The application of radio occultation observations for … Australian Meteorological and ceanographic Journal 62 The application of radio occultation observations for climate monitoring

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Australian Meteorological and Oceanographic Journal 62 (2012) 323–334

The application of radio occultation observations for climate monitoring and

numerical weather prediction in the Australian region

(Manuscript received October 2011; revised September 2012)

John Le Marshall1,2, Yi Xiao1, Robert Norman2, Kefei Zhang2, Anthony Rea3, Lidia Cucurull4, Rolf Seecamp3, Peter Steinle1, K. Puri1, E. Fu3,2 and Tan Le1

1Centre for Australian Weather and Climate Research (CAWCR)2RMIT University, Australia

3Bureau of Meteorology, Australia 4NASA, NOAA and Department of Defense Joint Center for Satellite Data Assimilation, USA

Introduction

The GPS/MET mission, the CHAMP (CHAllenging Minisatellite Payload) mission (Germany), the SAC-C mission (Argentina), the GRACE (Gravity Recovery And Climate Experiment) mission (two spacecraft, United States of America/Germany), the Constellation Observing System

for Meteorology, Ionosphere and Climate (COSMIC) mission and the Meteorological Operational satellite (MetOp) mission provide new observations for climate determination and operational meteorology which have been shown to provide significant information on the state of the atmosphere and ionosphere and have allowed significant improvements in the determination of the current and future atmospheric state. By employing the GPS constellation of satellites and radio receivers in low earth orbit, the radio occultation (RO) technique can provide valuable atmospheric temperature and moisture profile information (Melbourne et al. 1994,Ware

Corresponding author address: John Le Marshall, Centre for Australian Weather and Climate Research, GPO Box 1289, Melbourne, Vic. 3001, Australia. Email: [email protected]

Space-based GPS radio occultation observations provide significant information related to current and future atmospheric state. They also enable important ac-tivities such as examination of radiosonde performance and temperature trends. Three months of radio occultation observations have been assimilated using four dimensional variational assimilation into the global Australian Community Cli-mate Earth Systems Simulator, ACCESS-G, which is employed at the Australian Bureau of Meteorology to provide operational forecasts. The radio occultation data was found to improve the forecasts in the lower, middle and upper tropo-sphere. The improvement varies from small, to up to eight-hours improvement in 48-hour forecasts of mean sea level pressure. Because of the relatively unbi-ased nature of radio occultation observations, they have been used in a compara-tive study with radiosonde data to probe mean annual atmospheric temperature changes in the Australian region and the southern hemisphere. In particular, dif-ferences between radio occultation data based, area weighted annual average short-term (2007–2010) temperature trends over the Australian region and the southern hemisphere and those obtained by averaging data at radiosonde net-work sites were noted. In the Australian region a mid and upper tropospheric area averaged temperature increase from 2007 to 2010 was accompanied by an aver-age cooling at radiosonde sites. The radio occultation data have also been used to probe ionospheric content. Results documented here indicate the use of radio occultation data has the potential to improve operational analysis and forecasting in the Australian region and to make a very important and unique contribution to vital tasks such as climate monitoring.

324 Australian Meteorological and Oceanographic Journal 62:4 December 2012

et al. 1996, Kursinski et al. 1996, Rocken et al. 1997, Wickert et al. 2001). The study of planetary atmospheres has long used the RO technique (e.g. Fjeldbo et al. 1971) where it has provided much valuable information. The radio occultation technique is based on measuring the bending of radio waves by refractive index gradients in the atmosphere. Through observing this bending, a vertical profile of bending angle or refractive index can be derived and subsequently a profile of temperature/moisture. In the data sparse southern hemisphere with its dearth of conventional data, observations of this type are invaluable for analysis, forecasting and climate monitoring. These data have recently been employed in some analysis centres around the globe where they have been shown to improve operational global numerical weather prediction (Healy and Thepaut 2006, Cucurull and Derber 2008, Cucurull et al. 2008, Rennie 2010). Here we describe the use of COSMIC /FORMOSAT-3 Mission (Anthes et al. 2008), MetOp and GRACE radio occultation data in the ACCESS-G forecast system at the Bureau of Meteorology. The ACCESS system is based on the UK Met Office unified model (UM). We examine the utility of these data for forecasting over the southern hemisphere and in particular in the Australian region. We note the relatively small bias associated with the radio occultation data (Anthes 2011) and we characterise the bias and root mean square difference (RMS) between the radiosonde and GPSRO data used across the Australian region during these trials and earlier. In addition we point out the important ways these observations improve the database used for operational meteorology and for climate monitoring. In particular, we note their utility in both providing high vertical resolution profiles in southern hemisphere regions devoid of

conventional data, their utility for climate monitoring and also their ability, as a result of their limited bias, to improve the accuracy of radiative transfer calculations associated with using advanced sounders, which are often calibrated/tuned using model fields.

Radio occultation

The US global positioning system (GPS) comprises a constellation of 24 satellites that continuously transmit radio signals at frequencies of 1.57542 GHz (L1) and 1.22760 GHz (L2). When viewing these satellites from a low earth orbiting satellite (LEO) these GPS satellites are seen to rise or set behind the earth. As the GPS satellites rise or set, the signals measured by a receiver on the LEO satellites are directly affected by the earth’s ionosphere and atmosphere. Observations of the rising and setting of these satellites (GPS occultations) can be used to provide limb soundings of the earth’s atmosphere. These measurements have global coverage, high vertical resolution, and can provide information in almost all weather conditions. The active nature of the measurements and the inversion methodology provides an observation showing little bias through the upper troposphere and a significant part of the stratosphere. Several authors have provided the detailed theory of radio occultation using a receiver in space (Melbourne et al. 1994, Kursinski et al. 1997, Rocken et al. 1997). The methodology is based on measuring the amount of bending by refractive index gradients, of radio waves emitted by a GPS satellite when viewing the occultation process from a low earth orbiting satellite. The process is shown schematically in Fig.

Fig. 1. A schematic of a radio occultation observation. Retrieved profiles are attributed to the location of the tangent point.

Le Marshall et al: The application of radio occultation observations for climate monitoring and numerical weather prediction 325

1, where a radio wave from the GPS satellite is bent around the earth and received by a low earth orbiting satellite. The bending angle of the beam is alpha (α). The phase and amplitude of the transmitted GPS signal is measured by the receiver on the LEO satellite. Because the refractive index of the neutral atmosphere is greater than unity an excess phase delay results and the ray path is bent. As the satellites are in motion, the frequency of the received signal is Doppler shifted and if the position and velocity of the satellites are known accurately the excess Doppler shift can be inverted assuming spherical symmetry to yield bending angle and the impact parameter a = r0 n(r0), where r0 is the geocentric radial distance of the ray at closest approach and n is the refractive index. If the atmosphere is spherically symmetric, r is the geocentric radial distance, the bending angle may be written as;

...(1)α (a) = −2a ∫a∞ d lnn / dx( ) / x2 − a2( )1/2 dx

where x = nr. After using the two GPS satellite transmission frequencies to remove the bending due to the ionosphere (Vorob’ev and Krasil’nikova 1994), assuming a spherically symmetric

atmosphere, the variation of the corrected bending angle with the impact parameter can be inverted with an Abel transformation (Phinney and Anderson 1968, Fjeldbo et al 1971) to recover the refractive index profile.

n(x) = exp 1/π ∫ x∞ a(a) / x2 − a2( )1/2( )( )da. ...(2)

The refractive index profile, n(x), can be recovered numerically. The refractive index n is related to the refractivity N through n = 1 + 10–6 N. In the neutral atmosphere the refractivity is related to the temperature T and the partial pressure of dry air PD and water vapour PW through;

N = k1PD /T + k2PW /T + k3PW /T2, ...(3)

where the ki are experimentally determined constants (see for example Bevis et al. 1994). In a dry atmosphere (PW = 0), refractivity is proportional to density and the refractivity profile can be used to produce a temperature profile using the ideal gas law. In our case however, the refractivity measurement contains both temperature and water vapour information and this information can be derived using apriori profile information and a statistically optimal 4D variational inversion (eg. Healy and Eyre 2000).

Fig. 2. Mean annual atmospheric temperatures (°C) 2007 to 2010 in the Australian region (a) measured by radiosonde; (b) measured by radio occultation at a radiosonde sites; and (c) measured by area weighting all radio occultation observa-tions in the Australian region. Note: 2006 is an incomplete year, see text for more information.

(a) (b) (c)


The application of radio occultation data in the Australian region and the southern hemisphere

The dataAlmost five years of GPS radio occultation data over the Australian region (in this context 10°S to 50° S, 100°E to 150°E) and southern hemisphere have been studied. They have been compared to radiosonde data from 2006 onwards and have been used to monitor Australian region and southern hemisphere temperatures. They have also been used in data assimilation studies over the Australian region and in the southern hemisphere.

Monitoring regional and hemispherical temperature changesIn this study as part of documenting the utility of radio occultation observations, GPS radio occultation data have been compared to radiosonde data in the Australian region and over the southern hemisphere. Their utility for climate monitoring is also examined. Several papers have been published using radio occultation data for climate applications, a number of these are referred to in the 'Climate Applications' sections of Anthes et al. 2008 and Anthes 2011.

In the Australian region, annual average contemporaneous and co-located, radiosonde and radio occultation temperatures from UCAR (CDAAC V2010.2640) have been compared from day 194 in 2006 after the launch of the COSMIC constellation, to the end of 2010 (Fig. 2(a) and (b)). The radiosonde type used in the Australian region is the Vaisala RS92 (from 2006 onwards). It is important to note that average data labelled 2006 only represents data from day 194 to the end of that year, all other years have full annual coverage. Figure 2(c) shows the area weighted mean annual temperature over the Australian region when all radio occultation observations are used. The area weighted annual average has been calculated by averaging data in 10° squares, weighted by the area of the square. Figures 2(a) and (b) show the similarity of the annual average temperature record for radiosonde and radio occultation data at the radiosonde sites. This similarity is also reflected in Fig. 4(a) which shows that in the Australian region the magnitude of the bias between radiosonde and radio occultation data is generally less than 0.3 °C except in the lower troposphere and between 400 and 150 hPa and that a comparatively steady bias is seen from year to year. The RMS difference is generally between 1.5 °C to 1.7 °C. Figure 2(c) shows the area

Fig. 3. Mean annual atmospheric temperatures (°C) 2007 to 2010 over the southern hemisphere: (a) measured by radiosonde; (b) measured by radio occultation at a radiosonde sites; and (c) measured by area weighting all radio occultation observa-tions over the southern hemisphere. Note: 2006 is an incomplete year, see text for more information.

(a) (b) (c)


weighted annual average temperatures for all occultations over the Australian region, removing to a significant extent effects caused by the Australian region radiosonde geographic distribution. Figure 2(c) is largely consistent with Fig. 2(a) and (b), however temperature values are a little warmer due to enhanced sampling of the warmer tropical region. An interesting comparison can be made during this period between averages at radiosonde sites and those in the Australian region. The cooling trend between 2007 and 2010 shown in the mid to lower troposphere in the Australian region at radiosonde sites (e.g. 500 to 300 hPa) is reversed when looking at area weighted data over the region in Fig. 2(c) for the same period. Over the southern hemisphere annual average temperatures from contemporaneous and co-located radiosonde and radio occultation data are displayed in Fig. 3(a) and (b). The radiosonde types used over the southern hemisphere are Vaisala (~88 per cent), VIZ (~10 per cent) and others (~2 per cent). Again it should be noted the 2006 average represents the average from day 194 until the end of the year. Figure 3(a), (b) and 4(b) show good correspondence between radiosonde and radio occultation data with biases generally less than 0.25 °C except between 400 and 250 hPa and in the low troposphere. RMS values vary from 1.5 to 2.0 °C except in the low troposphere. A reduction in bias between contemporaneous and co-located radiosonde and radio occultation data is apparent in much of the troposphere when averaging across the southern hemisphere in comparison to the Australian region. Figure 3(c) shows the area weighted annual variation of average annual temperature determined from all radio occultation observations over the southern hemisphere. There appear some in flattening of trends in Fig. 3(c) (due to increased sampling) in comparison with data collected at the radiosonde sites however trends generally appear similar. An interesting comparison can be made during this period between hemispheric averages and those in the Australian region. The cooling trend shown in the mid to lower troposphere at radiosonde sites in the Australian region (e.g. 500–300 hPa) is reversed when looking at hemispheric data in Fig. 3(c). It was also found, for global coverage (all radio occultation observations)—not shown, that a modest warming trend in the period 2007 to 2010 in the lower troposphere was evident as with the southern hemisphere. Overall the radio occultation data have demonstrated a potential for climate monitoring. They show small biases (Kuo et al. 2004; He et al. 2009; Ho et al. 2009), show consistent trends to radiosonde data at radiosonde sites and importantly allow extension of the domain of climate quality observations into data sparse areas usually devoid of basic upper air temperature information providing new information such as tropospheric warming in the Australian region in the period studied and a flattening of southern hemisphere tropospheric warming in the period studied. Their full potential for climate monitoring however will be realised with a longer record, a larger constellation and

accounting for a changing data distribution because of an ever changing observation constellation.

The assimilation of GPS RO dataThree separate months of GPS radio occultation data from the COSMIC/FORMOSAT-3, MetOp and GRACE missions have been used in data impact studies. The first of period of study was from 26 February 2009 to 26 March, 2009. The majority of the data used came from the COSMIC constellation. The radio occultation data taken from the COSMIC constellation has been processed at UCAR to provide observation data including, pressure, temperature and moisture profile information, bending angle, refractivity, data quality and other important information. The data has been transmitted on the global telecommunications system (GTS) in BUFR format. The data coverage (1638 sounding positions) for 15 March 2009 is shown in Fig. 5 and it is typical of the period studied. A smaller amount of GRACE data processed at the German Research Centre for Earth Science (GFZ) and MetOp data processed by the GRAS Satellite Application Facility (DMI 2008) has also been used. The second period of study was from 13 October to 12

Fig. 4. Annual average mean and RMS temperature differ-ences between co-located and contemporaneous ra-diosonde and radio occultation observations in the (a) Australian region and (b) southern hemisphere. Note: 2006 is an incomplete year, see text for more information.

(a)

(b)


November 2010. The data coverage (1281 sounding positions) for 13 October 2010 is shown in Fig. 6. The reduction in the number of sounding positions in the second trial was caused by a loss of COSMIC data due to problems with part of the COSMIC constellation prior to the commencement of this study, namely, COSMIC FM3 was out of contact from 1 August 2010 and COSMIC FM5 was out of contact from 26 September

2010 and returned to service on 10 November 2010.In these first two studies we have assimilated refractivity. The expected fractional refractivity errors (Kursinski et al.1997, 2000) are around 0.002 at altitudes between 9 and 25 km and increase to near 0.01 as the profile descends from 9 km to the surface. This translates to better than one degree Kelvin temperature accuracy from the upper troposphere into the lower stratosphere (9 km to 25 km). GPS/MET temperatures have been found to be consistent with ECMWF analyses at the 1 to 1.5 K level RMS (Kursinski et al.1996, Rocken et al. 1997) while geopotential height comparisons with ECMWF have been found to be consistent at the 20 m level (Leroy 1997). GPS radio occultation data during the period studied (5 March to 20 March 2009) have been compared to radiosonde observations (within 400 km) and some results are shown in Fig. 7. The figure shows a plot of the mean temperature difference (red) between COSMIC and radiosonde observations for the period studied. Little bias is seen in the upper troposphere and lower stratosphere. This characteristic of the radio occultation data make them potentially very useful for numerical modelling as well as climate studies, where they can remove bias from the analysis process. In the third of these studies, the trial period was from 1 November to 30 November 2010. The data coverage in this trial was a little better than that in trial two with COSMIC FM5 being out of contact from 26 September 2010 and returning to service on 10 November 2010. In this study bending angle was assimilated.

The assimilation trialsThe ACCESS system (Puri et al. 2012) is now being employed at the Australian Bureau of Meteorology to provide real-time operational forecasts. Before being incorporated into this new operational numerical weather prediction (NWP) suite, radio occultation observations from COSMIC, MetOp and GRACE, taken over the globe, were used in assimilation trials undertaken using the global version of the ACCESS system (ACCESS-G). The system and assimilation methodology are described below. The trials examined the impact of using COSMIC, MetOp and GRACE radio occultation observations on forecast accuracy in the southern hemisphere and in particular over the Australian region. The results from the trials were consistent with earlier results over the southern hemisphere (for example Cucurull et al. 2008, Rennie 2010) and importantly provided quantification of the beneficial impact of the radio occultation data over the Australian region.

The assimilation systemThe ACCESS system was implemented to upgrade the Bureau's NWP and climate modelling capabilities. The system employed in this trial, ACCESS-G, is based on the atmospheric model of the UK Met Office (UKMO). The system was initially implemented in the Centre for Australian Weather and Climate Research (CAWCR) and is

Fig. 5. GPS radio occultation sounding positions for 15 March 2009. (Image courtesy of UCAR).

Fig. 6. GPS radio occultation sounding positions for 13 October 2010. (Image courtesy of UCAR).

Fig. 7. A plot of the mean temperature difference (red) between COSMIC and radiosonde observations for the period 5 March to 20 March 2009. The number of samples (blue) is also shown. (Data courtesy of UCAR).


now used operationally in real time in the Bureau’s National Meteorological and Oceanographic Centre (NMOC). The ACCESS-G system has a global domain and a horizontal resolution of around 100 km with 50 vertical levels. The ACCESS system has been configured using the UKMO’s suite control system, and uses the UK Met. Office unified model (UKUM), the 4DVAR analysis system, the UK Met. Office observation processing system (OPS) and the surface fields processing system (SURF). The ACCESS-G suite uses daily sea surface temperature (SST) analyses from the BLUElink (Brassington et al. 2007, Beggs et al. 2010) ocean forecasting system. The ACCESS-G suite runs with a six-hour cycle, four times per day (0000 UTC, 0600 UTC, 1200 UTC, 1800 UTC), using observations processed by OPS, with a 4DVAR analysis. The ACCESS-G system is summarised in the table below.

Table 1. A summary of the characteristics of the ACCESS-G system.

Domain Global

Unified model horizontal resolution

(217 x 288) ~125 km x 83 km

Analysis horizontal resolution (163 x 216) ~166 km x 111 km

Vertical resolution L50

Observational data used (6 h window)

AIRS, ATOVS, SCAT, AMV, SYNOP, SHIP, BUOY, AM-DAR, AIREP, TEMP, PILOT

Sea surface temperature (SST) analysis

Weekly global 1° SST analysis

Model time step 15 minutes

Analysis time step 40 minutes

The data assimilation methodologyIn these trials the impact of using COSMIC, MetOp and GRACE radio occultation observations on forecast accuracy in the southern hemisphere, and in particular over the Australian region, was examined. For the first two periods studied, four dimensional variational assimilation (4DVAR) was employed to assimilate radio occultation refractivity data into the global ACCESS system (ACCESS-G), which provided forecasts to five days. In the third period studied 4DVAR assimilation was employed to assimilate radio occultation bending angle data into the ACCESS-G system, which again provided forecast to five days. The forecasts for the control run and the experimental run were verified after five days against their own analyses. The control run contained the full operational database used at the Bureau of Meteorology and included the data listed in Table 1. The experimental run was identical to the control run in all aspects except that it had radio occultation data added to the full operational database for use in the analysis step. Typically in the first assimilation period 500 to 600 occultation observations were available to the system every six hours, of which about two-thirds of the data were assimilated usually between around 580 to 20 hPa. This number was reduced

during the second assimilation period (typically 300 to 400 occultations), influenced by the loss of COSMIC FM3 data from 1 August 2010 and COSMIC FM5 being out of contact from 26 September 2010 and not returning to service until 10 November 2010.

The results

In the first trial (26 February to 26 March 2009) refractivities associated with radio occultation events were assimilated into the ACCESS-G system. The S1 skill score, the root mean square errors and the anomaly correlations (ACs) have been computed for the forecasts to five days. An improvement in the forecast accuracy was seen in mean sea level pressure and also through the lower, middle and upper troposphere. The magnitude of the improvements was appreciable. Verifications for the Australian region (65.0°S 17.125°N, 65.0°E 184.625°E) are shown in Fig. 8. In terms of the root mean square errors and the ACs, the addition of the COSMIC, MetOp and GRACE radio occultation data has led to an improvement for the one to five-day period for MSLP, 500 hPa geopotential height and 200 hPa geopotential height. The magnitude of the improvement in the Australian region was around three points in AC or eight-hours at five days near the surface (MSLP) and in addition there was clear improvement at standard levels through the troposphere. As expected, the temperature fields, for example the 200 hPa temperature field (not plotted), shows consistent improvements. The degree of improvement in this study for example for a five-day forecast at 500 hPa over the southern hemisphere from using COSMIC data was consistent with that shown in Cucurull and Derber (2008), although the definition of the southern hemisphere annulus is a little different. Generally the magnitude of the impacts from COSMIC data shows their potential for improving forecasts in the Australian region and overall their use in other forecast offices has shown them to be of significant value for NWP. In the second trial (13 October to 12 November 2010) refractivities associated with radio occultation events were again assimilated into the ACCESS-G system. The S1 skill score, the root mean square errors and the anomaly correlations were again computed for the forecasts to five days. A modest improvement in the forecast accuracy was seen in mean sea level pressure and also through the lower, middle and upper troposphere. Verifications for the Australian region (65.0°S 17.125°N, 65.0°E 184.625°E) are shown in Fig. 9. In terms of the root mean square errors and the ACs, it can be seen the addition of the COSMIC, MetOp and GRACE radio occultation data has led to a modest improvement for the one to five-day period, for MSLP, 500 hPa geopotential height and 200 hPa geopotential height. The magnitude of the improvement in the Australian region was over one point in AC near the surface (MSLP), reduced from the previous study, and again there was improvement at standard levels through the troposphere.


AN-CORRELATION AN-CORRELATION AN-CORRELATION

Fig. 8. RMS errors and ACs for forecasts to five-days for the Australian region for (a) ACCESS-G MSLP; (b) ACCESS-G 500 hPa; and (c) ACCESS-G 200 hPa. Control (black) and GPS RO data (red) for the period 26 February to 26 March 2009.

96

RMS_ERRORS RMS_ERRORS RMS_ERRORS


0 24 48 72 12096Hours

0 24 48 72 12096Hours

0 24 48 72 12096Hours

0 24 48 72 12096Hours

0 24 48 72 12096Hours

0 24 48 72 12096Hours

3.48

3.09

2.7

2.32

1.58

1.54

1.16

0.772

0.386

0

69.2

61.5

53.8

46.1

38.5

30.8

23.1

15.4

7.69

0

41.8

37.1

32.5

27.8

23.2

18.6

13.9

9.28

4.64

0

100

98

96

94

92

90

88

86

84

82

100

98.4

96.7

95.1

93.5

91.8

90.2

88.6

86.9

85.3

100

98.1

96.2

94.4

92.5

90.6

88.7

86.9

85

83.1


Fig. 9. RMS errors and ACs for forecasts to five-days for the Australian region for (a) ACCESS-G MSLP; (b) ACCESS-G 500 hPa; and (c) ACCESS-G 200 hPa. Control (black) and GPS RO data (red) for the period 13 October to 12 November 2010.

100

97.8

95.7

93.5

91.4

89.2

87.1

84.9

82.8

80.6

43.7

38.8

34

29.1

24.3

19.4

14.6

9.71

4.85

024 48 72 12096

Hours0

24 48 72 12096Hours

0

3.53

3.14

2.75

2.35

1.96

1.57

1.18

0.785

0.392

024 48 72 12096

Hours0

100

97.9

95.9

93.8

91.8

89.7

87.7

85.6

83.6

81.524 48 72 12096

Hours0

53.9

47.9

41.9

35.9

30

24

18

12

5.99

0

100

98.1

96.2

94.3

92.4

90.5

88.5

86.6

84.7

82.8

24 48 72 12096Hours

0

24 48 72 12096Hours

0


Fig. 10. RMS errors and ACs for forecasts to five-days for the Australian region for (a) ACCESS-G MSLP; (b) ACCESS-G 500 hPa; and (c) ACCESS-G 200 hPa. Control (black) and GPS RO data (red) for the period 1 November to 30 November 2010.

Fig. 11. RMS errors and ACs for forecasts to five-days for the southern hemisphere for (a) ACCESS-G MSLP; (b) ACCESS-G 500 hPa; and (c) ACCESS-G 200 hPa. Control (black) and GPS RO data (red) for the period 1 November to 30 November 2010.





3.15

2.8

2.45

2.1

1.75

1.4

1.05

0.699

0.35

0

100

98.5

96.9

95.4

93.8

92.3

90.7

89.2

87.7

86.1

24 48 72 12096Hours

0

24 48 72 12096Hours

0

39.9

35.5

31.1

26.6

22.2

17.7

13.3

8.87

4.44

024 48 72 12096

Hours0

100

98.3

96.6

94.8

93.1

91.4

89.7

87.9

86.2

84.524 48 72 12096

Hours0

52.4

46.6

40.7

34.9

29.1

23.3

17.5

11.6

5.82

0 24 48 72 12096Hours

0

100

98.4

96.8

95.2

93.7

92.1

90.5

88.9

87.3

85.724 48 72 12096

Hours0

4.55

4.04

3.54

3.08

2.53

2.02

1.52

1.01

0.506

0

100

98.2

96.4

94.6

92.8

91

89.2

87.4

85.5

83.7

24 48 72 12096Hours

0

24 48 72 12096Hours

0

49.2

43.7

38.3

32.8

27.3

21.9

16.4

10.9

5.47

024 48 72 12096

Hours0

100

98.4

96.9

95.3

93.7

92.1

90.6

89

97.9

85.924 48 72 12096

Hours0

2.72

2.41

2.11

1.81

1.51

1.21

0.906

0.604

0.302

024 48 72 12096

Hours0

100

97.4

94.8

92.2

89.6

87

84.4

81.8

79.2

76.724 48 72 12096

Hours0


The magnitude of the improvement in the Australian region was probably influenced by the loss of COSMIC FM3 data from 1 August 2010 and COSMIC FM5 being out of contact from 26 September 2010 and not returning to service until 10 November 2010. In relation to the third trial (1 November to 30 November 2010) the bending angles associated with radio occultation events were assimilated into the ACCESS-G system. The S1 skill score, the root mean square errors and the anomaly correlations were again computed for the forecasts to five days. An improvement in the forecast accuracy was seen in mean sea level pressure and also through the lower, middle and upper troposphere. The magnitude of the improvements, as in the first trial, was appreciable. Verifications for the Australian region are shown in Fig. 10. In terms of the root mean square errors and the anomaly correlations, it can be seen the addition of the COSMIC, MetOp and GRACE radio occultation data has again led to an improvement for the one to five-day period, for MSLP, 500hPa geopotential height and 200 hPa geopotential height. The magnitude of the improvement in the Australian region was almost three points in AC or almost eight-hours at five days near the surface (MSLP) and in addition there was clear improvement at standard levels through the troposphere. In relation to the third trial (1 November to 30 November 2010) verification over the southern hemisphere annulus (60°S 20°S, 0°E 360°E) has also been shown for completeness. The S1 skill score, the root mean square errors and the ACs were again computed for the forecasts to five days. An improvement in the forecast accuracy was seen in mean sea level pressure and also through the lower, middle and upper troposphere. The magnitude of the improvements, as in the first trial, was appreciable. Verifications for the southern hemisphere annulus are shown in Fig. 11. In terms of the root mean square errors and the ACs, it can be seen the addition of the COSMIC, MetOp and GRACE radio occultation data has again led to an improvement for the one to five-day period, for MSLP 500 hPa geopotential height and 200 hPa geopotential height. The improvement for the southern hemisphere annulus was seen at standard levels through the troposphere.

The future

The use of GPS radio occultation data in meteorology for research and operational activities is expanding. In the Australian region GPS radio occultation data are improving meteorological analyses by providing atmospheric temperature observations which have little bias through most of the troposphere and the stratosphere. Their addition to the meteorological database in the southern hemisphere has resulted in improved predictability both within the Australian region and across the southern hemisphere. The comparison of radio occultation temperature observations with radiosonde temperature measurements also indicates their utility for extending the monitoring of climate at

radiosonde network sites, to a fuller monitoring across both the Australian region and the southern hemisphere with new information including trends being revealed. The beneficial impact of radio occultation data in the Australian region and southern hemisphere has generally been found to be larger than in the northern hemisphere. As a result the maintenance of the space-based constellations used to provide radio occultation observations in the Australian region is important for operational and research meteorology and climate monitoring. Simulation studies have been conducted to model the distributions of daily RO

Fig. 12. Distribution of simulated daily COSMIC II RO events in the Australasian region with GPS, Galileo, Glonass and QZS-1.

Fig.13. Ionospheric profile obtained from Fedsat radio wave occultation measurements on day 170, 2003, at 15.3N 170E.


events (ROEs) from future global navigation satellite system (GNSS) satellites, namely COSMIC II with GPS, Galileo, Glonass and QZS-1. With these GNSS satellites (Galileo, Glonass and QZS-1), the number of daily ROEs from COSMIC II (Fig. 12) will be increased significantly, reaching over 3000. In this regard it should be noted Australia has already launched a microsatellite, Fedsat, into low earth orbit (LEO) in 2002 and occultation measurements from GPS receivers on board this LEO satellite have been used to profile the atmosphere down to 100 km. This work is reported in Norman et al. (2011) and an occultation profile obtained for that mission showing electron density is displayed in Fig. 13. This early investment in Australia to develop the ability to integrate and use a GPS radio occultation capability, positions Australia well in terms of Australian support for future missions, in respect to ground-based reception, processing of mission data and to the possible provision of spaceborne hardware. This could be a modest investment ensuring a suitably sized LEO constellation, providing great benefit to the Australian community in terms of weather forecasting, prediction of severe weather, accurate climate monitoring and a number of other benefits.

Summary and conclusions

Data provided by the COSMIC/FORMOSAT-3, MetOp and GRACE missions have been used to limb sound the atmosphere. These observations have allowed high vertical resolution sounding of an accuracy which makes them beneficial for NWP and of particular value in the Australian region which is in the data sparse southern hemisphere. Here we have assimilated both refractivity and bending angle data produced from COSMIC observations by the COSMIC Data Analysis and Archive Center at UCAR, from GRACE data processed at the German Research Centre for Earth Science (GFZ) and from MetOp data processed by the GRAS Satellite Application Facility (DMI 2008) and this has resulted in an improvement in forecast accuracy in the lower middle and upper troposphere. The magnitude of the impact varies from small to up to eight-hours improvement at five days when examining 48-hour forecasts at mean sea level pressure. These data are unaffected by clouds and precipitation, do not show instrumental drift and do not require calibration. Their use is well suited to NWP and climate studies for a number of reasons. The improved vertical resolution of the radio occultation data near the tropopause complements the advanced sounder observations in that region, while for example the higher horizontal resolution of the advanced sounders assists in the effective use of radio occultation data. In addition their unbiased nature between the mid troposphere and the mid stratosphere allows them to potentially improve the bias tuning/calibration schemes used in numerical weather prediction and in climate reanalysis, and in conjunction with their direct use, to potentially reduce bias in the analysis process. In effect, it is anticipated

the unbiased nature of these observations will improve the absolute accuracy of reanalyses and also assist in the vertical resolution of features such as the tropopause. Both these aspects are of importance in climate studies. In addition, the comparison of radio occultation temperature observations with radiosonde temperature measurements indicates their utility for extending the monitoring of temperature from, around radiosonde network sites, to a fuller monitoring across both the Australian region and the southern hemisphere with new information including trends being revealed. In essence it shows radio occultation temperature observations exhibit consistent trends to radiosonde data at radiosonde sites and importantly allow extension of the domain of climate quality observations into data sparse areas often devoid of basic upper air temperature information. This has been shown to provide new information such as tropospheric warming in the Australian region in the period studied and a flattening of southern hemisphere tropospheric warming compared to radiosondes in the period studied. These results show that radio occultation data have the potential make a very important and unique contribution to vital tasks such as climate monitoring, providing a greater spatial and temporal coverage than the radiosonde network. Currently a follow-on mission to the COSMIC/FORMOSAT-3 mission is under consideration and several other GNSS related systems are being prepared to provide occultation data. Overall it would appear Australian meteorological, oceanographic, climate and environmental monitoring and prediction will clearly benefit from such missions. In relation to these endeavours, activity is under way in Australia which may lead to Australia making a contribution to a GNSS mission. In conclusion, GNSS radio occultation data have established themselves as an important component of a modern data assimilation system database. In the southern hemisphere, and in particular over the Australian region their contribution to Australian meteorological and climate monitoring has been and will continue to be considerable. As a result of these trials the radio occultation data was introduced into the NMOC operational database.


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