Retrieving hurricane wind speeds using cross-polarization C-band measurements

Abstract

Abstract. Hurricane-force wind speeds can have a large societal impact and in this paper microwave C-band crosspolarized (VH) signals are investigated to assess if they can be used to derive extreme wind-speed conditions. European satellite scatterometers have excellent hurricane penetration capability at C-band, but the vertically (VV) polarized signals become insensitive above 25 m s −1 . VV and VH polarized backscatter signals from RADARSAT-2 SAR imagery acquired during severe hurricane events were compared to collocated SFMR wind measurements acquired by NOAA's hurricane-hunter aircraft. From this data set a geophysical model function (GMF) at strong-to-extreme/severe wind speeds (i.e., 20 m s −1 < U 10 < 45 m s −1 ) is derived. Within this wind speed regime, cross-polarized data showed no distinguishable loss of sensitivity and as such, cross-polarized data can be considered a good candidate for the retrieval of strong-to-severe wind speeds from satellite instruments. The upper limit of 45 m s −1 is defined by the currently available collocated data. The validity of the derived relationship between wind speed and VH backscatter has been evaluated by comparing the cross-polarized signals to two independent wind-speed data sets (i.e., short-range ECMWF numerical weather prediction (NWP) model forecast winds and the NOAA best estimate 1-minute maximum sustained winds). Analysis of the three comparison data sets confirm that cross-polarized signals from satellites will enable the retrieval of strong-to-severe wind speeds where VV or horizontal (HH) polarization data has saturated. The VH backscatter increases exponentially with respect to wind speed (linear against VH [dB]) and a near-real-time assessment of maximum sustained wind speed is possible using VH measurements. VH measurements thus would be an extremely valuable complement on next-generation scatterometers for hurricane forecast warnings and hurricane model initialization.

Future Ocean Scatterometry: On the Use of Cross-Polar Scattering to Observe Very High Winds

“…Given the expense of hurricane evacuations and emergency preparations and the undermining effect of false alarms on population response, unnecessary hurricane warnings should be avoided. Recently, the analysis of RADARSAT-2 C-band SAR data and collocated wind measurements by the NOAA hurricane-hunter aircraft [7]- [11] has revealed that cross-polar scattering does not show any evident loss of sensitivity as the wind speed increases above 25 m/s, thus allowing accurate retrieval of strong to severe wind speeds. As a consequence, the use of cross polarization in addition to the most common VV or HH polarization becomes 0196-2892 © 2015 IEEE.…”

A C-band cross polarization geophysical model function

“…In the meantime, much effort is devoted to developing cross polarization geophysical model functions (GMFs) for hurricane wind retrieval using polarimetric SAR imagery. Several of the GMFs assume a linear relationship between VH in dB and wind speed U10 van Zadelhoff et al, 2013van Zadelhoff et al, , 2014. One drawback of these GMFs is the lack of incidence angle () dependence, at least for the high wind range (U10>21 m/s).…”

“…The result prompts the incorporation of VH channel in the next generation scatterometer designs for extending the retrieved wind speed range (Lin et al 2012;Belmonte Rivas et al, 2014). In the meantime, much effort is devoted to developing cross polarization geophysical model functions (GMFs) for hurricane wind retrieval using polarimetric SAR imagery.…”

“…Here, we present a C-band GMF using the piecewise powerlaw relationship. Presented in this study are three datasets van Zadelhoff et al, 2013van Zadelhoff et al, , 2014 composed of R2 dual-polarization (VV and VH) NRCS and:…”

“…shows a comparison of the proposed GMF with three recently published VH GMFs, referred to as vZ13S, vZ13E(van Zadelhoff et al, 2013(van Zadelhoff et al, , 2014 and Z14. In the same convention, the GMFs described in this paper are called H14S and H14E.…”