An 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 sigma 0 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. Plain Language Summary Satellite instruments called wind scatterometers send beams of microwave radiation to the ocean surface, where they are scattered by the rough surface. The more wind, the rougher the surface and the more microwave radiation is directed back to the satellite. This way, the backscattered power may be used to measure the wind at the ocean surface. Different wind scatterometers use slightly different microwave wavelength with different wind response. Geophysical intercalibration of the wind behavior is however needed for users to be able to use wind services from different wind scatterometers at the same time. This manuscript addresses the finding that cold waters appear less rough than warm sea surface temperatures (SST) for cm wavelength microwaves. It quantifies the effect of SST by comparing the 2-cm microwave measurements of the NASA RapidScat wind scatterometer on the International Space Station with collocated wind measurements of the European ASCAT wind scatterometer, operating with 5-cm wavelength microwaves.