Evaluation of Wind Vectors observed by ASCAT using Statistical Distributions

1. Evaluation of Wind Vectors observed by ASCAT using Statistical Distributions Naoto EBUCHI Institute of Low Temperature Science, Hokkaido University ebuchi@lowtem.hokudai.ac.jp MetOp-A

2. 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

3. Monthly Report on Buoy Comparison by KNMI

4. Buoy Data for Comparison with ASCAT • Data Period - From 20 Nov. 2008 to 31 Dec. 2010 • Collocation - Dr < 12.5 km, Dt<10 min. • Height and Stability Collections - Liu and Tang (1996) Code - 10-m height Neutral Equivalent Wind NDBC KEO NDBC TAO RAMA WHOI PIRATA

5. 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

6. 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, 60oS – 60oN • Collocated ECMWF winds (Non-EN wind) • 156 million data points

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

8. ECMWF Wind Speed Histograms Collocated with ASCAT observations

9. Statistics of ASCAT Wind Speed Distribution NWP ASCAT

10. Statistics of ASCAT Wind Speed Distribution 12.5 km Product NWP ASCAT

11. 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 ECMWF ASCAT Left Swath, Wind Speed Range: 7-9 m/s, Descending Paths

12. 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

13. Wind Speed Dependence of Normalized Wind Direction Histograms (Outer Cells) 3-5 m/s 5-7 m/s 7-9 m/s 9-11 m/s 11-13 m/s 13-15 m/s Descending Ascending Normalized (ASCAT/NWP), WVC#: 1-3 (Left Outer Swath)

14. Wind Speed Dependence of Normalized Wind Direction Histograms (Mid Cells) 3-5 m/s 5-7 m/s 7-9 m/s 9-11 m/s 11-13 m/s 13-15 m/s Descending Ascending Normalized (ASCAT/NWP), WVC#: 10-12 (Left Mid Swath)

15. Wind Speed Dependence of Normalized Wind Direction Histograms (Inner Cells) 3-5 m/s 5-7 m/s 7-9 m/s 9-11 m/s 11-13 m/s 13-15 m/s Descending Ascending Normalized (ASCAT/NWP), WVC#: 19-21 (Left Inner Swath)

16. 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

17. 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

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

19. ECMWF Wind Speed Histograms Collocated with OSCAT observations

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

21. OSCAT Wind Direction Histograms WVC# 1-3 WVC# 4-6 WVC# 7-9 WVC# 10-12 WVC# 13-15 WVC# 16-18 ECMWF OSCAT Left Swath, Wind Speed Range: 7-9 m/s, Ascending Paths

22. 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.

24. Comparison of wind speed

25. Residuals of Wind Speed and Direction

26. Residuals of Wind Speed and Direction Binning wind speed = (Ubuoy + Uscat)/2

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

28. Comparison of wind speed OSCAT

29. Residuals of Wind Speed and Direction OSCAT Binning wind speed = (Ubuoy + Uscat)/2

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

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

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

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

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

35. Rough estimation of incidence angle error • Based on the SASS-2 GMF (Wentz et al., 1984) • At an incidence angle of q = 57o, VV-pol., and wind speed of U10 = 10 m/s, d s 0 / d q = – 0.3 dB d s 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.