VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
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)
Italian National Research Council
Corso Stati Uniti 4, 35127 Padova, Italy
Corresponding author: F. De Biasio – email: [email protected]
Bari, 4-5 dicembre 2012 - Villa Larocca
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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: 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:
DATA:
SWATH:
PERIOD
1:
PERIOD 2:
2003 - present
0.25°x 0.25° gridded OWV data downloaded
from Remote Sensing Systems (RSS)
One swath 950 km wide
February 2003 – November 2009 (QuikSCAT)
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: 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:
DATA:
SWATH:
PERIOD
1:
1999 - 2009
12.5km OWV data provided by and
downloaded from the NASA's Physical
Oceanography Distributed Active Archive
Center (PO.DAAC)
One swath 1800 km wide
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: 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:
DATA:
SWATH:
PERIOD
2:
2006 - present
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)
Two swaths 550 km wide, central gap 672 km
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
Dataset description: temporal coverage
Number of monthly observations
October 2009
WindSat:
QuikSCAT:
ASCAT:
WindSat
QuikSCAT
ASCAT
62K observations
150K – 35k collocations
98K - 4K collocations
4-6 a.m. + 2-5 p.m.
2-6 a.m. + 3-7 p.m.
6-10 a.m. + 5-9 p.m.
WindSat + ASCAT cover 4-10 + 14-21
~ 2 hits per point per day
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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
QuikSCAT
 Max hits ~50
 Data up to near coast
ASCAT
 Max hits ~40
 Mostly homogeneous
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 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
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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:
•
ranges between -1.5 and 1.8 m/s
WindSAT-ASCAT
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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
WindSAT-ASCAT
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
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 correlation
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
1 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. 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
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Global statistics: wind speed bias vs. time
2003-2009: WindSAT-QuikSCAT wind speed bias:
WindSAT-QuikSCAT wind speed
•
Clear and stable annual fluctuation (±0.25 m/s)
bias amplitude spectrum: peak
•
Linear trend (from 0.05 to +0.25 m/s)
period ~ 1 year
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-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. 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
Bari, 4-5 dicembre 2012 - Villa Larocca
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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
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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).
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Global statistics: wind speed distributions
WindSAT/QuikSCAT/ECMWF:
•
QuikSCAT and ECMWF are in good
agreement
•
WindSat wind populates more the range 712 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 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
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
Global statistics: wind speed Taylor diagrams
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
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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)
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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
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...)
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VII Riunione Annuale CeTeM-AIT sul telerilevamento a Microonde
THANK TOU
FOR YOUR ATTENTION
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