Evaluation of Wind Vectors observed by Evaluation of Wind Vectors observed by ASCAT using Statistical DistributionsASCAT using Statistical Distributions

Naoto EBUCHIInstitute of Low Temperature Science, Hokkaido University

ebuchi@lowtem.hokudai.ac.jp

MetOp-A

OutlineOutline

• Comparison of ASCAT wind vectors with data from ocean buoys

• Assessment of statistical distributions of ASCAT wind speeds and directions – Global wind speed histograms– Directional distributions relative to antenna beams

• Applications to Oceansat-II scatterometer

Monthly Report on Buoy Comparison Monthly Report on Buoy Comparison by KNMIby KNMI

Buoy Data for Comparison with ASCATBuoy Data for Comparison with ASCAT

• Data Period- From 20 Nov. 2008 to 31 Dec. 2010

• Collocation- ∆r < 12.5 km, ∆t<10 min.

• Height and Stability Collections- Liu and Tang (1996) Code- 10-m height Neutral Equivalent Wind

NDBCNDBC

TAO

PIRATA

KEO

RAMA WHOI

Comparison of ASCAT Winds with Buoy DataComparison of ASCAT Winds with Buoy Data

# Bias RMS CorrelationWind Speed (m/s) 27,247 -0.15 1.42 0.885Wind Direction (deg.)

U > 0 m/s 26,976 3.0 26.2 0.957 U > 3 m/s 23,750 2.7 17.4 0.979 U > 5 m/s 18,672 2.7 14.1 0.985

Calculation of Global Statistics of Calculation of Global Statistics of ASCAT Wind Speeds and DirectionsASCAT Wind Speeds and Directions

• ASCAT 25 km Swath Products (L2)• 1 Jan. 2010 – 31 Dec. 2010 (1 year)• Global ocean, 60oS – 60oN• Collocated ECMWF winds (Non-EN wind)• 156 million data points

ASCAT Global Wind Speed HistogramsASCAT Global Wind Speed Histograms

Period: 1 Jan. 2010 – 31 Dec. 2010 (1 year)Area: global ocean, 60oS – 60oNBin size: 1 m/s

ECMWF Wind Speed HistogramsECMWF Wind Speed Histograms

Collocated with ASCAT observations

Statistics of ASCAT Wind Speed DistributionStatistics of ASCAT Wind Speed Distribution

ASCAT

NWP

Statistics of ASCAT Wind Speed DistributionStatistics of ASCAT Wind Speed Distribution

ASCAT

NWP

12.5 kmProduct

ASCAT Wind Direction HistogramsASCAT Wind Direction Histograms

Left Swath, Wind Speed Range: 7-9 m/s, Descending Paths

WVC# 1-3 WVC# 4-6 WVC# 7-9 WVC# 10-12

WVC# 13-15 WVC# 16-18 WVC# 19-21

ECMWFASCAT

Normalized ASCAT Wind Direction HistogramsNormalized ASCAT Wind Direction HistogramsWVC# 1-3 WVC# 4-6 WVC# 7-9 WVC# 10-12

WVC# 13-15 WVC# 16-18 WVC# 19-21

DescendingAscending

Normalized (ASCAT/NWP), Left Swath, Wind Speed Range: 7-9 m/s

Wind Speed Dependence of Normalized Wind Direction Wind Speed Dependence of Normalized Wind Direction Histograms (Outer Cells)Histograms (Outer Cells)

Normalized (ASCAT/NWP), WVC#: 1-3 (Left Outer Swath)

DescendingAscending

3-5 m/s 5-7 m/s 7-9 m/s

9-11 m/s 11-13 m/s 13-15 m/s

Wind Speed Dependence of Normalized Wind Direction Wind Speed Dependence of Normalized Wind Direction Histograms (Mid Cells)Histograms (Mid Cells)

Normalized (ASCAT/NWP), WVC#: 10-12 (Left Mid Swath)

DescendingAscending

3-5 m/s 5-7 m/s 7-9 m/s

9-11 m/s 11-13 m/s 13-15 m/s

Wind Speed Dependence of Normalized Wind Direction Wind Speed Dependence of Normalized Wind Direction Histograms (Inner Cells)Histograms (Inner Cells)

Normalized (ASCAT/NWP), WVC#: 19-21 (Left Inner Swath)

DescendingAscending

3-5 m/s 5-7 m/s 7-9 m/s

9-11 m/s 11-13 m/s 13-15 m/s

Applications to data Applications to data from Oceansat-II scatterometerfrom Oceansat-II scatterometer

Oceansat-II Scatteromter(OSCAT)Ku-band (13.51 GHz)Scanning dual pencil beamVV and HH pol50 km resolution1800 km swath

Comparison of Comparison of OSCATOSCAT Winds with Buoy Data Winds with Buoy Data

# Bias RMS CorrelationWind Speed (m/s) 8,938 0.56 1.65 0.893Wind Direction (deg.)

U > 0 m/s 8,835 -0.2 32.4 0.938 U > 3 m/s 7,550 -0.3 22.3 0.968 U > 5 m/s 6,130 -0.4 14.1 0.977

OSCATOSCAT Global Wind Speed Histograms Global Wind Speed Histograms

Period: 1 Jan. 2010 – 31 Dec. 2010 (1 year)Area: global ocean, 60oS – 60oNBin size: 1 m/s

ECMWF Wind Speed HistogramsECMWF Wind Speed Histograms

Collocated with OSCAT observations

Statistics of Statistics of OSCATOSCAT Wind Speed Distribution Wind Speed Distribution

OSCATAsc

NWPOSCATDec

ECMWFAscECMWFDec

OSCATOSCAT Wind Direction Histograms Wind Direction Histograms

Left Swath, Wind Speed Range: 7-9 m/s, Ascending Paths

WVC# 1-3 WVC# 4-6 WVC# 7-9

WVC# 13-15 WVC# 16-18WVC# 10-12

ECMWFOSCAT

SummarySummary

• ASCAT wind speeds and directions agrees well with buoy observations in general.

• ASCAT global wind speed histogram varies with cross-track cell location. This trend is clearly represented by statistical parameters, such as standard deviation, skewness and kurtosis.

• Directivity relative to the antenna beams is discernible for ASCAT wind directions.

• At low wind speeds and the outer cells (high incidence), most of wind vectors aligned to the flight direction or mid-beam direction.

• These results suggest the needs for further refinements of the wind retrieval algorithms and the C-band GMF.

Comparison of wind speedComparison of wind speed

Residuals of Wind Speed and DirectionResiduals of Wind Speed and Direction

Residuals of Wind Speed and DirectionResiduals of Wind Speed and Direction

Binning wind speed = (Ubuoy + Uscat)/2

Dependence of Wind Speed Residual Dependence of Wind Speed Residual on Wave Parameteron Wave Parameter

Significant Wave Height Inverse Wave Age

Correlation Coefficient = 0.300 Correlation Coefficient = -0.311

Comparison of wind speedComparison of wind speedOSCAT

Residuals of Wind Speed and DirectionResiduals of Wind Speed and Direction

Binning wind speed = (Ubuoy + Uscat)/2

OSCAT

Statistics of Statistics of NSCATNSCAT Wind Speed Wind Speed DistributionDistribution

ASCAT

ASCAT

ASCAT

ASCAT

(Ebuchi, J. Oceanogr., 2000)

Statistics of Statistics of QSCATQSCAT Wind Speed Distribution Wind Speed Distribution

ASCAT

ASCAT

ASCAT

ASCAT

Ascending

Descending

Histograms of Histograms of ERS-1ERS-1 Wind Directions Wind DirectionsCMOD-4 CMOD-IFR2

(Ebuchi & Graber, JGR, 1998)

Histograms of Histograms of NSCATNSCAT Wind Directions Wind Directions

(Ebuchi, JGR, 1999)

Histograms of Histograms of QSCATQSCAT Wind Directions Wind Directions

(Ebuchi, Proc. IGARSS 2000, 2000)

Rough estimation of incidence angle errorRough estimation of incidence angle error

• Based on the SASS-2 GMF (Wentz et al., 1984)

• At an incidence angle of θ = 57o, VV-pol., and wind speed of U10 = 10 m/s,

d σ 0 / d θ = – 0.3 dB

d σ 0 / d U10 = 0.7 dB

• An error of 1o in incidence angle may cause an error of 0.4 m/s in wind speed.