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

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: [email protected]



Outline

Motivations for the study

Datasets description

Local statistics

Global statistics

Conclusions



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



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



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



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



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



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



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



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



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



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.



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



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



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



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



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).



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)



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



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



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)



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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Documents

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