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Statistical issues in radiometer- and scatterometer-derived winds over the Mediterranean Sea

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)

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Statistical issues in radiometer- and scatterometer-derived winds over the Mediterranean Sea

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  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

  2. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Outline • Motivations for the study • Datasets description • Local statistics • Global statistics • Conclusions

  3. 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

  4. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Dataset description: WindSat WindSat 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 - present • DATA: 0.25°x 0.25° gridded OWV data downloaded from Remote Sensing Systems (RSS) • SWATH: One swath 950 km wide • PERIOD 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

  5. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Dataset description: QuikSCAT Quick Scatterometer (QuikSCAT) is a satellite of National Aeronautics and Space Administration (NASA) The onboard SeaWinds instrument is a Ku-band microwave scatterometer. • OPERATIVE: 1999 - 2009 • DATA: 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 wide • PERIOD 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

  6. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Dataset description: ASCAT ASCAT: 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 - present • DATA: 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 km • PERIOD 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

  7. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Dataset description: temporal coverage Number of monthly observationsOctober 2009 • WindSat: 62K observations • QuikSCAT: 150K – 35k collocations • ASCAT: 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

  8. QuikSCAT • Max hits ~50 • Data up to near coast ASCAT • Max hits ~40 • Mostly homogeneous coverage VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Dataset description: spatial coverage 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

  9. 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 Basin WindSAT-QuikSCAT WindSat-ASCAT: • ranges between -0.2 and 1 • lowest correlations in the Ionian Sea • higer variability than with QuikSCAT, but also shorter period WindSAT-ASCAT VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Local statistics: wind speed correlation

  10. Windsat-QuikSCAT: • ranges between -0.3 and 0.9 m/s • little variations throughout the basin WindSAT-QuikSCAT Windsat-ASCAT: • ranges between -1.5 and 1.8 m/s WindSAT-ASCAT VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Local statistics: wind speed bias

  11. 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-QuikSCAT ASCAT: • several spots of high values. Higher variability than QuikSCAT • ranges in [0.3-1.9] m/s WindSAT-ASCAT 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

  12. WindSat-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-ASCAT VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Local statistics: wind direction correlation 1 Crosby, Breaker, and Gemill, 1993. J. Atmos. Oceanic Technol., 10, 355–367.

  13. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Global statistics: wind speed correlation vs. time 2003-2009: 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

  14. 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 VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Global statistics: wind speed bias vs. time 2003-2009: 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

  15. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Global statistics: wind speed RMSD vs. time 2003-2009: 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

  16. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Global statistics: wind direction correlation vs. time 2003-2009: 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

  17. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Global statistics: wind direction bias and RMSD vs. time 2003-2009: 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).

  18. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Global statistics: wind speed distributions WindSAT/ASCAT/ECMWF: • The situation is similar to QuikSCAT (but less robust statistics: March 2009 - November 2010) WindSAT/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

  19. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Global statistics: wind direction rose plots WindSAT/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

  20. WindSAT/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 VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Global statistics: wind speed Taylor diagrams

  21. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde 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)

  22. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde Conclusions 2/2 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: WIND SPEED • correlation steady (>0.85) with annual variability (lower for ASCAT) • bias increases steadily with annual variability (from 0.05 to +0.25 m/s) • CRMSD steady or slightly negative around 1.35 m/s (higher for ASCAT) • WINDSAT has higher velocities WIND DIRECTION • correlation decreases steadily (from 1.7 to 1.4) with annual variability (lower correlation for ASCAT) • |bias| increases steadily (from 1 to 2 deg, higher for ASCAT) • CRMSD increases (more steeply from 2008, from 28 to 45 deg, higher for ASCAT) • unexpected low population of sectors 310 and 130 degrees (130+180=310...)

  23. VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde THANK TOU FOR YOUR ATTENTION

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