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Statistical issues in radiometer- and scatterometer-derived winds over the Mediterranean Sea. F. De Biasio, S. Zecchetto Istituto di Scienze dell'Atmosfera e del Clima (ISAC) Institute of Atmospheric Sciences and Climate Consiglio Nazionale delle Ricerche (CNR) - PowerPoint PPT Presentation
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Bari, 4-5 dicembre 2012 - Villa Larocca 1
VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Statistical issues in radiometer- and scatterometer-derivedwinds over the Mediterranean Sea
F. De Biasio, S. Zecchetto
Istituto di Scienze dell'Atmosfera e del Clima (ISAC)Institute of Atmospheric Sciences and Climate
Consiglio Nazionale delle Ricerche (CNR)Italian National Research Council
Corso Stati Uniti 4, 35127 Padova, Italy
Corresponding author: F. De Biasio – email: f.debiasio@isac.cnr.it
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Outline
Motivations for the study
Datasets description
Local statistics
Global statistics
Conclusions
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Motivations
• Need to resolve the sea surface wind flow in the spatial range 2-200 km (meso- and meso- scales), especially where the flow interacts with the orography
• Lack of OWV observations: temporal and spatial gaps
• Need of specific and complete OWV datasets for use in numerical weather and storm-surge prediction models (ESA DUE projects: eSurge-Venice1 and eSurge2)
1 http://www.esurge-venice.eu
2 http://www.storm-surge.info
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Dataset description: WindSatWindSat is a polarimetric microwave radiometer onboard the Coriolis satellite, developed by the U.S. Naval Research Laboratory (NRL) .Polarimetric radiometer operating at 6.8, 10.7, 18.7, 23.8 and 37GHz
OPERATIVE: 2003 - presentDATA: 0.25°x 0.25° gridded OWV data downloaded
from Remote Sensing Systems (RSS)SWATH: One swath 950 km widePERIOD 1: February 2003 – November 2009 (QuikSCAT)PERIOD 2: March 2009 – November 2010 (ASCAT)
Data processing: only validated data ≥ 3 ms-1 and ≤ 25 ms-1
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Dataset description: QuikSCATQuick Scatterometer (QuikSCAT) is a satellite of National Aeronautics and Space Administration (NASA)The onboard SeaWinds instrument is a Ku-band microwave scatterometer.
OPERATIVE: 1999 - 2009DATA: 12.5km OWV data provided by and
downloaded from the NASA's Physical Oceanography Distributed Active Archive Center (PO.DAAC)
SWATH: One swath 1800 km widePERIOD 1: February 2003 – November 2009 (QuikSCAT)
Data processing: gridded on a 0.125°x 0.125° regular grid. Data flagged for rain are discarded. Downsampled to the 0.25°x 0.25° grid. Data ≥ 3 ms-1 and ≤ 25 ms-1 are kept
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Dataset description: ASCATASCAT: C-band scatterometer onboard the MetOp-A satellite of the European Space Agency (ESA) operated by the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT)The ASCAT instrument is a C-band microwave scatterometer
OPERATIVE: 2006 - presentDATA: 12.5km OWV data provided by the Royal
Netherlands Meteorological Institute (KNMI), downloaded from the NASA's Physical Oceanography Distributed Active Archive Center (PO.DAAC)
SWATH: Two swaths 550 km wide, central gap 672 kmPERIOD 2: March 2009 – November 2010 (ASCAT)
Data processing: gridded on a 0.125°x 0.125° regular grid, downsampled to the 0.25°x 0.25° grid. Data ≥ 3 ms-1 and ≤ 25 ms-1 are kept
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Number of monthly observationsOctober 2009
WindSat: 62K observationsQuikSCAT: 150K – 35k collocationsASCAT: 98K - 4K collocations
WindSat 4-6 a.m. + 2-5 p.m.QuikSCAT 2-6 a.m. + 3-7 p.m.ASCAT 6-10 a.m. + 5-9 p.m.
WindSat + ASCAT cover 4-10 + 14-21~ 2 hits per point per day
Dataset description: temporal coverage
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
WindSat Max hits ~50 Very sensitive to the
presence of land up to ~50 km from coast
The Adriatic and Aegean seas are totally uncovered
QuikSCAT Max hits ~50 Data up to near coast
ASCAT Max hits ~40 Mostly homogeneous
coverage
Dataset description: spatial coverage
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Local statistics: wind speed correlation
WindSat-QuikSCAT :• ranges between 0.7 and 1• highest correlation in the Gulf of Lion and in the
Levantine Basin, east of Crete (steady and strong winds)
• homogeneous throughout the BasinWindSAT-QuikSCAT
WindSAT-ASCAT
WindSat-ASCAT:• ranges between -0.2 and 1• lowest correlations in the Ionian Sea• higer variability than with QuikSCAT, but also shorter
period
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Local statistics: wind speed bias
Windsat-QuikSCAT:• ranges between -0.3 and 0.9 m/s• little variations throughout the basin
WindSAT-QuikSCAT
WindSAT-ASCAT
Windsat-ASCAT:• ranges between -1.5 and 1.8 m/s
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Local statistics: wind speed RMS deviation (centred)Centred root mean square deviation is calculated from zero-mean samples
WindSAT-QuikSCAT
QuikSCAT:• high values where the orography distorts the
main flow: Gulf of Lion, Bonifacio Strait, southeast of Sicily and Calabria, south of Crete, Black Sea
• ranges in [0.8-1.7] m/s
WindSAT-ASCAT
ASCAT:• several spots of high values. Higher variability than QuikSCAT• ranges in [0.3-1.9] m/s
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Local statistics: wind direction correlationWindSat-QuikSCAT :• ranges between 1.1 to 1.7 (theory: 0-2.
Crosby et al., 19931)• is homogeneous throughout the Basin, but
statistics over 7 years, while ASCAT is only 1 year
WindSAT-QuikSCAT
WindSat-ASCAT:• ranges between 0.7 and 1.9• low correlation in Lybian Sea, central Ionian
Sea and Tyrrhenian Sea
WindSAT-ASCAT1 Crosby, Breaker, and Gemill, 1993. J. Atmos. Oceanic Technol., 10, 355–367.
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Global statistics: wind speed correlation vs. time2003-2009: WindSAT/QuikSCAT :• 7 year mean > 0.87• no trends• annual signal
2009-2010: WindSAT/ASCAT :• 1+1/2 year mean correlation ~0.8• positive trend• ½ year signal
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Global statistics: wind speed bias vs. time2003-2009: WindSAT-QuikSCAT wind speed bias:• Clear and stable annual fluctuation (±0.25 m/s)• Linear trend (from 0.05 to +0.25 m/s)
2009-2010: WindSAT-ASCAT wind speed bias:• The annual fluctuation is less evident• ASCAT bias (+0.29 m/s) in agreement with QuikSCAT values
WindSAT-QuikSCAT wind speed bias amplitude spectrum: peak period ~ 1 year
WindSAT-ASCAT wind speed bias amplitude spectrum: 2 peaks: ~1/2 yr and ~ 1 yr
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Global statistics: wind speed RMSD vs. time2003-2009: WindSAT/QuikSCAT :• 7 year mean 1.35 m/s• light negative trend• annual signal
2009-2010: WindSAT/ASCAT :• 1+1/2 year mean 1.6 m/s• negative trend• annual signal
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Global statistics: wind direction correlation vs. time2003-2009: WindSAT/QuikSCAT:• correlation decreases (negative trend), annual signal is visible
2009-2010: WindSAT/ASCAT:• correlation lower than that of QuikSCAT, with negative trend, annual signal visible
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Global statistics: wind direction bias and RMSD vs. time2003-2009: WindSAT/QuikSCAT:• |bias| increases, light annual variability• RMSD increases, annual signal is visible
2009-2010: WindSAT/ASCAT:• bias mean value similar to that of QuikSCAT, but higher variability• RMSD agrees with that of QuikSCAT (in 2009).
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Global statistics: wind speed distributionsWindSAT/QuikSCAT/ECMWF:• QuikSCAT and ECMWF are in good
agreement• WindSat wind populates more the range 7-
12 m/s and less the range 4-7 m/s, with respect to QuikSCAT
WindSAT/ASCAT/ECMWF:• The situation is similar to QuikSCAT
(but less robust statistics: March 2009 - November 2010)
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Global statistics: wind direction rose plotsWindSAT/QuikSCAT:• Mean direction is 335°±85°
(northwesterly wind)• The angular sectors around 310° and
130° are less populated for WindSat:310° =130° +180°: could be a privileged direction
WindSAT/ASCAT:• Mean direction is 310°±85° (northwesterly
wind)• Even here the angular sectors 310° and
130° are less populated• Less robust statistics
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Global statistics: wind speed Taylor diagramsWindSAT/QuikSCAT/ECMWF:• STD dev similar (3 m/s)• Good agreement for correlation (0.9)• Centred RMSD is 1.3 m/s
WindSAT/ASCAT/ECMWF :• STD dev similar (2.8 m/s)• Agreement for correlation (0.8)• Centred RMSD is 1.75 m/s
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Conclusions 1/2
• WindSat temporal coverage is similar to that of QuikSCAT and does not overlap that of ASCAT (WindSat + ASCAT cover 4-10 am + 2-9 pm)
• WindSat spatial coverage is unsuitable for coastal applications (no data in the Adriatic and Aegean seas, no data within 50/60 km from coast)
• Together WindSat and ASCAT guarantee an average number of samples per grid point of about 2/day (OWV, ws≥3 m/s)
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Conclusions 2/2
WIND SPEED
1. correlation steady (>0.85) with annual variability (lower for ASCAT)
2. bias increases steadily with annual variability (from 0.05 to +0.25 m/s)
3. CRMSD steady or slightly negative around 1.35 m/s (higher for ASCAT)
4. WINDSAT has higher velocities
WIND DIRECTION
1. correlation decreases steadily (from 1.7 to 1.4) with annual variability (lower correlation for ASCAT)
2. |bias| increases steadily (from 1 to 2 deg, higher for ASCAT)
3. CRMSD increases (more steeply from 2008, from 28 to 45 deg, higher for ASCAT)
4. unexpected low population of sectors 310 and 130 degrees (130+180=310...)
Correlation of wind speed (0.8-0.9) and direction (1.2-1.6) are acceptable, low biases (0.2 m/s), but some statistical parameters show deterioration with time:
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
THANK TOU
FOR YOUR ATTENTION
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