High-Resolution Hurricane Vector Winds from C-Band Dual-Polarization SAR Observations
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272 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 31 High-Resolution Hurricane Vector Winds from C-Band Dual-Polarization SAR Observations BIAO ZHANG School of Marine Sciences, Nanjing University of Information Science and Technology, Nanjing, Jiangsu, China WILLIAM PERRIE Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada JUN A. ZHANG Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida ERIC W. UHLHORN NOAA/AOML/Hurricane Research Division, Miami, Florida YIJUN HE School of Marine Sciences, Nanjing University of Information Science and Technology, Nanjing, Jiangsu, China (Manuscript received 21 December 2012, in final form 1 November 2013) ABSTRACT This study presents a new approach for retrieving hurricane surface wind vectors utilizing C-band dual- polarization (VV, VH) synthetic aperture radar (SAR) observations. The copolarized geophysical model function [C-band model 5.N (CMOD5.N)] and a new cross-polarized wind speed retrieval model for dual polarization [C-band cross-polarized ocean surface wind retrieval model for dual-polarization SAR (C-2POD)] are employed to construct a cost function. Minimization of the cost function allows optimum estimates for the wind speeds and directions. The wind direction ambiguities are removed using a parametric two-dimensional sea surface inflow angle model. To evaluate the accuracy of the proposed method, two RADARSAT-2 SAR images of Hurricanes Bill and Bertha are analyzed. The retrieved wind speeds and directions are compared with collocated Quick Scatterometer (QuikSCAT) winds, showing good consis- tency. Results suggest that the proposed method has good potential to retrieve hurricane surface wind vectors from dual-polarization SAR observations. 1. Introduction tracks and intensities, particularly for the protection of coastal residents and infrastructure. Better under- Large tropical storms form as typhoons in the Pacific standing of the hurricane surface wind fields is also Ocean and hurricanes in the Atlantic Ocean. Recent essential. hurricanes such as ‘‘Superstorm’’ Sandy (2012) caused The first applications of microwave remote sensing significant damage with intense rainfall and storm surges, technology to hurricanes involved an airborne active as well as serious flooding. At least 286 people were scatterometer and passive radiometer during Hurri- killed along Sandy’s path in seven countries. Therefore, it cane Allen, demonstrating their potential for reliable is important to accurately forecast and monitor hurricane measurements (Jones et al. 1981). Although scatter- ometers and radiometers can also observe the ocean surface by day or night, they can only provide coarse- Corresponding author address: Dr. William Perrie, Bedford In- stitute of Oceanography, 1 Challenger Drive, P.O. Box 1006, resolution measurements, and thus their observations Dartmouth NS B2Y 4A2, Canada. are not very suitable to probe high-resolution wind gra- E-mail: [email protected] dient variations between the hurricane eye and eyewall. DOI: 10.1175/JTECH-D-13-00006.1 Ó 2014 American Meteorological Society Unauthenticated | Downloaded 10/06/21 05:33 AM UTC FEBRUARY 2014 Z H A N G E T A L . 273 Yueh et al. (2003) examined the collocated Quick Scat- without the 1808 ambiguity (Zecchetto and De Biasio terometer (QuikSCAT) winds and Special Sensor Mi- 2002, 2008). crowave Imager (SSM/I) rain rates, and showed that the There can be alias features, such as oceanic or atmo- 2 regions of high rain rate (.20 mm h 1)havealower spheric internal waves, that produce similar linear fea- normalized radar cross section (NRCS) than the neigh- tures on the same spatial scales as wind rows. These boring areas, indicative of the attenuation effects of nonwind streak features are not generally aligned with rain. Compared to these sensors, C-band synthetic ap- the local wind vectors and can therefore contaminate erture radar (SAR) is not only able to contribute fine- wind direction retrievals inferred directly from the resolution sea surface wind field information but also SAR image. In hurricane wind regimes, although the is much less influenced by rain perturbations resulting wind direction dependence of the NRCS for CMOD5 from volume scattering and attenuation by raindrops is much weaker than that of CMOD4 (Horstmann et al. in the atmosphere than the Ku-band sensors men- 2005), inaccurate wind directions are still able to cause tioned above (Tournadre and Quilfen 2003; Weissman serious errors in wind speed retrievals. To avoid the and Bourassa 2011). Although SAR images of typhoons influence of inaccurate wind directions on wind speed and hurricanes occasionally show dark spiral-shaped retrieval, Shen et al. (2006) proposed an approach to features due to extreme precipitation events (Reppucci determine hurricane wind vectors, assuming constant et al. 2008, 2010), the advantages of C-band SAR make wind speeds and equal-distant wind directions in three it a potentially excellent instrument to investigate meso- neighboring subimage blocks of any specific concentric scale hurricane intensity and structure. circle around the hurricane eye. However, this method Great efforts have been devoted to extract hurri- needs to be modified in some circumstances, for ex- cane surface wind fields using spaceborne SAR mea- ample, for degraded hurricanes, which tend to gradu- surements. Horstmann et al. (2005) used a C-band ally lose their circular structure, and for SAR images RADARSAT-1 HH-polarized SAR image of Hurri- not containing the hurricane eye. cane Ivan to investigate the feasibility of hurricane sur- Saturation of the VV-polarized NRCS can be a prob- face wind retrieval using conventional VV-polarized lem for high wind retrieval from SAR. As wind speed geophysical model functions (GMFs), such as C-band increases, VV-polarized NRCS values also increase, even- models 4 and 5 (CMOD4 and CMOD5, respectively). tually reaching a maximum and then starting to decrease However, studies have shown that CMOD4 is inaccurate for higher values of wind speeds. Thus, the VV-polarized for high wind speed retrieval under extreme weather NRCS experiences saturation under high wind condi- conditions (Migliaccio et al. 2003; Horstmann et al. 2005; tions, which creates a wind speed ambiguity problem Signell et al. 2010). In any case, whether CMOD4 or (multiple solutions) for SAR wind speed retrieval CMOD5 is used, ultimately, wind direction has to be (Shen et al. 2009). Investigations from global positioning determined prior to wind speed retrieval from SAR. system (GPS) dropwindsonde observations and labo- In Seasat imagery, linear features were first observed ratory experiments, as well as airborne scatterometer to be aligned with the direction of the wind on the measurements, suggest that sea surface roughness, and scale of a few kilometers (Gerling 1986). Moreover, the air–water drag coefficient, approaches limiting values, marine atmosphere boundary layer (MABL) rolls are and also suggest that the NRCS acquired at VV polari- frequently observed in SAR images as linear streaks zation does not continue to increase as winds increase (Alpers and Brummer 1994). These steaks are caused beyond marginal hurricane strength (Powell et al. 2003; by the convergence or divergence of MABL rolls in Donelan et al. 2004; Fernandez et al. 2006; Black et al. the planetary boundary (Brown 2000). Therefore, wind 2007). Bubble annulus experiments suggest that the direction can be estimated using the linear features bubble layer in saltwater impedes the transfer of mo- associated with wind streaks, in combination with spec- mentum from the wind (Lundquist 1999). Altimeter tral analysis of the SAR image through a discrete Fourier observations also showed that the reflectivity is strongly transform (Vachon and Dobson 1996), or measurement impacted by foam layer and whitecap coverage (Quilfen of SAR image streaks via wavelet transform methods et al. 2010; Toffoli et al. 2011). In addition to the pos- (Du et al. 2002), or by acquisition of the orthogonal of sible effects of foam, sea spray is hypothesized to sig- the most frequent gradient direction using the local nificantly influence the transfer of momentum, heat, gradient method (Koch 2004). However, these methods and moisture in tropical cyclones (Kepert et al. 1999; all produce wind direction with inherent 1808 ambigu- Perrie et al. 2004a,b). A significant weakness of the ities, which can be resolved using the two-dimensional wind speed retrieval using VV-polarized SAR images continuous wavelet transform technique to isolate wind- of hurricanes or typhoons is the CMOD5 saturation related cells and features, and to extract wind direction under high wind conditions. Shen et al. (2009) developed Unauthenticated | Downloaded 10/06/21 05:33 AM UTC 274 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 31 a technique to remove the wind speed ambiguity based wind speed, one NRCS is associated with four rela- on the dominant hurricane wind structure. Reppucci tive wind directions over the range (08;3608). In princi- et al. (2010) proposed a method to overcome the prob- ple, wind directions can be acquired from the wind lem of saturation in the tropical cyclone wind regime, streaks and standard image processing techniques basing the technique on the combined use of SAR mea- (Vachon and Dobson 1996; Du et al. 2002; Koch 2004; 2 surements for areas of wind speed of 20 m s 1 or less, Zecchetto and De Biasio 2002, 2008). Though often and a parametric