On the use of Sentinel-1 cross-polarization imagery for wind speed retrieval

The use of co-polarization (HH or VV) synthetic aperture radar (SAR) normalized radar cross section (NRCS) measurements to estimate winds speed has matured to an operational product. However, these wind speed retrievals have only been well validated for wind speeds less than 20 m/s and require some a priori estimated of the wind direction. At higher wind speeds, the radar cross section for co-polarization imagery becomes less sensitive to increases in wind speed. The use of cross-polarization (HV or VH) NRCS measurements is difficult because of the low NRCS values for wind speeds less than 5-10 m/s. However, as wind speeds become higher, the radar cross section becomes significantly larger than the system noise. Cross-polarization data from Radarsat-2 SAR imagery has shown promise for estimating wind speeds above 10 m/s and the retrievals can be independent of knowledge of the wind direction. In this paper, we apply a published cross-polarization wind speed algorithm to Sentinel-1 imagery. We find improved wind speed retrievals over co-polarization retrievals in very high winds especially in hurricanes. However, the noise floor in the imagery is a significant problem, especially for low wind conditions. In addition, the specification of the noise floor at beam boundaries in multi-beam imagery causes wind speed image discontinuities.