The document presents a new technique called C-2PO for retrieving wind speeds from cross-polarized synthetic aperture radar (SAR) data during hurricanes. C-2PO uses the cross-polarization ratio, which is insensitive to wind direction and incidence angle, making it easier to map to wind speed compared to existing techniques. Comparisons show C-2PO provides more accurate wind speed retrievals than existing methods for hurricanes, with lower biases and errors. Reasons include existing models becoming saturated at high winds and not accounting for rain and other factors that impact radar backscatter.
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1. Cross-Polarized SAR: A New Potential Technique for Hurricanes Will Perrie and Biao Zhang Bedford Institute of Oceanography Dartmouth, Nova Scotia, Canada
2. PR vs. incidence angle, from quad-polarization data. Compared to other PR models … PR dependence on incidence angle We develop a polarization ratio model to improve wind speed retrievals… SAR-wind models
3. SAR PR ocean wind models Nonlinear Least Squares fit …. PR Model I only incidence angle dependence PR Model II with additional wind speed dependence -0.0802 C 0.0333 B 0.5474 A Fitted values Coefficient -0.0505 Q2 -0.0006 Q1 0.9559 P3 -0.0162 P2 0.0012 P1 Fitted values Coefficient
4. Dataset Mouche et al #2 Our #1model Mouche et al #1 downwind Mouche et al #1 crosswind Johnsen et al Thompson et al Elfouhaily Our #2 model Vachon&Dobson PR PR from RADARSAT-2
5. Dataset 17 SAR winds from CMOD5.N and our model#I + R2 HH pol image 14.1 m/s 12.9 m/s 12.4 m/s 11.6 m/s 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:10.1029/2010JC006522.
6. Co-Polarization Model: CMOD5.N Cross-Polarization Ocean model: C-2PO 1:wind speed, wind direction, incidence angle. 2: NRCS_VV saturated under high winds. 1: only wind speed dependence. 2: NRCS_VH not saturated under high winds. Correlation coefficient for C-2PO = 0.91 VV HH HV VH U 10 U 10 C-band models
7. Dataset R2 Quad -Polarization Ocean Backscatter Measurements NRCS_VV saturates no NRCS_HV saturation NRCS_HV, NRCS_VH not sensitive to incidence angle, wind direction NRCS_VV, NRCS_HH depend on incidence angle, wind direction U 10 U 10 U 10 U 10 C-2PO corr coeff = 0.91
8. Hurricane Bertha – 12 July 2008 Hurricane wind-speed retrievals with C-2PO
12. Hurricane Earl on Sep 02, 2010 at 22:59 UTC VV polarization VH polarization RADARSAT-2 dual-polarization SAR image
13. Hurricane Earl on Sep 02, 2010 at 22:59 UTC H*Wind (m/s) Winds -buoy #41001 is 18.1 (m/s) C-2PO is 16.0 CMOD5.N is 17.4 H*Wind is 16.8 CMOD5.N (m/s) C-2PO model (m/s)
14. Comparison of C-2PO and CMOD5. SAR wind retrievals Along track-SFMR (hr) time series SFMR-measured 10s rain rates (mm/hr) time series (hr) eyewall? rain? gradients?
15. Hurricane Earl Comparisons of C-2PO and CMOD5.N SAR- retrieved winds U10 ( Sep 02, 2010 at 22:59 UTC) with collocated H*Wind C-2PO CMOD5.N
16. dual-polarization SAR image at 23:56 UTC on Sep 10, 2008 Hurricane Ike VV polarization VH polarization CMOD5.N + wind directions via H*Wind C-2PO model U10
17. Comparisons of C-2PO and CMOD5.N SAR- retrieved winds U10 (23:56 UTC, on September 10, 2008) with collocated H*Wind Hurricane Ike CMOD5.N bias of -4.89 m/s and RMS error of 6.51 m/s C-2PO bias of -0.88 m/s and RMS error of 4.47 m/s C-2PO CMOD5.N
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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.
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).
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) .
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.
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.
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 e xternal 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.
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).
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 plot s
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 e xternal wind directions from NOAA HRD H*Wind are overlaid , and (d) from the C-2PO model and .
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 .