A SST-dependent Ku-band geophysical model function for RapidScat

A new Ku-band Geophysical Model Function (GMF) which includes Sea Surface Temperature (SST) dependence, named NSCAT-5, is developed for improved RapidScat wind retrieval. The RapidScat scatterometer instrument mounted on the International Space Station (ISS) provides near-global wind data over the oceans. Starting from the existing NSCAT-4 GMF, the variation of sigma0 is approximated as a second-order Taylor expansion in sea surface temperature T for each given wind speed V, and the fitting coefficients are obtained from both observed and simulated radar cross sections, using ASCAT winds, of either vertical or horizontal polarizations. Furthermore, an intercalibration is performed which aligns the distribution of RapidScat wind speeds to that of ASCAT. NSCAT-5 is obtained by correcting NSCAT-4 with SST dependencies and intercalibration information. Validation of the new RapidScat wind products retrieved using NSCAT-5 shows clear improvements over those obtained with NSCAT-4: Wind inversion residuals no longer depend on SST, and the wind speed Probability Density Functions (PDFs) are closely overlapping with those of ASCAT. Also, RapidScat NSCAT-5 minus ASCAT wind speed differences show no SST dependence. The work presented here opens a door for further improving the quality of wind products from Ku-band backscatter measurements, and helps to build a long-term and consistent essential Climate Data Record (CDR) of scatterometer winds.