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

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…

SAR PR ocean wind models Nonlinear Least Squares fit …. Coefficient Fitted values A 0.5474 B 0.0333 C -0.0802 PR Model I only incidence angle dependence PR Model II with additional wind speed dependence Coefficient Fitted values P1 0.0012 P2 -0.0162 P3 0.9559 Q1 -0.0006 Q2 -0.0505

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

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:10.1029/2010JC006522. 17

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.

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

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

Hurricane Ike – 10 Sept 2008

Hurricane Bill – 22 Aug 2009

Hurricane Danielle – 28 Aug 2010

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

Hurricane Earl on Sep 02, 2010 at 22:59 UTC Winds -buoy #41001 is 18.1 (m/s) C-2PO is 16.0 CMOD5.N is 17.4 H*Wind is 16.8 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)

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)

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

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

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 -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

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= -0.89 m/s, RMSE= 3.23 m/s, CMOD5.N bias= -4.14 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.

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:10.1029/2009GL041780. Vachon, P.W. and J. Wolfe, (2010), C-band cross-polarization wind speed retrieval, IEEE Geosci. Remote Sens. Lett., 7, 456-459. 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:10.1029/2010JC006522. 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).