1. WIND RETRIEVAL WITH CROSS-POLARIZED SAR RETURNS
Paul A. Hwang
Remote Sensing Division
Naval Research Laboratory
Washington DC, USA
William Perrie and Biao Zhang
Fisheries and Oceans Canada
Bedford Institute of Oceanography
Dartmouth, NS, Canada

2. Radar backscatter from the ocean surface
NRCS (0)  roughness  wind velocity
Scatterometer (wind)
Polarimetric returns (VV, HH; VH & HV)
RADARSAT-2 quad-pol
Co-pol (VV, HH):
Good agreement with Bragg theory
Wind sensitivity decreases toward high wind
De-pol (VH & HV) :
>>Bragg theory
Increased sensitivity with wind speed (cubic)
Quad-pol (25 km x 25 km); Dual-pol (scan SAR mode 300 km x 300 km or 500 km x 500 km)
Passive and Active systems (scatterometer vs. radiometer)

3. (LP-Filtered roughness)
Altimeter
(LP-Filtered roughness)
According to the 2θ deviation, the phase shift causes constructive (left figure) or destructive (right figure) interferences (Wikipedia)
Scatterometer
(Bragg roughness)
Composite surface Bragg theory
GO: Geometric(al) optics 3

4. CB theory matured by 1960s, but …
Hwang 2011
Hwang 2008
Plant 2002
Elfouhaily 1997

5. CB theory matured by 1960s, but …
Hwang 2011
Hwang 2008
Plant 2002
Elfouhaily 1997

6. RADARSAT-2 quad-polarimetric dataHH HV VV
Decreasing sensitivity
with wind
Saturation/dampening
Increasing sensitivity
with wind:
Linear: low wind
Cubic: high wind
R2 and CB in good agreement
R2 >> CB
Breaking contribution missing in CB computation
Hwang et al. 2010

7. rms 1.50
rms 1.62
rms 1.67
rms 1.65
VV,HH:
CMOD5
Hwang et al. 2010 7

8. Hwang et al. 2010 8

9. Dual-pol (300 km x 300 km or 500 km x 500 km)
Black: Quad-pol (# 427); Blue: Dual-pol (# 372)
Quad-pol (25 km x 25 km)
Dual-pol (300 km x 300 km or 500 km x 500 km)

10. Dual-pol (300 km x 300 km or 500 km x 500 km)
Noise subtracted
P2 (dual pol): to
R2 (quad pol): to
Quad-pol (25 km x 25 km)
Dual-pol (300 km x 300 km or 500 km x 500 km)

12. VH noise sub. B: -0.022, c': 0.965, D: 1.901, R: 0.837, SI: 0.250
VH w/ noise B: , c': 0.965, D: 1.912, R: 0.835, SI: 0.251
Linear U10(VH) B: 0.000, c': 0.971, D: 1.989, R: 0.819, SI: 0.261

13. -31 dB noise subtracted

14. Vachon and Wolfe 2011

15. Radar: Co-pol saturation problem
22
32.5
43.5
54
Wind speed U m/s
Normalized Radar Cross Section (dB)
Airborne high wind data
Donnelly et al. 1999

16. WindSat global analysis
Radiometer: if saturation, it’s at a much higher wind speed because the foam factor.
WindSat
global analysis
Data: Meissner and Wentz (2009)
Hwang (2011, TGRS, submitted)

17. If saturation, it’s at a much higher wind speed.
Radiometer:
If saturation, it’s at a much higher wind speed.
Hwang (2011, TGRS, submitted)

18. If saturation, it’s at a much higher wind speed.
Radiometer:
If saturation, it’s at a much higher wind speed.
V: black; H: red
Hwang (2011, TGRS, submitted)

19. RADARSAT-2 polarimetric dataHH VV VH VH
-noise
Quad-pol
(25 km x 25 km)
Dual-pol
(300 km x 300 km or 500 km x 500 km) VH

20. Scatterometer wind retrieval
Wind velocity dependence of radar returns NRCS (0)  roughness  wind speed
Co-pol (VV, HH): high wind less sensitive (saturation problem)
De-pol (VH, HV): high wind more sensitive (may not saturate)
VH wind retrieval: dual-pol scan SAR high wind speed range from active radar may approach that of passive radiometer if de-pol is used