1. Cross-Polarized SAR: A New Potential Technique for Hurricanes
Will Perrie and Biao Zhang
Bedford Institute of Oceanography
Dartmouth, Nova Scotia, Canada

2. SAR-wind models PR dependence on incidence angle
PR vs. incidence angle, from quad-polarization data.  Compared to other PR models …
PR dependence on incidence angle
Figure 2a. Polarization ratio (PR) as a function of incidence angle, from RADARSAT-2 quad-polarization observations and our PR models, as well as selected empirical and theoretical PR models from the literature.
We develop a polarization ratio model to improve wind speed retrievals…

3. SAR PR ocean wind models Nonlinear Least Squares fit ….
Coefficient
Fitted values A
0.5474 B
0.0333 C
PR Model I
only incidence angle dependence
PR Model II
with additional wind speed dependence
Coefficient
Fitted values P1
0.0012 P2 P3
0.9559 Q1 Q2

4. Dataset Mouche et al #2 Our #1model Mouche et al #1 downwind
Mouche et al #1 crosswind
Johnsen et al
Thompson et al
Comparisons of observed NRCS in VV polarization (x-axis) from RADARSAT-2 measurements with equivalent values (y-axis) calculated from the various empirical and theoretical PR models in the literature: (a) Model 2 from Mouche et al. [2005] (M2005), (b) Model with incidence angle dependence from this study, (c) Model 1 from Mouche et al. [2005] (M2005), (d) Model with from Horstmann et al. [2000] (H2000), (e) Model from Johnsen et al. [2008] (J2008), (f) Model with from Thompson et al. [1998] (T1998), (g) Model with Eq.(5) from Elfouhaily [1996] (E1996), (h) Model with wind speed and incidence angle dependence from this study, and (i) Model with from Vachon and Dobson [2000] (V2000).
Elfouhaily
Our #2 model
Vachon&Dobson PR
PR from RADARSAT-2

5. SAR winds from CMOD5.N and our model#I + R2 HH pol image
Dataset
14.1 m/s
12.9 m/s
12.4 m/s
11.6 m/s
SAR wind map retrieved by our PR model with only incidence angle dependence and CMOD5.N, from a RADARSAT-2 SAR image as shown in Figure 6a. The colorbar denotes wind speed, in units of m/s. The black arrow indicates wind direction (with 0 to the east).
15.9 m/s
16.6 m/s
Zhang, B., W. Perrie, and Y. He (2011), Wind speed from RADARSAT-2 quad-polarization images using a
new polarization ratio model, J. Geophys. Res., doi: /2010JC 17

6. C-band models Co-Polarization Model: CMOD5.N HH  VV HV VH
1:wind speed, wind direction, incidence angle.
2: NRCS_VV saturated under high winds.
VH
Cross-Polarization Ocean model: C-2PO
U10
U10
Correlation coefficient for C-2PO = 0.91
1: only wind speed dependence.
2: NRCS_VH not saturated under high winds.

7. Dataset R2 Quad-Polarization Ocean Backscatter Measurements
NRCS_VV, NRCS_HH
depend on incidence
angle, wind direction
NRCS_VV saturates
U10
U10
NRCS_HV, NRCS_VH
not sensitive to incidence
angle, wind direction
no NRCS_HV saturation
a)-b) Normalized radar cross section (NRCS) versus wind speed, for six 5 incidence angle bins between 20 and 50 for (a) VV polarization, (b) HH polarization. In (a), include CMOD4, CMOD5, CMOD5.N.
c)-d) Normalized radar cross section (NRCS) versus wind speed, for six 5 incidence angle bins between 20 and 50 for (a) HV polarization, (b) VH polarization. The solid line corresponds to a linear fit, with correlation coefficient of 0.97.
C-2PO corr coeff = 0.91
U10
U10

8. Hurricane wind-speed retrievals with C-2PO
Hurricane Bertha – 12 July 2008

9. Hurricane Ike – 10 Sept 2008

10. Hurricane Bill – 22 Aug 2009

11. Hurricane Danielle – 28 Aug 2010

12. RADARSAT-2 dual-polarization SAR image
Hurricane Earl on Sep 02, 2010 at 22:59 UTC
RADARSAT-2 dual-polarization SAR image acquired over Hurricane Earl at 22:59 UTC on September 2, 2010, (a) VV polarization and (b) VH polarization.
VH polarization
VV polarization
RADARSAT-2 dual-polarization SAR image

13. Hurricane Earl on Sep 02, 2010 at 22:59 UTC
Winds -buoy #41001 is 18.1
(m/s) C-2PO is
CMOD5.N is
H*Wind is
RADARSAT-2 dual-polarization SAR image acquired over Hurricane Earl at 22:59 UTC on September 2, 2010, (a) VV polarization and (b) VH polarization. Colorbar shows sigma-naught in VV polarization () and in VH polarization () in dB, respectively. SAR-retrieved wind speeds from (c) the CMOD5.N model and , with external wind directions from NOAA HRD H*Wind are overlaid, and (d) from the C-2PO model and . Colorbar shows wind speeds at 10-m height () in m/s.
H*Wind (m/s)
CMOD5.N (m/s)
C-2PO model (m/s)

14. Comparison of C-2PO and CMOD5. SAR wind retrievals
eyewall? rain? gradients?
Along track-SFMR (hr) time series
Comparisons of C-2PO and CMOD5.N SAR-retrieved wind speeds (at 22:59 UTC, on September 2, 2010) with collocated SFMR-measured along-track 10s averaged surface winds during 22:30~23:30 UTC, on September 2, 2010, (a) time series plot, (d) gives SFMR-measured 10s path-integrated rain rates (mm/hr).
SFMR-measured 10s rain rates (mm/hr) time series (hr)

15. Hurricane Earl C-2PO CMOD5.N
Comparisons of C-2PO and CMOD5.N SAR-retrieved wind speeds (at 22:59 UTC, on September 2, 2010) with collocated SFMR-measured along-track 10s averaged surface winds during 22:30~23:30 UTC, on September 2, 2010, (b) and (c) scatter plots
Comparisons of C-2PO and CMOD5.N SAR-retrieved winds U10 (Sep 02, 2010 at 22:59 UTC) with collocated H*Wind

16. Hurricane Ike dual-polarization SAR image at 23:56 UTC on Sep 10, 2008
VV polarization
VH polarization
CMOD5.N + wind directions via H*Wind
A RADARSAT-2 dual-polarization SAR image acquired over Hurricane Ike at 23:56 UTC on September 10, 2008, (a) VV polarization and (b) VH polarization. Colorbar shows sigma-naught in VV polarization () and in VH polarization () in dB, respectively. SAR-retrieved wind speeds from (c) the CMOD5.N model and , with external wind directions from NOAA HRD H*Wind are overlaid, and (d) from the C-2PO model and .
C-2PO model U10

17. Hurricane Ike C-2PO CMOD5.N
Comparisons of C-2PO and CMOD5.N SAR-retrieved wind speeds (at 23:56 UTC, on September 10, 2008) with collocated H*Wind derived wind speeds at 01:30 UTC, on September 11, 2008.
Comparisons of C-2PO and CMOD5.N SAR-retrieved winds U10 (23:56 UTC, on September 10, 2008) with collocated H*Wind
CMOD5.N  bias of m/s and RMS error of 6.51 m/s C-2PO  bias of m/s and RMS error of 4.47 m/s

18. Summary C-2PO model presented
insensitive to wind direction, radar incidence angle
easy mapping of observed cross-pol NRCS to wind speed
avoids errors in wind speed retrievals that occur in CMOD5.N
in quad-pol data, C-2PO does not seem to saturate
potential for hurricane wind retrievals
dual-pol Earl: high wind comparisons R2 SAR – airborne SFMR
Earl: C-2PO bias= m/s, RMSE= 3.23 m/s, CMOD5.N bias= m/s, RMSE= 6.24 m/s.
Reasons for under-estimates:
CMOD5.N is saturated in high winds
co- and cross-polarized NRCS calibration error
CMOD5.N and C-2PO do not account for rain, high sea states, eye gradients, e.g. dampen the NRCS  biases in wind speeds
inaccurate wind directions for CMOD5.N.

19. Some other sources…
Paul Hwang et al. TU2.T10.4 (11:20) Wind retrieval with cross-polarized SAR returns
Vladimir Zabeline et al. TH2.T02.2 RADARSAT Application in ocean wind measurements
Hwang, P. A., B. Zhang, and W. Perrie, 2010: Depolarized radar return for breaking wave measurements and hurricane wind retrieval. Geophys. Res. Lett., 37, L01604, doi: /2009GL
Vachon, P.W. and J. Wolfe, (2010), C-band cross-polarization wind speed retrieval, IEEE Geosci. Remote Sens. Lett., 7,
Zhang, B., and W. Perrie (2010), C-band Quad-Polarization ocean backscatter measurements: A new polarization ratio model. SEASAR2010, January 25-29, Frascati, Italy.
Zhang B., W. Perrie, and Y. He, 2011: Wind speed retrieval from RADARSAT-2 quad-polarization images using a new polarization ratio model. J. Geophys. Res., 116, doi: /2010JC
Zhang, B., W. Perrie (2011), Cross-polarized synthetic aperture radar: a new measurement technique for Hurricanes, under review - Bulletin of the American Meteorological Society (BAMS).