter la navigation.

Il ne suffit pas de connaître la température de l’eau lors d’opérations MetOc, mais aussi de localiser des entités océanographiques présentant une signature thermique, en particulier des fronts, de la turbulence et des limites de masse d’eau, entités qui ont toutes une incidence sur le rendement des sonars. Sur la côte Est, par exemple, les données sur les entités océanographiques thermiques fournies par les installations MetOc indiquent aux forces déployées l’emplacement du mur nord du Gulf Stream.

Lors d’opérations civiles ou militaires, la principale technique employée pour détecter des entités océanographiques thermiques d’échelle moyenne consiste en l’observation de la Terre, en particulier au moyen de capteurs de rayonnement infrarouge thermique spatioportés. Des organismes fédéraux dépendent de la constellation de capteurs de rayonnement infrarouge thermique du type AVHRR de la NOAA. Toutefois, depuis le lancement des satellites environnementaux Terra et Aqua de la NASA, ils recourent également aux canaux thermiques des capteurs MODIS.

Malheureusement, toute couverture nuageuse rend les capteurs de rayonnement infrarouge thermique moins efficaces, voire inefficaces, sans compter qu’on soupçonne les techniques de composition d’images thermiques utilisées dans le cadre d’opérations du gouvernement fédéral canadien d’être moins bonnes que celles de la U.S. Navy.

À la lumière de ces observations, RDDC Ottawa, en collaboration avec l’Agence spatiale canadienne et le centre MetOc de Halifax, a chargé OEA Technologies Incorporated

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d’évaluer i) le potentiel de détection d’entité océanique de RADARSAT, ii) les techniques de composition d’images thermiques de la U.S. Navy et iii) les liens entre ces deux éléments.

Résultats

En 2006, 35 images ont été prises par RADARSAT au-dessus des eaux canadiennes orientales et analysées pour évaluer la possibilité d’utiliser ce satellite dans le cadre d’opérations MetOc. Des entités thermiques ont été détectées sur des images de RADARSAT en fusionnant ces dernières à des données de la température à la surface de l’océan et à des données bathymétriques. Sans ces données auxiliaires, il est impossible d’identifier sans équivoque des entités thermiques océaniques sur des images de RADARSAT. On en déduit que les images de RADARSAT ne peuvent jouer qu’un rôle complémentaire dans les opérations maritimes MetOc rattachées à l’utilisation de sonars et qu’elles doivent, par conséquent, être conjuguées à d’autres données recueillies depuis l’espace et in situ. Par exemple, on a évalué la possibilité d’estimer les vecteurs de courant de surface associés à des entités thermiques de petite échelle en fusionnant des images de RADARSAT aux plus récentes images composites de la température à la surface de l’océan en pixels en gamme de gris, ainsi qu’à des données bathymétriques, au moyen des outils ENVI Zoom.

La détection d’entités thermiques s’est avérée remarquablement plus efficace lorsque conjuguée aux plus récentes images composites de la température à la surface de l’océan en pixels du U.S. Naval Research Laboratory plutôt qu’aux données composites moyennes sur la température à la surface de l’océan qui sont actuellement utilisées lors d’opérations maritimes du gouvernement fédéral canadien.

Cependant, la température de l’eau, à proprement dit, ne constitue pas un facteur déterminant en ce qui concerne la détection d’entités océanographiques thermiques au moyen de RADARSAT. Lorsque RADARSAT détecte de telles entités, il détecte en fait d’autres paramètres physiques liés à celles-ci, à savoir des courants, des formes naturelles, des vents de surface ou un ensemble donné de ceux-ci.

Par conséquent, RADARSAT ne détecte de manière générale que certaines entités océanographiques présentant une signature thermique en surface. Par exemple, il a constamment détecté des entités thermiques au large de la côte sud-ouest de la Nouvelle-Écosse, en août et en septembre, et au large du banc Georges, en octobre, alors qu’il n’en a détecté aucune de grande échelle au large de la côte nord du Maine, où circule le courant côtier de l’est du Maine.

Pour expliquer de manière concluante cette lacune, il faut analyser de façon approfondie les courants de surface et des conditions dans les couches limites atmosphériques liés à ces entités, ce qui va au-delà de la portée de la présente étude. Toutefois, les données disponibles laissent supposer que le gradient thermique de surface de ces entités frontales présente un seuil d’environ 0,4 à 0,5 ºC/km, sous lequel RADARSAT ne peut détecter ces entités.

D’après certains résultats, il pourrait également y avoir des distorsions quant à l’orientation des entités thermiques superficielles détectées par RADARSAT, lesquelles seraient attribuables à des distorsions relatives à la direction des gradients de courant de surface détectés par le capteur du satellite.

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Une des hypothèses examinées dans le cadre de la présente étude prévoit l’utilisation des produits RADARSAT sur les vents de surface de RDDC Ottawa à titre de masque de détection, c’est-à-dire comme seuil en dessous ou au-dessus duquel il est inutile de chercher une cible, compte tenu que les vents ne s’y prêtent pas. Certains résultats montrent que cette hypothèse n’est pas valable lorsqu’une cible se compose d’entités océanographiques présentant une signature thermique superficielle, ces dernières étant détectées lorsque les données de RADARSAT sur le vent indiquent une vitesse nulle. Ce phénomène a été observé à presque tous les angles d’incidence du capteur utilisé aux fins de la présente étude (de 18 à 40 degrés).

On a généralement observé que les images de RADARSAT sont en grande partie ou entièrement composées d’entités atmosphériques lorsque la vitesse du vent dépasse 9 à 10 m/s environ. Toutefois, RADARSAT a continuellement détecté des fronts de vent modéré à fort aux environs et au large du banc Georges, fronts qui coïncidaient avec des entités thermiques attribuables au front du plateau et du talus ou à des entités à noyau chaud du Gulf Stream. On suppose donc que la détection de fronts de vent persistants pourrait permettre celle des fronts thermiques auxquels ils sont liés, comme le mur nord du Gulf Stream. On ne se concentrerait alors pas sur la vitesse du vent en soi, mais plutôt sur le fait que son gradient peut servir d’indicateur d’entité océanique émettant un signal météorologique, comme un front thermique fort d’échelle synoptique. Il serait d’autant plus pertinent de développer cette technique qu’elle pourrait fonctionner dans des conditions qui rendent très peu efficaces, voire inutiles, les techniques classiques de détection de front thermique d’échelle synoptique, en présence d’une couverture nuageuse vaste et temporairement persistante.

Importance

Depuis le lancement du satellite SeaSat de la NASA, en 1978, et la mise en orbite ultérieure de satellites civils dotés de radars à synthèse d’ouverture, on a constaté la capacité de ces appareils de détecter des fronts maritimes, de la turbulence, des limites de masse d’eau, des zones de remontée des eaux et des cheminées d’eau douce, à des échelles spatiales se chiffrant dans les dizaines de mètres. Des techniques recourant aux radars à synthèse d’ouverture ont été élaborées pour détecter des ondes superficielles et internes et des nappes d’hydrocarbures naturelles, ainsi que pour recueillir des données bathymétriques et mesurer des gradients de courant de surface. Pourtant, à notre connaissance, la seule installation au monde où l’on utilise ces capteurs civils pour produire des données sur les entités océanographiques à des fins opérationnelles est le Centre européen pour les prévisions météorologiques à moyen terme, qui n’utilise, en outre, qu’un produit de cartographie des ondes de surface.

Les résultats de la présente étude portent sur les obstacles techniques à franchir pour régler les problèmes susmentionnés. Bien que la portée de cette initiative de recherche et de développement n’ait pas permis d’élaborer des produits opérationnels, ses résultats délimitent et orientent des travaux qui en permettront l’élaboration, conformément aux recommandations qui ont été présentées, qui comportent toutes des recommandations secondaires.

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Perspectives

Les résultats de la présente étude seront intégrés à un réseau envisagé de capteurs spatioportés visant à recueillir des données océanographiques pour fournir un appui MetOc supplémentaire aux opérations maritimes fédérales.

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Table of contents

Abstract ............................................................................................................................................ i Résumé ............................................................................................................................................ ii Executive summary ........................................................................................................................ iii Sommaire....................................................................................................................................... vii Table of contents ............................................................................................................................ xi List of figures ................................................................................................................................ xii List of tables ................................................................................................................................. xiv Acknowledgements ....................................................................................................................... xv 1. Introduction............................................................................................................................... 1 2. Methodology............................................................................................................................. 3

2.1 Approach ....................................................................................................................... 3 2.2 Project area .................................................................................................................... 5

3. Results....................................................................................................................................... 7 3.1 Thermal features in AVHRR imagery........................................................................... 7 3.2 Oceanic, atmospheric and bathymetric features in RADARSAT imagery ................. 12

3.2.1 Southern Nova Scotia and Bay of Fundy ...................................................... 12 3.2.2 Outer reaches of Georges Bank..................................................................... 22 3.2.3 Sable Island ................................................................................................... 27

3.3 Tidal currents............................................................................................................... 27 4. Discussion and summary ........................................................................................................ 30

4.1 Wind ............................................................................................................................ 30 4.2 Thermal signatures and current shear .......................................................................... 31 4.3 Currents and bathymetry ............................................................................................. 33 4.4 Software Tools............................................................................................................. 34

5. Recommendations................................................................................................................... 36 5.1 Meteorological features ............................................................................................... 36 5.2 Surface thermal features and currents ......................................................................... 36 5.3 Bathymetry .................................................................................................................. 37

References ..................................................................................................................................... 38 List of symbols/abbreviations/acronyms/initialisms ..................................................................... 40

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List of figures

Figure 1. Map of main project area, reproduced with permission [13]. The Eastern Maine Coastal Current passes through red dot "I", which like "E" shows the location of in situ current meters. .................................................................................................... 6

Figure 2. Cropped 7-day SST mean composite for week of 13 Aug. 06. EMCC = Eastern Maine Coastal Current; GM = Grand Manan Island; BI = Brier Island........................ 7

Figure 3. Seven-day SST mean composites (deg C): blue = 1-3, green = 6-8, yellow = 11-13, orange = 15-18, red = 21-24, deep red > 26................................................................ 10

Figure 4. Seven-day SST mean for week of 24 Sept. (top) and 1-day mean for 27 Sept. ........... 11 Figure 5. Re-projected 1-day SST mean composite and RADARSAT image for 27 Aug............ 15 Figure 6. Corresponding RADARSAT-derived surface wind field for Figure 5 images. ............. 16 Figure 7. Images from Figure 5 blended together. ........................................................................ 17 Figure 8. RADARSAT for 27 (top) and 30 Aug. and 1-day SST mean composite for 30 Aug. ... 18 Figure 9. Blend of 30 Aug. RADARSAT and SST images presented in Figure 8........................ 19 Figure 10. RADARSAT and its corresponding surface wind field for 27 Sept. ........................... 20 Figure 11. One-day SST mean composite for 27 Sept. (top); RADARSAT image with SST

frontal contours (in blue) from top image; University of Maine NOAA15 AVHRR pass at 1052 UTC (bottom; different projection than RADARSAT image). .............. 21

Figure 12. RADARSAT image, corresponding wind field and 7-day SST mean composite for 1 Oct. ........................................................................................................................... 23

Figure 13. RADARSAT image, wind field and 7-day SST mean for 4 Oct. ................................ 24 Figure 14. Top: 4 Oct. RADARSAT image with 50 fathom (92 m) isobaths; Middle: 7-day

MODIS SST latest-pixel composite for 28 Sept. - 4 Oct. in grey scale; Bottom: same 7-day MODIS SST LPC with bathymetry. ........................................................ 25

Figure 15. RADARSAT (top left), 7-day MODIS SST LPC (top right, 5-11 Oct.); 1-day MODIS SST LPC (bottom left) for 11 Oct.; 7-day AVHRR SST mean composite for week of 8 Oct......................................................................................................... 26

Figure 16. WebTide sites: blue-Bay of Fundy, red-Grand Manan, pink-EMCC, green-Brier Island, grey-Jordan Basin, black-south shore, yellow-off Cape Sable, aqua-tip of Georges Bank, gold-outer Georges Bank, salmon-off Sable Island............................ 27

Figure 17. Modelled tidal currents off Cape Sable and Brier Island presented as east/west (u) and north/south (v) components during the period 20 Aug. to 31 Oct........................ 28

Figure 18. u/v ratio versus time for tidal currents presented in Figure 17. Values of 100 = beyond scale. ............................................................................................................... 29

Figure 19. Seasonal front detection probability averaged over the period 1985 to 1996, reproduced with permission [12]. Summer = Jul.-Sep.; Autumn = Oct.-Dec. ............ 33

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Figure 20. SST mean gradient magnitude at frontal pixels for data presented in Figure 19, reproduced with permission [12]................................................................................. 33

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List of tables

Table 1. Thermal features in RADARSAT imagery (yes or no) determined by comparing to 7-day and when available 1-day SST mean composites. Results presented as 7-day/1-day comparison followed by RADARSAT incidence angle in brackets. Where just one SST is presented, it is the 7-day. A blank indicates sub-area not in RADARSAT image, whereas an * indicates no thermal feature in the SST. A "?" indicates questionable results. ..................................................................................... 13

Table 2. Auxiliary data to Table 1. Time = start time of RADARSAT acquisition. Wind range (knots) derived from RADARSAT image................................................................... 14

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Acknowledgements

This work was made possible by the cooperation of the Canadian Ice Service, Ottawa, which provided required RADARSAT imagery, and by the Canadian Space Agency, through its Government Related Initiatives Program. The author also wishes to express his gratitude to Dr. Paris Vachon (DRDC Ottawa), LCdr. Wayne Renaud and Lt.(N) Darryl Williams (Canadian Forces) for the direction and insight they provided to this work, to Dr. Neal Pettigrew (University of Maine) and Dr. David Ullman (University of Rhode Island) for the use of published graphics, to Pierre Larouche (Fisheries and Oceans Canada), whose eleventh hour rescue resulted in the sea-surface temperature mean composites presented in this report, to Dr. Robert Arnone and Dr. Paul Martinolich (U.S. Naval Research Laboratory, Stennis) for the demonstration sea-surface temperature latest-pixel composite products, and to Dr. Andrew Thomas (University of Maine) and Richard Olsen (Norwegian Refence Research Establishment) for their comments and constructive review of this report.

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DRDC Ottawa CR 2007-101 1

1. Introduction

In the maritime environment, the state of the atmosphere is influenced by the state of the ocean and vice versa, thus maritime defence and security operations are supported by integrated MetOc (meteorology and oceanography) facilities. In the case of the Canadian Forces, these facilities are concentrated in two regional offices, one in Halifax and the other in Esquimalt. Traditionally, these offices have focused their operations on forecasting atmospheric conditions and monitoring water temperature.

The temperature of seawater influences the state of the atmosphere and the performance of sonar technologies, and therefore the rapid environmental assessment component of Canadian Forces antisubmarine warfare, mine countermeasures and submarine operations. It also influences joint Canadian Coast Guard / Canadian Forces search and rescue operations, from a hypothermia perspective, and it can be used as an aid to navigation.

Rapid environmental assessment is NATO's term for providing environmental information to deployed forces in littoral waters in tactical time frames and formats. It was initiated in 1995 and indoctrinated in 2001 [1]. The Canadian Forces published its rapid environmental assessment guidelines in 2005 [2]. Throughout this document we use the military definition of the word littoral, not the oceanographic definition (i.e. inter-tidal). Within NATO this is ill-defined but in general pertains to waters in which the bottom influences the performance of acoustic sensors (i.e. coastal waters, from the shore out to perhaps the continental slope) [3].

The water temperature aspect of MetOc operations does not simply determine water temperature. It also locates oceanographic features that have a thermal signature, specifically, fronts, eddies and water mass boundaries, all of which can influence sonar performance. On the East coast, for example, MetOc's ocean feature product provides deployed forces with information pertaining to the location of the northern wall of the Gulf Stream.

In addition to these ocean features, littoral waters experience coastal upwelling and freshwater plumes, both of which can have dramatic effect upon water temperature. In general, littoral waters also experience finer temporal and spatial scales of variability.

The primary method used by both civilian and military operations to detect meso-scale thermal oceanographic features is Earth observation, specifically, spaceborne thermal IR sensors. Federal agencies rely on NOAA's constellation of AVHRR thermal IR sensors, however, with the launch of NASA's Terra and Aqua environmental satellites in 1999 and 2002, respectively, this approach has expanded to include the thermal channels on the MODIS sensors. Theoretically, the European Space Agency's ATSR and AATSR sensors aboard their ERS and ENVISAT satellites could also be used for such purpose, but we are not aware of any operational organization doing so in North America. ATSR data has proven to be very difficult to obtain in operational time frames.

Unfortunately, thermal IR sensors are impeded or rendered useless by cloud cover, and thermal image compositing techniques used by Canadian federal operations are suspected of being inferior to those used by the U.S. Navy.

2 DRDC Ottawa CR 2007-101

In recognition of these observations, DRDC Ottawa, in co-operation with the Canadian Space Agency and MetOc Halifax, contracted OEA Technologies Incorporated to investigate (i) RADARSAT's ocean feature detection potential, (ii) thermal image compositing techniques used by the U.S. Navy and (iii) inter-relationships between the two. This document summarizes OEA Technologies' findings in these areas. A related investigation that focused on inter-relationships between RADARSAT and the ocean colour channels on MODIS is published elsewhere [4].

Since the launch of NASA's SeaSat satellite in 1978, and every civilian spaceborne synthetic aperture radar (SAR) sensor since, it has been recognized that SAR sensors are capable of detecting marine fronts, eddies, water mass boundaries, upwelling zones and freshwater plumes, at spatial scales of tens of metres [5]. And SAR techniques have been developed to detect surface and internal waves, bathymetry, surface current gradients and natural slicks. Several recent publications summarize this field [5, 6, 7], the role of SAR sensors in ocean intelligence [8] and RADARSAT's potential relevance to Canadian Forces MetOc operations [4, 9, 10].

Two operational advantages of RADARSAT over thermal sensors for ocean feature monitoring are that (a) it is not impeded by cloud cover and (b) with a 50 to 100 m spatial resolution in SCANSAR mode, it provides the spatial resolution required to monitor nearshore and coastal features. In comparison, referenced thermal IR sensors have a spatial resolution of 1,100 m.

Yet, to the best of our knowledge, the only facility world wide that is using spaceborne SAR derived ocean feature products operationally is the European Centre for Medium-range Weather Forecasting, and it is only using a surface wave mapping product. In part, this situation is due to deficiencies in required operational infrastructure, the limited temporal resolution of single satellite SAR programs and the cost associated with acquiring large volumes of SAR imagery [9]. Collectively, the Canadian Forces' Polar Epsilon project, Environment Canada's oil spill monitoring program and the Canadian Space Agency's RADARSAT-2 data pre-purchase and proposed constellation programs will overcome some of these barriers.

It is also understood that detecting ocean features with synthetic aperture radar sensors (e.g. RADARSAT) is dependent upon surface wind conditions. There is a minimum and maximum wind threshold below and above which ocean features are not detected by this type of sensor. The literature reports a minimum threshold of 2-3 m/s and maximum of 8 to 14 m/s [5].

There remains, however, a need to further understand the conditions under which SAR sensors detect ocean features that have thermal signatures, and the infrastructure required to operationalize this aspect of spaceborne SAR.


2. Methodology

2.1 Approach

The project investigates RADARSAT's ability to detect ocean features that have a thermal signature, through visual comparison of RADARSAT images with spaceborne thermal IR products and auxiliary data. No attempt is made to develop new algorithms.

Although surface currents and atmospheric boundary conditions are recognized as significant parameters for ocean feature detection with RADARSAT, such analyses were beyond the scope of the project. WebTide Version 0.65, which is a tidal current prediction model produced and maintained by Fisheries and Oceans Canada (Bedford Institute of Oceanography), was run for locations and periods covered by the RADARSAT data, but these data received very limited analyses. WebTide calculates the east/west (u) and north/south (v) components of tidal currents and also tidal heights. Results can be calculated based on individual tidal harmonics, however, for this project the model was run based on total tidal constituents. WebTide was downloaded from the Internet along with the required data set [www.mar.dfo-mpo.gc.ca/science/ocean/coastal_hydrodynamics/main.html]. The WebTide model solutions used for this particular project are described by Dupont et al. [11].

RADARSAT analyses were performed using DRDC Ottawa's IAPro software (Windows® Version 1.5). RADARSAT-derived surface wind fields were produced by DRDC Ottawa using software based on a variant of the CMOD4 model (cmod4ifr2k). Wind barb spacing was set at 12.5 km. Wind masking was done by visual inspection of the wind product.

IAPro Version 1.5 does not blend the geotiff and HDF file formats commonly used by the civilian oceanographic community. This resulted in development of a procedure that produced desired R&D results but also required the use of ENVI® and ENVI ZOOM® (Version 4.3) image analysis and blending software. Adobe Photoshop® was also used to format imagery for print media.

RADARSAT imagery from Environment Canada's East coast operational oil spill monitoring program was provided by the Canadian Ice Service. It covered shelf and slope waters between Cape Cod and the Grand Banks of Newfoundland. Specifically, 23 images of the Grand Banks, 45 images of southern Newfoundland waters, 29 images of the Gulf of St. Lawrence, five images of waters off outer regions of Georges Bank, 22 images over southwest Nova Scotia / Bay of Fundy / Gulf of Maine, 25 images in the area of Sable Island, and 16 images over other Nova Scotian waters. In total, this comprised 165 images, however, most were simply archived.

A preliminary investigation [9] concluded that the waters of southwest Nova Scotia, Bay of Fundy, Gulf of Maine and Georges Bank should be used for development and demonstration purposes. Thus, analyses focused on these areas. Eight of the 25 images around Sable Island were also analyzed, for a total data set of 35 images. All of the analyzed images were collected in ScanSAR narrow (W1, W2) mode – 50 m spatial resolution, approximately 350 km x 350 km, both descending (approximately 6 am local) and ascending (approximately 6 pm local). Imaging incident angles varied between 18 and 40 degrees.


Data were collected during the period of 20 August to 31 October 2006. This time period was chosen for logistics reasons and to minimize the effects of cloud cover on required thermal IR and multispectral Earth-observation imagery. In eastern Canadian waters, cloud cover is a dominating factor in all but a few months of the year. In December 2005, for example, 92 % of the MODIS true colour imagery collected over southwest Nova Scotia was largely covered by clouds, or missing [4]. This finding is consistent with a 12 year time series of thermal IR imagery collected over these waters, which found that, on average, in winter a mere 10 % of pixels were clear [12]. The clearest period was September / October with 30 - 40% clarity.

AVHRR sea-surface temperature (SST) products were provided by Fisheries and Oceans Canada through the Maurice Lamontagne Institute (MLI). They included seven-day mean composites and selected one-day mean composites, both in geotiff format [geographic lat/long projection and NAD-83/WGS-84 datum]. The choice of one-day composites was limited by cloud cover over the project area. Prior to use these files were converted to an IAPro-compatible format using ENVI 4.3. One-day choices were determined by visual inspection of individual AVHRR passes [http://www.seasurface.umaine.edu/].

MODIS true colour images were provided by the University of Wisconsin in geotiff format [UTM Zone 20N projection and WGS-84 datum] via anonymous ftp [aqua.ssec.wisc.edu with initial path of pub/terra/modis]. These files loaded and blended in IAPro as provided.

MODIS Aqua and Terra chlorophyll_a, SST, total absorption at 443 nm and backscatter at 551 nm one-day and seven-day latest-pixel composites were provided by NRL Stennis in jpg and geotiff [Mercator projection and WGS-84 datum] formats using their proprietary Automated Processing System (APS). These APS products were provided on a trial basis and only for the month of October 2006. They are unique in that they were produced using a latest-pixel compositing (LPC) technique, as opposed to the common mean compositing technique.

Unfortunately, the grey-scale geotiff versions of the NRL products could not be used in IAPro. During the loading process the software performed an automated enhancement on the image which rendered it almost featureless. Thus, these products could only be analyzed using ENVI 4.3 and ENVI ZOOM, however, neither of these two software products load RADARSAT images properly as provided. Thus, when comparing the NRL products with RADARSAT using ENVI it was necessary to first load the RADARSAT image in IAPro and save a screen shot of the image as a grey-scale IAPro geotiff file, which is compatible with ENVI 4.3 and ZOOM.

Canadian hydrographic charts of the project area were purchased in bsb format, which is common for such products and surprisingly loaded and blended in IAPro as provided.

In summary, the RADARSAT image analyses procedure comprised the following steps:

ftp RADARSAT images from Environment Canada server, convert file extensions to IAPro compatible extensions, load each image into IAPro, identify and catalogue images

provide DRDC Ottawa a list of images requiring wind analysis and subsequently obtain RADARSAT wind products from DRDC Ottawa via ftp


provide MLI a list of dates requiring SST analysis and obtain corresponding products; once obtained convert to ENVI geotiff format (compatible with IAPro)

ftp corresponding MODIS Terra true colour images from University of Wisconsin anonymous server (Aqua images only provided when Terra pass not available)

download MODIS APS latest-pixel composite products via NRL Web site

load RADARSAT image into IAPro, perform either linear stretch or equalization enhancement and visually locate surface features; view corresponding wind image

blend corresponding SST mean composites with IAPro-loaded RADARSAT image, locate thermal features and compare with underlying RADARSAT image

When necessary, blend MODIS true colour image with IAPro-loaded RADARSAT image; compare features seen in images

blend hydrographic chart with IAPro-loaded RADARSAT image, compare features located in the RADARSAT image with underlying bathymetric / topographic features

use IAPro to save individual and blended screen viewings as either grey-scale or colour geotiff files

view jpeg versions of NRL latest-pixel MODIS products for presence of data over project area, load corresponding grey-scale geotiff versions into ENVI 4.3, crop to project area, enhance as required and convert to ENVI geotiff format (compatible with Photoshop); in ENVI ZOOM blend cropped and enhanced ENVI geotiff version with geotiff screen shot of RADARSAT image produced by IAPro, locate thermal features and compare with underlying RADARSAT image; use additional ZOOM blending tools not found in IAPro

import IAPro, ENVI 4.3 and ENVI ZOOM geotiff screen viewings into Photoshop, crop as desired and save as jpeg files for subsequent export to print media

create tide markers in WebTide for locations having thermal features in SST imagery; for each marker run WebTide on 60 minute time-step intervals; calculate u/v for each point, plot results and compare with thermal feature, bathymetry and RADARSAT analyses.

2.2 Project area

The 35 analyzed images are mostly of waters shown in Figure 1, which is reproduced from and with the permission of Pettigrew et al. (2005) [13]. Sable Island is located just off the right-hand edge of this figure. Grey vectors show springtime geostrophic circulation patterns for the upper 40 metres, however, tidal forces dominant much of the shown area. Tidal currents cause strong vertical mixing off southwest Nova Scotia, within the Bay of Fundy and around Grand Manan Island, Georges Bank and Nantucket Shoals [14]. This mixing brings relatively cold deep waters to the surface.


Figure 1. Map of main project area, reproduced with permission [13]. The Eastern Maine Coastal Current

passes through red dot "I", which like "E" shows the location of in situ current meters.

Wind causes periodic vertical mixing along the northern coast of Maine and over Georges Bank [15], and the Gulf of Maine receives significant freshwater input, particularly in springtime and to a lesser extent late fall [13, 14, 16, 17]. Most of the freshwater enters the area from the Scotian Shelf and slope between southwest Nova Scotia and Browns Bank and between Brown's and Georges Bank. This is attributable to freshwater from the Labrador Sea and St. Lawrence River. Local riverine input is small in comparison and largely attributable to the St. John River (Andrew Thomas, personal communication). Other local rivers include the Kennebec-Androscoggin and Penobscot River in Maine, the Merrimac River near the New Hampshire / Massachusetts border and rivers flowing into the Bay of Fundy itself.

In addition to surface thermal features resulting from these forces, a persistent shelf-slope front just offshore of Nantucket Shoals and Georges Bank is well documented [12]. This front is less well defined off the Scotian Shelf [12]. Episodic warm-core rings emanating from the Gulf Stream are also observed in the outer shelf area [17]. In the SST images presented herein it is difficult to distinguish remnants of warm-core rings from the shelf-slope front as the Gulf Stream itself is not within the project area.


3. Results

3.1 Thermal features in AVHRR imagery

The 7-day mean sea-surface temperature (SST) composites are presented in Figures 2 and 3. They show oceanographic features identified in the previous section and demonstrate advantages and limitations of using thermal IR imagery for operational purposes. These composites extend well beyond the project area and thus include international waters beyond Environment Canada jurisdiction, but within that of the Canadian Forces and Canadian Coast Guard. Thus, for example, they may show components of the Gulf Stream and much more of the shelf-slope front than observed in the corresponding project RADARSAT imagery.

Figure 2. Cropped 7-day SST mean composite for week of 13 Aug. 06. EMCC = Eastern Maine Coastal

Current; GM = Grand Manan Island; BI = Brier Island; temperature in oC.

Several of the weekly composites show offshore areas of confused colors intermixed with black (e.g. weeks of 27 August, 10 September, 22 October), sometimes in regions where the northern wall of the Gulf Stream would be expected. This pattern is due to artifacts of the cloud-masking routine used to produce this imagery (Pierre Larouche, IML, personal communication).

The composites show general cooling of Atlantic surface waters as the project progresses from mid-August through to October, likely due to seasonal effects of solar heating. Even in August, however, the strong surface cooling effects of tidal mixing are evident off southwest Nova Scotia,


within the Bay of Fundy and around Brier Island, Grand Manan, Georges Bank and Nantucket Shoals. Thermal effects of the Eastern Maine Coastal Current (EMCC) and shelf-slope front are also visible.

The composites presented in Figure 3 also show that large areas of eastern Canadian waters, such as off central and northern Nova Scotia, landward of Sable Island, and towards southern Newfoundland, do not display strong or persistent surface thermal fronts, but occasionally show diffuse filaments of cool Labrador water in northeastern regions of the Scotian Shelf (e.g. the 7-day mean composites for the weeks of 10th and 17th September). Thus, although these waters are monitored by Environment Canada, their corresponding RADARSAT imagery are of little value to this investigation of surface thermal fronts.

Figure 3 shows strong surface thermal features within the project area in August, progressing to generally homogeneous sea-surface temperature conditions on the shelf in October. This lack of surface thermal structure on the shelf in mid-to-late October resulted in an inability to use thermal imagery from this area during this period to investigate potential MetOc merits of RADARSAT.

7-day SST mean, week of 13 Aug. 06 7-day SST mean, week of 20 Aug. 06

7-day SST mean, week of 27 Aug. 06 7-day SST mean, week of 3 Sep. 06


7-day SST mean, week of 10 Sep. 06 7-day SST mean, week of 17 Sep. 06

7-day SST mean, week of 24 Sep. 06 7-day SST mean, week of 1 Oct. 06

7-day SST mean, week of 8 Oct. 06 7-day SST mean, week of 15 Oct. 06


7-day SST mean, week of 22 Oct. 06 7-day SST mean, week of 29 Oct. 06 Figure 3. Seven-day SST mean composites (deg C): blue = 1-3, green = 6-8, yellow = 11-13, orange = 15-

18, red = 21-24, deep red > 26.

The general cooling of surface shelf waters in October resulted in the clearest depictions of thermal features associated with the shelf-slope front appearing in the latter part of the sampling program. Particularly, note the seven-day mean composites for the weeks of 8th and 29th of October. However, absence of frontal features in a seven-day mean SST composite is not conclusive as cloud cover can still be an issue (e.g. week of 22 October), and the mean compositing technique tends to blend away dynamic (i.e. varies on time scales significantly shorter than the compositing period) thermal features. This situation is demonstrated in Figure 4.

In the one-day SST composite in Figure 4, note the distinct valley-like thermal feature to the south of Georges Bank and the finger of cold water pointing to the southeast off Cape Sable, Nova Scotia, both of which are largely absent in the seven-day composite. The one-day composite also shows a distinct filament of relatively cold water across the outer shelf and slope of Nova Scotia in the vicinity of Sable Island that is very diffuse in the seven-day composite.

The composite for 27 September was chosen deliberately for this example as its corresponding RADARSAT image over the project area is spectacular, as shown in the subsequent section. The present point being that in comparison to SST composites, the RADARSAT image essentially represents a point in time, thus for purposes pertaining to this project it should only be compared to thermal features that are stable over the compositing period, such as the large thermal frontal feature located to the south of Cape Cod in Figure 4.

This point can be made at finer temporal resolution by comparing individual NOAA AVHRR passes, which are hours apart, with such composites. In general, acute meteorological events, such as strong winds or heavy rains can cause significant changes in otherwise stable surface thermal properties, over periods of just a few hours, as observed during certain periods discussed in the next section. Unfortunately, individual AVHRR passes were not available to the project in compatible (i.e. georeferenced) formats and therefore could not be blended with RADARSAT imagery.


Figure 4. Seven-day SST mean for week of 24 Sept. (top) and 1-day mean for 27 Sept.


3.2 Oceanic, atmospheric and bathymetric features in RADARSAT imagery

3.2.1 Southern Nova Scotia and Bay of Fundy

Results for the 22 RADARSAT images collected over southwest Nova Scotia, the Bay of Fundy and Gulf of Maine are presented in Tables 1 and 2 by sub-area, as follows: the Nova Scotia shore of the Bay of Fundy (BoF/NS), the Eastern Maine Coastal Current and waters around Grand Manan Island (EMCC), Brier Island (Brier), Jordan Basin (Jordan), waters off Cape Sable (C. Sable – i.e. southwest Nova Scotia, not Sable Island) and the south shore of Nova Scotia (S. Shore). A blank in the table indicates the sub-area was not in the particular RADARSAT image.

Thermal fronts were located by visual inspection of the SST composite. An asterisk (*) in Table 1 indicates no thermal front was observed in the identified sub-area. Note, for example, the lack of thermal fronts within these waters in October, as discussed in the previous section. A "yes" indicates the thermal feature was identified in both the SST composite and RADARSAT image whereas a "no" indicates the feature was observed in the SST but not RADARSAT image.

Table 1 indicates thermal features off southwest Nova Scotia (C. Sable) were observed consistently in RADARSAT imagery whereas thermal features off Grand Manan and Brier islands and along the northeastern shore of Maine (EMCC) were not observed in any of the RADARSAT scenes. The Bay of Fundy and Jordan Basin analyses were in between these two extremes in that sometimes thermal features observed within these areas were detected by RADARSAT while other times they were not.

Although upwelling along the south shore of Nova Scotia (S. Shore) is probable in summer months [12], it was only observed on two days in August in which corresponding RADARSAT images were available. Comparison was inconclusive.

The numbers in brackets in Table 1 are the corresponding RADARSAT incidence angles for the thermal feature. These results suggest that although SAR's sensitivity to surface winds and ocean features varies with incidence angle (Paris Vachon, personal communication), incidence angle does not appear to be a determining factor in these results. Thermal features were observed off southwest Nova Scotia at incidence angles ranging between 23 and 40 degrees, but were not observed in waters of the Eastern Maine Coastal Current at incidence angles ranging between 18 and 39 degrees.

Auxiliary data for results presented in Table 1 are presented in Table 2. Time represents the start time for acquisition of the RADARSAT scene. Acquisitions beginning with "10" are descending whereas those beginning with "22" are ascending. Thus, thermal features were observed off southwest Nova Scotia in both ascending and descending modes, and as also shown, in all stages of the tidal cycle – flood, ebb and slack water. Fine-scale wind speeds extracted from the RADARSAT imagery range between 0 and 38 knots for these 22 images.


Table 1. Thermal features in RADARSAT imagery (yes or no) determined by comparing to 7-day and when available 1-day SST mean composites. Results presented as 7-day/1-day comparison followed by RADARSAT incidence angle in brackets. Where just one SST is presented, it is the 7-day. A

blank indicates sub-area not in RADARSAT image, whereas an * indicates no thermal feature in the SST. A "?" indicates questionable results.

DATE BoF/NS EMCC Brier Jordan C. Sable S. Shore

20 Aug no (25-30) no (23) no (24)

20 Aug ? ?

24 Aug ?/yes

27 Aug yes/yes (23) no/no (30-35) */? (28) */yes (30) yes/yes (23)

30 Aug no/* (35) no/* (22-27) no/* no/* yes/yes (31) *

3 Sep no (28) no (35-39) no (34) yes (34) * *

6 Sep yes (29) no (18-22) no (23) no (21) * *

10 Sep */* */* */? */? *

16 Sep no/no (28-33) */no (34) */no (32) yes/yes (36) *

20 Sep * ? * * * *

23 Sep * ? * * * *

27 Sep */* */* */* */yes (34) */yes (28) */*

30 Sep */* */* */* */* ? *

4 Oct * * * *

10 Oct * * * * * *

14 Oct * * * * * *

17 Oct * * * * * *

21 Oct * * * * * *

24 Oct * * * * * *

24 Oct

28 Oct * * * *

31 Oct * * *


Table 2. Auxiliary data to Table 1. Time = start time of RADARSAT acquisition. Wind range (knots) derived from RADARSAT image.

DATE ISTOP

File No.

TIME

(UTC)

WIND

(knots)

INTERNAL

WAVES

SURFACE

SLICKS

TIDE at

C. SABLE

20 Aug G046 5943 1041 0-13 yes no flood

20 Aug G046 6176 2201 0-17 yes yes flood

24 Aug G046 5846 1025 0-10 yes yes slack

27 Aug G046 7355 1037 0-11 yes yes ebb/slack

30 Aug G046 6242 2209 0-12 yes yes ebb

3 Sep G046 8293 1033 0-14 yes yes ebb

6 Sep G046 7381 2205 0-18 yes yes flood

10 Sep G046 9080 1029 3-18 yes no flood

16 Sep G046 8831 2213 0-10 yes yes ebb

20 Sep G046 9067 1037 0-12 yes yes flood

23 Sep G047 1018 2209 4-25 yes no flood

27 Sep G047 1467 1033 0-13 yes yes slack

30 Sep G047 0639 2205 0-13 yes no ebb

4 Oct G047 2329 1029 1-12 yes no

10 Oct G047 2130 2213 0-15 ? yes

14 Oct G047 3859 1037 0-10 no yes

17 Oct G047 2958 2209 0-14 no yes

21 Oct G047 5405 1033 20-38 no no

24 Oct G047 4193 2205 6-21 no no

24 Oct G047 3852 1046 6-21 yes no

28 Oct G047 5252 1029 0-16 yes no

31 Oct G047 5353 1041 3-24 no no

Table 2 indicates internal waves are common in these waters and that the classic natural surface slick pattern in SAR imagery was observed frequently. Although not indicated in the table, the internal waves were largely observed in the vicinity of the shelf slope and Georges Bank.


The greatest agreement between thermal features found in a SST composite and its corresponding RADARSAT scene occurred on 27 August 2006 (Table 1), however, simply visualizing RADARSAT imagery next to SST imagery does not readily permit identification of features in the SAR image. Figure 5 presents the one-day SST composite for 27 August, re-projected to the projection of the corresponding RADARSAT image (geographic lat/long, WGS-84).

Figure 5. Re-projected 1-day SST mean composite and RADARSAT image for 27 Aug.

The corresponding thermal features in the RADARSAT image appear as black patches, such as is observed in areas of no wind. To visualize this, compare the RADARSAT pattern with the yellow-coloured thermal patterns. In other words, the match is visualized at the front of the thermal pattern – not at the coldest (blue) inner features [visualization in print media less than photo quality is difficult]. For example, compare the inverted T-like structure seen in the lower


right-hand corner of the RADARSAT image with its corresponding inverted T-like structure in the SST composite, which appears similar to a dipole eddy structure. Other recognizable matches are the blue-yellow tongue of relatively cold water emanating from the Nova Scotia shore of the Bay of Fundy, and the tongue of relatively warm (orange) Jordan Basin water penetrating into colder coastal waters off Yarmouth. Cool waters off the south shore of Nova Scotia, just past Cape Sable, also match with their corresponding RADARSAT pattern, however, it is not certain whether this shows continuation of tidal mixing off Cape Sable or upwelling off the south shore, and part of the south shore is beyond the image. Thus, in Table 1 we identify this with a blank.

On the other hand, note the total lack of agreement between the RADARSAT pattern off the coast of Maine and its corresponding SST pattern. The confused SST pattern on the far left-hand side of the SST image is an artifact of the cloud masking routine and should be ignored (Pierre Larouche, personal communication).

Thus, with the exception of waters in the vicinity of the Eastern Maine Coastal Current, the large-scale surface patterns in the RADARSAT image for 27 August match very well with those seen in the one-day SST composite. Although SST matches are observed in other images, this 27 August image set is the only set for these waters to have such a broadly-based match.

The DRDC Ottawa wind field extracted from the RADARSAT image presented in Figure 5 is shown in Figure 6. Ocean features are present in the RADARSAT image in areas indicated as having zero wind. This was also observed in other images and thus the understanding that such features are not observed in SAR images below 2-3 m/s was ignored by this project. This likely reflects error associated with the RADARSAT wind extraction process at extremely low wind speeds rather than disagreement with the literature (Paris Vachon, personal communication). On the other hand, results were consistent with reported upper wind thresholds of 8 to 14 m/s [5] (i.e. above such speeds ocean features were not observed in the RADARSAT imagery).

Figure 6. Corresponding RADARSAT-derived surface wind field for Figure 5 images.

Comparison of the wind image with its corresponding SST image raises the question of whether the RADARSAT-derived wind image is also showing oceanographic features rather than the surface wind field in low wind areas (< 2-3 m/s). Specifically, compare the surface wind pattern


in the lower right hand corner of the wind image with its corresponding SST image, and also compare the two within the Bay of Fundy. If this is indeed the case, then it suggests the wind-from-SAR algorithm cannot be relied upon to detect and differentiate wind speeds over waters where the SAR is primarily responding to oceanographic phenomenon because these phenomenon are causing surface SAR patterns similar to those observed at very low to zero (i.e. undetermined) wind speeds (i.e. appear black). This type of pattern is also caused by surface slicks (natural or man-made), thus such oceanographic phenomenon could also influence (i.e. cause false detection in) oil spill monitoring programs that use synthetic aperture radar.

The RADARSAT image and SST composite shown in Figure 5 are presented blended together in Figure 7. This technique does not present well in print media, but certain additional observations can be made by observing the blended product.

Figure 7. Images from Figure 5 blended together.

The blended image demonstrates that the blackest sections of the inverted T-like structure in the RADARSAT image co-locate with the outer right-hand side of the corresponding SST feature and that the interior and coldest (blue-green) water associated with this feature is not presenting strongly in the RADARSAT image. It is speculated that this reflects the fact that the radar is not detecting water temperature per se, but physical forces associated with the thermal feature that affect surface roughness; in this case, surface current or marine atmospheric boundary layer patterns caused by strong tidal mixing. Note that the radar is detecting the eastern and southern fronts of this thermal feature, thereby suggesting surface current or atmospheric boundary differences between the eastern and western front of the feature. In this regard, note that the SST image itself (Figure 5) shows different surface thermal patterns for the eastern and western extremities. Variations in biological activity and associated surface slicks may also be influencing patterns observed in this RADARSAT image (Andrew Thomas, personal communication).

Another T-like feature is present in this general area three days later, on 30 August (Figure 8), but it is rotated approximately 45 degrees counter clockwise to the previous, and further offshore.


Figure 8. RADARSAT for 27 (top) and 30 Aug. and 1-day SST mean composite for 30 Aug.

Again, this is not obvious from looking at the separate images, especially since the RADARSAT image for 30 August cuts off the lower part of the "T", and most of the left half of the SST is either cloud covered or shows artifacts of cloud masking. However, the blended version of the RADARSAT and one-day SST for 30 August (Figure 9) show that the feature in the RADARSAT image off southwest Nova Scotia co-locates with thermal (yellow-coloured) features in the one-day SST. Figure 9, like Figure 7, again demonstrates that the blending step is an essential component of feature identification – it cannot be accomplished with simple visual inspection of individual images.


Figure 9. Blend of 30 Aug. RADARSAT and SST images presented in Figure 8.

The final example from this dataset is presented in Figures 10 and 11. It shows a remarkable feature off southwest Nova Scotia in the RADARSAT image, but the seven-day SST mean composite for this period (not shown) has an almost featureless surface thermal pattern. The one-day SST composite (Figure 11), however, shows structure that matches the feature detected by RADARSAT, and the individual NOAA pass for 27 September, obtained 19 minutes before the RADARSAT scene, shows even greater detail. This is a rare single pass in that it is almost void of cloud cover. Other individual passes collected prior to this pass, but not shown here, demonstrate fine-scale changes in the frontal zone of the surface thermal pattern off southwest Nova Scotia, occurring on time-scales of hours. In the same series of passes, changes on time-scales of hours are also visible in other tidally mixed areas such as the Nantucket shoals and off Georges Bank, whereas other observed thermal features, such as the shelf-slope front, are very stable during this period.


Figure 10. RADARSAT and its corresponding surface wind field for 27 Sept.

Observation of the corresponding surface wind field derived from the RADARSAT image suggests the pattern seen in the RADARSAT image largely reflects surface wind patterns. However, the fact that similar features co-locate in the corresponding SST image and that the SST patterns for the area are known to be caused by strong tidal mixing suggest that surface currents should also be considered as a contributing factor. This again raises the question of when wind speeds are less than about five knots, is the SAR-derived wind field inadvertently presenting variations in mesoscale oceanographic features as variations in atmospheric features (i.e. wind speed), or are the atmospheric variations real and in response to water temperature variations?


Figure 11. One-day SST mean composite for 27 Sept. (top); RADARSAT image with SST frontal contours

(in blue) from top image; University of Maine NOAA15 AVHRR pass at 1052 UTC (bottom; different projection than RADARSAT image).


3.2.2 Outer reaches of Georges Bank

The project received five RADARSAT images of waters bordering the outer reaches of Georges Bank. As previously discussed, Georges Bank is prone to tidal mixing, which is evident in SST imagery, and it is common to see aspects of the shelf-slope front off the outer edge of the Bank.

The images were collected on 1, 4, 11, 25 and 28 October. All were morning (descending) passes and all five detected frontal features, however, some of these features are clearly of atmospheric as opposed to oceanic origin. Slicks of natural surfactants were not detected in any of the images and only one clearly showed internal wave patterns. RADARSAT-derived wind speeds were generally much stronger in these images than in those presented in the previous section, with each image presenting maximum winds between 18 and 27 knots (9 to 14 m/s). Such speeds are at the limit of or beyond speeds at which ocean features are detected by RADARSAT. Atmospheric features, however, are detected at such speeds [5, 6]. These maximum speeds did not persist throughout the entire image and thus regions of lower wind speeds were also observed.

The 1 October images are presented in Figure 12. They show a stark atmospheric front, with winds near zero on one side and approximately 20 knots on the other. The seven-day SST mean composite for the week of 1 October shows a diffuse transition zone in the area imaged by RADARSAT. A one-day composite was not available due to cloud cover, but individual AVHRR passes of the area show a thermal front and associated thermal features in the area imaged by RADARSAT (individual AVHRR passes were not available in georeferenced format and therefore were of minimal analytical value to the project). Although the SST presented in Figure 12 is a seven-day composite and obviously contains compositing artifacts in the area imaged by RADARSAT, the diffuse frontal zone depicted in the SST composite resides in the general vicinity of the front detected by RADARSAT. This front may be the shelf-slope front or perhaps a remnant of the Gulf Stream. Time series analysis over a larger area would provide a definitive answer.

Data from 4 October result in a remarkable set of images, as presented in Figures 13 and 14, with the remarkable aspect arising from comparison of the two. Similar to the imagery for 1 October, the RADARSAT-derived wind field for 4 October shows a wind front in its lower right-hand corner, although weaker than that for 1 October and geographically further to the west. Again, a one-day SST mean composite was not available and the seven-day SST mean composite shows a diffuse transition zone in the general vicinity of the shelf-slope front, which generally co-locates with the wind front seen in the RADARSAT data.

In addition, however, the RADARSAT image shows large dark patches to the left of centre and a series of unidentified finger-like features bordering one of these patches. (Figure 13 – left of centre). Blending this image with bathymetric data (Figure 14 – top) shows that two of the three large dark patches co-locate with Georges Bank and the general area of Browns Bank and that the finger-like structures are all off the outer edge of Georges Bank.

The most remarkable image of the project is the middle image in Figure 14. It is a seven-day SST latest-pixel composite for the period 28 September to 4 October – generated from the thermal channels on MODIS Aqua. Thus, unlike all other SST products presented in this report, it is not


Figure 12. RADARSAT image, corresponding wind field and 7-day SST mean composite for 1 Oct.

generated from AVHRR data and it is not a mean composite. Also, it is based solely on MODIS/Aqua (i.e. does not include MODIS/Terra) thermal channels.

Note that whereas the seven-day mean (AVHRR) SST composite (Figure 13) is void of fine-scale features, the seven-day latest-pixel (MODIS) SST is not, and the features present in the latest-pixel composite match those seen in the RADARSAT image. In fact, comparison of the RADARSAT/bathy blend and latest-pixel SST/ bathy blend products in Figure 14 (i.e. top and bottom images) shows that the finger-like features common to both images appear almost identical. In addition, the latest-pixel SST provides a thermal signature of the Northeast Channel (i.e. the deep channel between Georges and Browns Bank) and even appears to capture the wind/thermal front present in the lower right-hand side of the RADARSAT image. The shelf-slope front is also discernible in the latest-pixel product, thereby permitting identification of this feature in the RADARSAT image.

On its own, it is not possible to identify features seen in the 4 October RADARSAT image, with the exception of the wind front in the lower right quadrant. Including the mean SST data provides little additional insight. In combination with the bathymetric and latest-pixel composite SST data, however, it is possible to identify these features as shown in Figure 14 (top). In addition, through such comparison it becomes evident that the RADARSAT image is also showing an outline of the Northeast Channel, albeit faintly as presented here.


Figure 13. RADARSAT image, wind field and 7-day SST mean for 4 Oct.

With these comparisons, it is evident that the finger-like features present in the RADARSAT-derived wind image reflect oceanographic and bathymetric features, which again raises the question of whether at low wind speeds this product is detecting variations in wind speed or mistakenly detecting other features as wind features. Another possibility is that it is correctly detecting fine-scale variations in wind velocity caused by variations in oceanographic conditions.

The most impressive blending exercise of the project cannot be presented in print media as it is an animation performed using ENVI ZOOM. Unlike IAPro, ENVI ZOOM loads and permits analysis of grey-scale HDF files, however, it does not load RADARSAT files properly. This requires the analyst to first load the RADARSAT image in either ENVI 4.3 or IAPro and change its file format to a ZOOM compatible format. ENVI ZOOM also has several different blending routines not present in IAPro, one of which is an automated transparency routine which gives the illusion of an animated video of the finger-like features off Georges Bank moving towards the Northeast Channel, when blending the RADARSAT and latest-pixel composite grey-scale SST images. This blending process suggests grey-scale SST products are preferred over false-colour SST products when comparing SST imagery to grey-scale RADARSAT imagery. It is easier for the eye to make the transition from one image to the other when both images are in grey-scale.


Figure 14. Top: 4 Oct. RADARSAT image with 50 fathom (92 m) isobaths; Middle: 7-day MODIS SST

latest-pixel composite for 28 Sept. - 4 Oct. in grey scale; Bottom: same 7-day MODIS SST LPC with bathymetry.


This exercise suggests that if an SST product of a precise time (e.g. individual pass or short-term composite) could be generated then surface current vectors associated with frontal thermal features could be estimated by blending RADARSAT and SST imagery.

Other images from this area and period show similar results. The RADARSAT image for 11 October (Figure 15) shows a wind front in the lower right corner and contains dark areas over Georges Bank and a finger-like feature emanating from the outer Bank, also with an east-west orientation. The seven-day SST mean composite was similar to that of 4 October. The seven-day latest-pixel composite MODIS SST composite for 5-11 October resolved fine-scale thermal features similar to that from the previous seven day latest-pixel composite, and although the one-day latest-pixel MODIS SST composite for 11 October was mostly cloud covered, it detected shelf-slope features also imaged by RADARSAT.

Figure 15. RADARSAT (top left), 7-day MODIS SST LPC (top right, 5-11 Oct.); 1-day MODIS SST LPC

(bottom left) for 11 Oct.; 7-day AVHRR SST mean composite for week of 8 Oct.

The RADARSAT image for 25 October is located well off the shelf and dominated by distinctive pock-marked features typical of convective cells observed near atmospheric fronts [e.g. 18]. Strong winds upward of 27 knots include a distinctive wind front running almost the full length of the image. Both mean and latest pixel SST composites are heavily cloud covered, although the latest-pixel composite MODIS SST shows recognizable thermal features in places.

Both the mean and latest-pixel SST composites corresponding to the RADARSAT image for 28 October were either featureless, cloud covered or otherwise lacked data (i.e. the NRL latest-pixel compositing sampling program ended), thus although the RADARSAT image showed a distinct


and large-scale frontal feature perpendicular to the outer edge of Georges Bank, there are no auxiliary data to further identify this feature.

3.2.3 Sable Island

The project analyzed eight of the 25 RADARSAT scenes collected in the vicinity of Sable Island, specifically, those for 28 August, 10, 17, 21 and 27 September, 1, 8 and 15 October. As discussed in Section 3.1, during the sampling period these waters were largely homogeneous from a surface thermal perspective, with the exception of traces of Labrador Sea water and occasional signs of the shelf-slope front off the shelf break.

Individual NOAA passes provided by the University of Maine do not cover these waters and the latest-pixel MODIS SST composites were only available for the month of October. In combination, this reduction in available SST imagery and relatively homogenous surface thermal patterns resulted in nothing remarkable for the eight RADARSAT images from this area. The exception being that one of the RADARSAT images showed a feature emanating from the mouth of Halifax Harbour, but it could not be identified with available data.

We present no further discussion of these eight images except to note that in a previous (2003) RADARSAT study of shelf waters around Sable Island, it was concluded that relatively weak currents within these waters result in limited success in determining RADARSAT's potential to resolve surface current features [19].

3.3 Tidal currents

WebTide analyses were conducted at locations identified in Figure 16.

Figure 16. WebTide sites: blue-Bay of Fundy, red-Grand Manan, pink-EMCC, green-Brier Island, grey-

Jordan Basin, black-south shore, yellow-off Cape Sable, aqua-tip of Georges Bank, gold-outer Georges Bank, salmon-off Sable Island.

As surface current analysis was not a component of the project's original scope, presentation is restricted to certain results off Cape Sable and Brier Island. As summarized previously, these waters experience persistent vertical mixing due to strong tidal currents, but RADARSAT only


detected thermal features off Cape Sable, not Brier Island. Figure 17 provides a comparison of the east/west (i.e. + u) and north/south (+ v) components of these tidal currents.

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Figure 17. Modelled tidal currents off Cape Sable and Brier Island presented as east/west (u) and

north/south (v) components during the period 20 Aug. to 31 Oct.

Modelled currents presented in Figure 17 show the east/west component of tidal currents off Cape Sable and Brier Island to be similar in magnitude, with those off Cape Sable being slightly stronger and approximating 0.5 m/s (1 knot). In contrast, the north/south component of tidal currents off Brier Island is several times greater than that off Cape Sable, varying about 0.8 m/s (1.6 knots). Off Cape Sable the north south component has a high of about 0.1 m/s (0.2 knots) and often is 0.05 m/s or less.


These same data are presented as u/v in Figure 18.

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Figure 18. u/v ratio versus time for tidal currents presented in Figure 17. Values of 100 = beyond scale.


4. Discussion and summary

In this study, oceanographic features are detected by their associated surface thermal signature and differentiated by geographic location and bathymetry. Results demonstrate that RADARSAT consistently detects certain oceanographic features, but not others.

RADARSAT consistently detected thermal features located off southwest Nova Scotia in August and September and Georges Bank in October. These waters are well known for their strong tidal currents and associated vertical mixing [14], the latter of which results in an associated surface thermal signature caused by relatively cold deep water being brought to the surface.

Throughout the months of August and September, RADARSAT never detected the large-scale thermal feature off the northern coast of Maine which Ullman and Cornillon [12] attribute to the Eastern Maine Coastal Current. This is a geostrophic current [13], where the horizontal pressure gradient is balanced by the Coriolis force.

This identification of underlying physical forces does not, however, explain why RADARSAT detected one type but not the other. For example, during August and September, RADARSAT did not detect surface thermal features in waters off Brier Island, which like the waters off southwest Nova Scotia are subject to vertical mixing caused by strong tidal currents. Another example would be that in October RADARSAT consistently detected thermal features off the outer reaches of Georges Bank that are attributable to the shelf-slope front and aspects of the Gulf Stream, the latter of which, like the Eastern Maine Coastal Current, is a geostrophic current.

MetOc parameters that influence the performance of synthetic aperture radar include surface wind speed, gradient and direction; surface current speed, gradient and direction; natural and anthropogenic surfactants and rain cells [5]. Off southwest Nova Scotia and Georges Bank, one or more of these parameters were consistently conducive to detection by RADARSAT, whereas off Brier Island, Grand Manan and within the Eastern Maine Coastal Current they were not. These MetOc parameters influence performance because they influence surface roughness.

Within the project area, rain cells are unlikely to be a significant factor in the months of August, September and October, thus rain cells are not given further consideration. And although small oil slicks could be present, no major oil spills were reported for these waters during the project period and the spatial scale of ocean features discussed in this report is much larger than that of small oil spills, therefore anthropogenic surfactants will also not be considered further.

4.1 Wind

A hypothesis investigated by this study is that DRDC Ottawa's RADARSAT-derived surface wind product can be used to create a detection mask – i.e. threshold, below or above which there is no point searching for the target of interest because wind conditions are not conducive to detection. The literature suggests a lower cut off of 2 to 3 m/s and upper cut off in the range of 8 to 14 m/s [5].


Results indicate that this hypothesis is not valid when the target of interest is oceanographic features that have a surface thermal signature. Such ocean features were detected when the RADARSAT-derived wind data indicated zero wind speed. This was observed almost over the entire range of sensor incidence angles used in this study (18 to 40 degrees).

Although a rigorous investigation was not conducted, it was generally observed that the RADARSAT images were dominated or consumed by atmospheric features (i.e. lacked discernable oceanographic features) above wind speeds of about 18 to 20 knots (9 to 10 m/s), which agrees with the above noted literature. However, although only five images were available for such purpose, RADARSAT consistently detected moderate to strong wind fronts in the vicinity of and to seaward of Georges Bank. And these wind fronts co-located with thermal features attributable to either the shelf-slope front [12] or warm core features (e.g. rings) of Gulf Stream origin [17]. These images involved upper wind speeds in the range of 18 to 27 knots (9 to 18 m/s) with wind differentials across the front ranging from a low of 5 to 10 knots (2.6 to 5.1 m/s) to a high of almost 20 knots (10.2 m/s). This observation suggests that detection of persistent wind fronts associated with certain thermal fronts, such as the northern wall of the Gulf Stream, could be used as an indicator of the front itself. This would be valid even at wind speeds in excess of 8 to 14 m/s.

Note that the observed upper wind speeds overlap with the upper cut off range reported in the literature. The point here is not to focus on wind speed per se but on the fact that wind speed, specifically wind-speed gradients, can be used as an indicator for oceanographic features that have a meteorological signal, such as strong, synoptic-scale thermal fronts. An additional incentive for developing this technique is that it involves conditions under which surface thermal techniques are rendered useless or of greatly diminished value due to spatially extensive and temporally persistent cloud cover.

It is observed that although both the Eastern Maine Coastal Current and Gulf Stream are geostrophic currents, and although both have a clear thermal signature, only the Gulf Stream is detected by means of an associated wind front.

One final comment pertaining to the surface wind vector is that the ocean does not respond to large-scale wind forcing instantaneously and therefore the RADARSAT-derived wind product may not reflect existing ocean features at time of satellite over pass. This would only be relevant to wind-driven oceanographic features, such as coastal upwelling.

4.2 Thermal signatures and current shear

Clearly, water temperature per se is not the determining factor when oceanographic features having thermal signatures are observed in RADARSAT imagery. When RADARSAT detects these type of oceanographic features it is actually detecting other physical parameters associated with the thermal feature. In this case, either surface wind, current or natural surfactant patterns or some combination thereof.

The ideal sea-surface temperature product is not a composite but a single image collected over time periods significantly shorter than the time period of environmental variability, which in maritime waters can be as short as a few hours. Tidally generated features, for example, change


on time scales of hours, whereas other features like the shelf-slope front and Gulf Stream change over significantly longer time periods. In Canadian waters, most of the time this ideal product cannot be realized with single passes of thermal or optical sensors due to cloud cover, and therefore compositing is required. Synthetic aperture radar does not have the cloud cover problem, but it does not detect water temperature per se.

The project demonstrated remarkable improvement in thermal feature detection by using latest-pixel composite (LPC) SST products instead of the available mean composite SST products. In specific instances, this may reflect applied cloud masking techniques as cloud masking artifacts were observed in some of the mean SSTs. The project also found that a grey-scale SST product was better for blending with a grey-scale RADARSAT product than a colourized SST product.

The project demonstrated an ability to identify thermal features in RADARSAT imagery by blending this imagery with grey-scale LPC SSTs and bathymetry. Without auxiliary information it is not possible to unequivocally identify such oceanographic features in RADARSAT imagery. This suggests that the role of RADARSAT in MetOc maritime sonar-related operations is complimentary – it should be used in combination with other spaceborne and in situ data. For example, the project observed a potential ability to estimate surface currents associated with fine-scale thermal features by blending RADARSAT imagery with grey-scale LPC SST imagery and bathymetric data using ENVI ZOOM blending tools. This could occur in near-real time and could involve highly isolated regions that are otherwise difficult to monitor.

Certain surface thermal oceanographic features were never detected by RADARSAT while others were consistently detected during the three month sampling period. A 12 year time series of surface thermal imagery of the project area suggests that the parameter that should be compared to RADARSAT imagery is not SST per se, but the gradient of SST. This is demonstrated in Figures 19 and 20, which are reproduced with permission from Ullman and Cornillon [12].

Thermal features observed in this study are consistent with results obtained over a 12 year period and presented in Figure 19, however, RADARSAT's ability to detect these features is not reflected in this figure. It is reflected in Figure 20, which shows the strength (i.e. magnitude) of the temperature gradient associated with these features. Figure 20 demonstrates that in summer thermal gradients in waters off Brier Island and Grand Manan, and in waters associated with the Eastern Maine Coast Current are weak in comparison with those associated with waters off southwest Nova Scotia, Georges Bank and in the vicinity of the northern wall of the Gulf Stream. When compared with RADARSAT imagery presented in this report, Figure 20 suggests that thermal gradients less than 0.4-0.5 deg/km are not detected by RADARSAT. It is possible that the thermal gradient is an indicator of current shear or cross-front variations in the atmospheric boundary layer, both of which can affect surface roughness, however, this is speculation as the project's scope did not permit investigation of these subjects.


Figure 19. Seasonal front detection probability averaged over the period 1985 to 1996, reproduced with

permission [12]. Summer = Jul.-Sep.; Autumn = Oct.-Dec.

Figure 20. SST mean gradient magnitude at frontal pixels for data presented in Figure 19, reproduced with

permission [12].

4.3 Currents and bathymetry

RADARSAT consistently detected bathymetric features of Georges Bank, which as already stated experiences strong tidal currents. At lowest normal tide, Georges Bank has depths of less than 15 metres and in certain sections less than 10 metres. Synthetic aperture radar sensors are known to detect bathymetric features to depths of tens of metres in waters experiencing strong currents and winds above 2 to 3 m/s but less than 8 to 10 m/s [20]. In this case, the sensor is detecting variations in surface current caused by topographic features.


In one of the five images collected in the vicinity of Georges Bank, RADARSAT also detected a feature which appears to be co-located with the outer edge of Browns Bank. In combination with features associated with Georges Bank, this resulted in a silhouette of the Northeast Channel, which separates Georges and Browns Bank. Browns Bank is not as shallow as Georges Bank, but has depths of 50 to 75 metres over large areas. And although not shown in Figure 1, a clockwise gyre has been observed on the western cap of Browns Bank [17], which as shown in Figure 1 would indicate opposing surface current directions within the area associated with the Northeast Channel. In short, like Georges Bank itself, variations in surface currents in combination with bathymetric features are the suggested cause of features observed in the areas of Browns Bank and the Northeast Channel.

RADARSAT consistently observed features off southwest Nova Scotia, which also involve shallow waters and strong tidal currents, but these features did not have the shape of underlying bathymetric features, often occurred in waters of 100 to 150 m depth, and had shapes often associated with eddies, jets and plumes. Similarly, although waters just off Brier Island and Grand Manan experience strong tidal currents and have shallow areas, RADARSAT never detected their associated thermal features. In short, strong tidal currents over relatively shallow waters do not always result in detection of bathymetric features, and strong tidal currents over relatively deep water can also result in ocean feature detection.

Limited tidal current analysis based on modelled conditions demonstrate that a fundamental difference between the currents off southwest Nova Scotia and those off Brier Island is that the southwest Nova Scotia currents flow predominantly in an east–west direction and have a relatively minor north–south component, whereas those off Brier Island flow predominantly and very strongly in a north–south direction. Thus, the directional flow of currents off southwest Nova Scotia approximate the range direction of the RADARSAT antenna whereas those off Brier Island approximate the azimuth direction. Such sensors are known to detect surface current gradients in the range direction (Paris Vachon, personal communication). These observations raise the possibility of project results exhibiting directional bias in the sensor's ability to detect oceanographic features.

In conclusion, although consistent detection of features off southwest Nova Scotia and a consistent lack thereof off Brier Island, Grand Manan and in waters of the EMCC are fundamental observations of the study, and although there is strong reason to support a hypothesis that the magnitude of the SST gradient at frontal pixels can be used as an indicator of thermal feature detection by RADARSAT, the underlying physical causes of observations presented in this study cannot be explained unequivocally with presented data. This aspect requires substantive additional analysis which focuses on surface current and atmospheric boundary conditions at time of satellite overpass.

4.4 Software Tools

The software requirements of this project were fulfilled by using a combination of four software products – IAPro, ENVI 4.3, ENVI ZOOM and Adobe Photoshop. IAPro is a research and development tool developed and owned by the government of Canada, whereas the other three are commercial products. ENVI is owned by ITT Visual and Photoshop by Adobe, both of which are based in the USA.


During post project-related meetings, representatives of the Canadian Ice Service and MetOc Halifax operations indicated they have no desire for a separate image display and manipulation system. Additional spaceborne capabilities, as recommended below, need to be integrated into existing or planned operational infrastructure rather than supplied as a separate entity requiring additional analyst time and training. In other words, even if IAPro, or an alternative product, could perform the desired functions, it is questionable as to whether federal operations would use this software in a stand-alone format.

A striking observation when viewing ENVI ZOOM and IAPro is how similar they look. Clearly, they target similar objectives, and clearly, they both have strengths and weaknesses when applied to the field of maritime MetOc feature analysis.

IAPro does not blend or properly display HDF files and almost all geotiff file formats used by the civilian oceanographic community. It also lacks key blending capabilities present in the existing version of ENVI ZOOM and its file searching, naming and retrieval interface requires improvement. ENVI ZOOM, on the other hand, performs all of these functions, very well.

But ENVI ZOOM also lacks functions required by this project and by maritime defence and security operations. Particularly, like ENVI 4.3, it does not load synthetic aperture radar imagery properly. ENVI ZOOM also does a poor job of blending images when there is little or no land present, which is very common to maritime operations. And although it loaded the NDI electronic hydrographic chart, it decimated its data and thereby projected a poor quality bathy chart. IAPro, on the other hand, performs all of these functions, very well.


5. Recommendations

5.1 Meteorological features

A. Using RADARSAT data, develop an automated process to detect meteorological features associated with the northern wall of the Gulf Stream and perhaps shelf-slope front. Once developed, test the algorithm on other oceanographic systems that persistently present a strong (i.e. thermal gradient greater than about 0.5 deg/km), synoptic-scale thermal front. Be certain to develop the algorithm in a manner that permits future integration into Canadian Forces MetOc automated feature extraction infrastructure.

B. RADARSAT imagery obtained through the Canadian Ice Service alone will be insufficient to implement recommendation 5.1(A) because it does not include the Gulf Stream and infrequently images the shelf-slope front. Canadian Ice Service RADARSAT imagery will need to be augmented with (i) more frequent sampling of Georges Bank and (ii) sampling of Gulf Stream edge waters. The latter could prove to be problematic if these waters are also imaged by the Canadian Forces for ship detection, which uses a different RADARSAT mode than that preferred for ocean feature and oil detection. Thus, expansion to include Gulf Stream waters will require co-ordination with the Canadian Forces Polar Epsilon project offices.

C. Do not differentiate RADARSAT-derived wind speeds below 2.5 m/s (5 knots). Just present as a single colour attributable to speeds less than 2.5 m/s and identify as "undetermined".

D. When conducting similar studies in the future, if feasible also collect air temperature and air temperature profile data over waters imaged by RADARSAT.

E. Do not report RADARSAT-derived wind speeds in knots. Use m/s.

5.2 Surface thermal features and currents

A. Obtain and install the U.S. Navy's automated processing system (APS) for thermal and optical satellite data as a means to automatically generate latest pixel composite products in support of federal maritime operations.

B. Once installed, modify the APS system such that it also produces a thermal gradient product. Blend the thermal gradient product with RADARSAT imagery as a means to further development of an ocean feature from RADARSAT product. Develop in a manner that, when operational, permits integration into Canadian Forces MetOc automated feature extraction infrastructure.

C. Blend the APS SST product and RADARSAT data to estimate surface current vectors associated with oceanographic features that have both a surface thermal and RADARSAT signal. Be certain to develop the algorithm in a manner that, when operational, permits integration into Canadian Forces MetOc automated feature extraction infrastructure.

D. When conducting similar studies in the future, if feasible also collect synoptic surface current data over waters imaged by RADARSAT. This can be accomplished through high-resolution


synoptic sampling (i.e. coastal HF radar) or preferably a combination of high-resolution synoptic sampling and modelling. Given the location of existing and planned Canadian civilian HF radar installations, this suggests future studies of this nature should also be carried out in the Gulf of Maine and waters off Southwest Nova Scotia.

5.3 Bathymetry

Develop an ESRI shape file of coastal bathymetric features, which when displayed with grey-scale and colourized spaceborne imagery permits co-visualization of bathymetric, oceanographic and meteorological features. This requirement likely dictates a need to develop isobath overlays that can be viewed in (i) different colours and (ii) different spatial resolutions. The latter is required to prevent excessive clutter that can inhibit feature viewing.

Ensure Crown intellectual property rights, thereby facilitating open and free distribution among federal agencies. Be certain to develop these products in a manner that, when operational, permits integration into Canadian Forces MetOc automated feature extraction infrastructure.

It is possible that this product already exists within the Canadian Forces Hydrographic Office or DRDC Atlantic, thus, implementation of this recommendation needs to be co-ordinated with representatives of these organizations.


References

[1] anon. (2001). Rapid environmental assessment warfare support. NATO EXTAC 777 (unclassified), NATO SACLANTCEN, La Spezia, Italy, p 14 plus annexes.

[2] anon. (2005). Rapid environmental assessment (REA). Canadian Forces TACNOTE 1520 (unclassified), Department of National Defence, Ottawa, Canada, p 14.

[3] Whitehouse, B.G. (2004). Survey of DRDC Atlantic's underwater warfare R&D opportunities in rapid environmental assessment. (DRDC Atlantic CR 2004-276). Defence R&D Canada – Atlantic, p 42.

[4] Vachon, P.W., Whitehouse, B.G., Renaud, W.M., De Abreu, R. and Billard, D. (2006). Polar Epsilon MODIS and fused MODIS / RADARSAT MetOc products for national defence and domestic security. (DRDC Ottawa TM 2006-067). Defence R&D Canada – Ottawa, p 48.

[5] Jackson, C.R. and Apel, J.R., (Eds.), (2004). Synthetic aperture radar marine user's manual. NOAA/NESDIS, U.S. Department of Commerce, ISBN 0-16-073214-X, p 464.

[6] Beal, R.C., Young, G.S., Monaldo, F.M., Thompson, D.R., Winstead, N.S. and Scott, C.A. (2005). High resolution wind monitoring with wide swath SAR: a user's guide. NOAA/NESDIS, U.S. Department of Commerce, p 155 plus Appendix.

[7] Lacoste, H., (Ed.), (2004). Proceedings of the second workshop on coastal and marine applications of SAR, Svalbard, Norway. European Space Agency publication SP-565, ESTEC, The Netherlands.

[8] Whitehouse, B.G. and Hutt, D. (2004). Ocean intelligence in the maritime battlespace: the role of spaceborne sensors and HF radar. Can. Mil. J., 5(1): 35-42.

[9] Whitehouse, B.G., Vachon, P.W., Thomas, A.C. and Quinn, R.J. (2005). Developing RADARSAT's METOC capabilities in support of Project Polar Epsilon. (DRDC Ottawa TM 2005-119). Defence R&D Canada – Ottawa, p 44.

[10] Whitehouse, B.G. (2003). Ocean intelligence in the maritime battle space – analysis and recommendations for a Canadian Forces maritime environmental assessment program. (DRDC Atlantic CR 2003-176). Defence R&D Canada – Atlantic, p 63.

[11] Dupont, F., Hannah, C.G. and Greenberg, D. (2005). Modelling the sea level in the upper Bay of Fundy. Atmosphere-Ocean, 43: 33-47.

[12] Ullman, D.S. and Cornillon, P.C. (1999). Satellite-derived sea surface temperature fronts on the continental shelf off the northeast U.S. coast. J. Geophys. Res., 104(C10): 23,459-23,478.


[13] Pettigrew, N.R., Churchill, J.H., Jansen, C.D., Mangum, L.J., Signell, R.P., Thomas, A.C., Townsend, D.W., Wallinga, J.P. and Xue, H. (2005). The kinematic and hydrographic structure of the Gulf of Maine Coastal Current. Deep-Sea Res. II, 52: 2369-2391.

[14] Garrett, C.J.R., Keeley, J.R. and Greenberg, D.A. (1978). Tidal mixing versus thermal stratification in the Bay of Fundy and Gulf of Maine. Atmosphere-Ocean, 16: 403-423.

[15] Loder, J.W. and Greenberg, D.A. (1986). Predicted positions of tidal fronts in the Gulf of Maine region. Continental Shelf Res., 6(3): 397-414.

[16] Townsend, D.W., Thomas, A.C., Mayer, L.M. and Thomas, M.A. (2004). Oceanography of the northwest Atlantic continental shelf (1,W). In A.R. Robinson and K.H. Brink (Eds.), The sea: the global coastal ocean: interdisciplinary regional studies and syntheses, Chapter 5, Harvard University Press.

[17] Smith, P.C., Houghton, R.W., Fairbanks, R.G. and Mountain, D.G. (2001). Interannual variability of boundary fluxes and water mass properties in the Gulf of Maine and on Georges Bank: 1993-1997. Deep-Sea Res. II, 48: 37-70.

[18] Young, G.S., Sikora, T.D. and Winstead, N.S. (2005). Use of synthetic aperture radar in finescale surface analysis of synoptic-scale fronts at sea. Weather and Forecasting, 20:311-327.

[19] Hutt, D., Stockhausen, J., Vachon, P.W. and Mosher, D. (2003). Capability of RADARSAT-1 for estimation of ocean surface currents on the Scotian Shelf. (DRDC Atlantic TM 2003-268). Defence R&D Canada – Atlantic, p 23.

[20] Alpers, W., Campbell, G., Wensink, H. and Zhang, Q. (2004). Underwater topography, In C.R. Jackson and J.R. Apel (Eds.) Synthetic aperture radar marine user's manual, Chapter 10, pp. 245-262. NOAA/NESDIS, U.S. Department of Commerce, ISBN 0-16-073214-X.


List of symbols/abbreviations/acronyms/initialisms

APS Automated Processing System (A)ATSR (Advanced) Along Track Scanning Radiometer AVHRR Advanced Very High Resolution Radiometer BoF Bay of Fundy BI Brier Island EMCC Eastern Maine Coastal Current GM Grand Manan Island IR Infra-red LPC Latest Pixel Composite MetOc Meteorology and Oceanography MLI Maurice Lamontagne Institute NASA National Aeronautic and Space Administration NATO North Atlantic Treaty Organization NOAA National Oceanic and Atmospheric Administration NRL (U.S.) Naval Research Laboratory R&D Research & Development SAR Synthetic Aperture Radar SST Sea-surface Temperature


Distribution list

Document No.: DRDC Ottawa CR 2007-101

LIST PART 1: Internal Distribution by Centre: 2 Library DRDC Ottawa (1 HC and 1 CD) 1 Gary Geling (1 HC) 1 Jeff Secker (1 HC) 2 Paris Vachon (1 HC and 1 CD)

6 TOTAL LIST PART 1

LIST PART 2: External Distribution by DRDKIM 1 DRDKIM (1 CD)

10 Brian Whitehouse, OEA Technologies Inc. (5 HC and 5 CD) 3650 Hammonds Plains Road, Unit 14, Suite 393 Upper Tantallon, NS B3Z 4R3

4 Daniel De Lisle, Canadian Space Agency (2 HC and 2 CD) 6767, Route de l'Aéroport Longueuil (St-Hubert), QC, J3Y 8Y9

2 LCdr Wayne Renaud, D MetOc Halifax (1 HC and 1 CD) P.O. Box 99000 Station Forces Halifax, NS, B3K 5X5

2 Lt(N) Darryl Williams, D MetOc Halifax (1 HC and 1 CD) P.O. Box 99000 Station Forces Halifax, NS, B3K 5X5

2 Roger DeAbreu, Canadian Ice Services (1 HC and 1 CD) 373 Sussex Drive Block E Ottawa, Ontario K1A OH3

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P.O. Box 1012 Dartmouth, Nova Scotia B2Y 3Z7


2 Robert Arnone, Naval Research Laboratory (1 HC and 1 CD) Stennis Space Center Stennis, MS 39248 USA

4 Andrew Thomas and Neal Pettigrew, School of Marine Sciences, Libby Hall (2 HC and 2 CD) University of Maine Orono, ME 04469-5706 USA

2 Dave Ullman, Graduate School of Oceanography (1 HC and 1 CD) University of Rhode Island South Ferry Road Narragansett, RI 02882 USA

2 Todd D. Sikora, Department of Earth Sciences (1 HC and 1 CD) Millersville University, P.O. Box 1002 Millersville, PA 17551-0302 USA

2 Richard Olsen, Forsvarets forskningsinstitutt (1 HC and 1 CD) (Norwegian Defence Research Establishment) P.O. Box 25, NO-2027 Kjeller Norway

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DOCUMENT CONTROL DATA (Security classification of title, body of abstract and indexing annotation must be entered when the overall document is classified)

1. ORIGINATOR (The name and address of the organization preparing the document. Organizations for whom the document was prepared, e.g. Centre sponsoring a contractor's report, or tasking agency, are entered in section 8.) Defence R&D Canada - Ottawa 3701 Carling Avenue Ottawa, Ontario K1A 0Z4

2. SECURITY CLASSIFICATION (Overall security classification of the document

including special warning terms if applicable.) UNCLASSIFIED

3. TITLE (The complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation (S, C, R or U) in parentheses after the title.) MetOc products for maritime operations: Thermal fronts, eddies and water mass boundaries with RADARSAT

4. AUTHORS (last name, followed by initials – ranks, titles, etc. not to be used) Whitehouse, B.G.

5. DATE OF PUBLICATION (Month and year of publication of document.) May 2007

6a. NO. OF PAGES (Total containing information, including Annexes, Appendices, etc.)

60

6b. NO. OF REFS (Total cited in document.) 20

7. DESCRIPTIVE NOTES (The category of the document, e.g. technical report, technical note or memorandum. If appropriate, enter the type of report, e.g. interim, progress, summary, annual or final. Give the inclusive dates when a specific reporting period is covered.) Contract Report

8. SPONSORING ACTIVITY (The name of the department project office or laboratory sponsoring the research and development – include address.) Canadian Space Agency (CSA) Government Related Initiative Program (GRIP) Project: "Impacts of the Marine Environment on Ship Detection by Commercial Spaceborne SAR"

9a. PROJECT OR GRANT NO. (If appropriate, the applicable research and development project or grant number under which the document was written. Please specify whether project or grant.) 15ec06-02

9b. CONTRACT NO. (If appropriate, the applicable number under which the document was written.) W7714-050947/001/SV

10a. ORIGINATOR'S DOCUMENT NUMBER (The official document

number by which the document is identified by the originating activity. This number must be unique to this document.) OEA10107

10b. OTHER DOCUMENT NO(s). (Any other numbers which may be assigned this document either by the originator or by the sponsor.) DRDC Ottawa CR 2007-101

11. DOCUMENT AVAILABILITY (Any limitations on further dissemination of the document, other than those imposed by security classification.)

( X ) Unlimited distribution ( ) Defence departments and defence contractors; further distribution only as approved ( ) Defence departments and Canadian defence contractors; further distribution only as approved ( ) Government departments and agencies; further distribution only as approved ( ) Defence departments; further distribution only as approved ( ) Other (please specify):

12. DOCUMENT ANNOUNCEMENT (Any limitation to the bibliographic announcement of this document. This will normally correspond to the

Document Availability (11). However, where further distribution (beyond the audience specified in (11) is possible, a wider announcement audience may be selected.)) Unlimited

13. ABSTRACT (A brief and factual summary of the document. It may also appear elsewhere in the body of the document itself. It is highly desirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the security classification of the information in the paragraph (unless the document itself is unclassified) represented as (S), (C), (R), or (U). It is not necessary to include here abstracts in both official languages unless the text is bilingual.)

Thirty-five RADARSAT images collected over eastern Canadian waters in 2006 were analyzed to determine this sensor's potential application to MetOc (meteorology and oceanography) operations. Analyses focused on blending RADARSAT, sea-surface temperature and bathymetric data as a means of detecting oceanographic features that have surface thermal signatures. Results demonstrate remarkable improvement in thermal feature detection by using latest-pixel composite sea-surface temperature products instead of the mean composite products presently used by federal maritime operations. RADARSAT consistently detected certain oceanographic features having surface thermal signatures, but not others. Conclusive explanation of this difference requires analysis of associated surface currents and atmospheric boundary conditions, however, results suggest there is a threshold in the magnitude of the surface thermal gradient associated with these frontal features, of about 0.4 to 0.5 oC/km, below which RADARSAT does not detect the feature. Although preliminary, these results suggest that under conditions of low to moderate wind speeds, the surface thermal gradient can be used as an indicator of ocean features detected by RADARSAT. Results also suggest a possible feature orientation bias in RADARSAT's ability to detect surface thermal features which stems from the sensor's directional bias in detecting the surface current gradient. RADARSAT consistently detected moderate to strong wind fronts to seaward of Georges Bank and these wind fronts co-located with thermal features attributable to either the shelf-slope front or aspects of the Gulf Stream. This observation suggests that detection of persistent wind fronts associated with certain thermal fronts, such as the northern wall of the Gulf Stream, could be used as an indicator of the thermal front itself.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (Technically meaningful terms or short phrases that characterize a document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus, e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus identified. If it is not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title.) MetOc, RADARSAT, AVHRR, MODIS, SST, sea-surface temperature, fronts, eddies, surface currents, water mass boundaries

Documents

MetOc products for maritime operations · data using ENVI ZOOM blending tools. The project observed remarkable improvement in thermal feature detection by using latest-pixel composite