EVALUATIONS OF WIND VECTORS OBSERVED BY ASCAT USING STATISTICAL DISTRIBUTIONS

Evaluation of Wind Vectors observed by ASCAT using Statistical Distributions Naoto EBUCHI Institute of Low Temperature Science, Hokkaido University [email_address] MetOp-A

Outline 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 by KNMI

Buoy 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 NDBC NDBC TAO PIRATA KEO RAMA WHOI

Comparison of ASCAT Winds with Buoy Data # Bias RMS Correlation Wind Speed (m/s) 27,247 -0.15 1.42 0.885 Wind 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 ASCAT Wind Speeds and Directions ASCAT 25 km Swath Products (L2) 1 Jan. 2010 – 31 Dec. 2010 (1 year) Global ocean, 60 o S – 60 o N Collocated ECMWF winds (Non-EN wind) 156 million data points

ASCAT Global Wind Speed Histograms Period: 1 Jan. 2010 – 31 Dec. 2010 (1 year) Area: global ocean, 60 o S – 60 o N Bin size: 1 m/s

ECMWF Wind Speed Histograms Collocated with ASCAT observations

Statistics of ASCAT Wind Speed Distribution ASCAT NWP

Statistics of ASCAT Wind Speed Distribution ASCAT NWP 12.5 km Product

ASCAT 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 ECMWF ASCAT

Normalized ASCAT Wind Direction Histograms WVC# 1-3 WVC# 4-6 WVC# 7-9 WVC# 10-12 WVC# 13-15 WVC# 16-18 WVC# 19-21 Descending Ascending Normalized (ASCAT/NWP), Left Swath, Wind Speed Range: 7-9 m/s

Wind Speed Dependence of Normalized Wind Direction Histograms (Outer Cells) Normalized (ASCAT/NWP), WVC#: 1-3 (Left Outer Swath) Descending Ascending 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 Histograms (Mid Cells) Normalized (ASCAT/NWP), WVC#: 10-12 (Left Mid Swath) Descending Ascending 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 Histograms (Inner Cells) Normalized (ASCAT/NWP), WVC#: 19-21 (Left Inner Swath) Descending Ascending 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 from Oceansat-II scatterometer Oceansat-II Scatteromter (OSCAT) Ku-band (13.51 GHz) Scanning dual pencil beam VV and HH pol 50 km resolution 1800 km swath

Comparison of OSCAT Winds with Buoy Data # Bias RMS Correlation Wind Speed (m/s) 8,938 0.56 1.65 0.893 Wind 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

OSCAT Global Wind Speed Histograms Period: 1 Jan. 2010 – 31 Dec. 2010 (1 year) Area: global ocean, 60 o S – 60 o N Bin size: 1 m/s

ECMWF Wind Speed Histograms Collocated with OSCAT observations

Statistics of OSCAT Wind Speed Distribution OSCAT Asc NWP OSCAT Dec ECMWF Asc ECMWF Dec

OSCAT 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# 16-18 WVC# 10-12 ECMWF OSCAT

Summary 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.

Residuals of Wind Speed and Direction

Residuals of Wind Speed and Direction Binning wind speed = (U buoy + U scat )/2

Dependence of Wind Speed Residual on Wave Parameter Significant Wave Height Inverse Wave Age Correlation Coefficient = 0.300 Correlation Coefficient = -0.311

Comparison of wind speed OSCAT

Residuals of Wind Speed and Direction Binning wind speed = (U buoy + U scat )/2 OSCAT

Statistics of NSCAT Wind Speed Distribution ASCAT ASCAT ASCAT ASCAT (Ebuchi, J. Oceanogr ., 2000)

Statistics of QSCAT Wind Speed Distribution ASCAT ASCAT ASCAT ASCAT Ascending Descending

Histograms of ERS-1 Wind Directions CMOD-4 CMOD-IFR2 (Ebuchi & Graber, JGR , 1998)

Histograms of NSCAT Wind Directions (Ebuchi, JGR , 1999)

Histograms of QSCAT Wind Directions (Ebuchi, Proc. IGARSS 2000 , 2000)

Rough estimation of incidence angle error Based on the SASS-2 GMF (Wentz et al., 1984) At an incidence angle of  = 57 o , VV-pol., and wind speed of U 10 = 10 m/s, d  0 / d  = – 0.3 dB d  0 / d U 10  = 0.7 dB An error of 1 o in incidence angle may cause an error of 0.4 m/s in wind speed.

EVALUATIONS OF WIND VECTORS OBSERVED BY ASCAT USING STATISTICAL DISTRIBUTIONS

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EVALUATIONS OF WIND VECTORS OBSERVED BY ASCAT USING STATISTICAL DISTRIBUTIONS