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

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
NDBC
KEO
NDBC
TAO
RAMA
WHOI
PIRATA
Data Period
- From 20 Nov 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

5. Comparison of ASCAT Winds with Buoy Data
# Bias RMS Correlation
Wind Speed (m/s) 27,
Wind Direction (deg.)
U > 0 m/s 26,
U > 3 m/s 23,
U > 5 m/s 18,

6. Calculation of Global Statistics of ASCAT Wind Speeds and Directions
ASCAT 25 km Swath Products (L2)
1 Jan – 31 Dec (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 – 31 Dec (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. Normalized (ASCAT/NWP), WVC#: 1-3 (Left Outer Swath)
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. Normalized (ASCAT/NWP), WVC#: 10-12 (Left Mid Swath)
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#: (Left Mid Swath)

15. Normalized (ASCAT/NWP), WVC#: 19-21 (Left Inner Swath)
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#: (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,
Wind Direction (deg.)
U > 0 m/s 8,
U > 3 m/s 7,
U > 5 m/s 6,

18. OSCAT Global Wind Speed Histograms
Period: 1 Jan – 31 Dec (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
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 =

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.