On the impact of sea surface temperature and swell to WindSAT foam fraction and retrieved wind speed

WindSAT derived foam fraction and sea surface wind speed dependence on sea surface temperature (sst) is explored. This global analysis is performed, for the first time, in relation to swell direction of propagation relative to the local wind direction. The results show that WindSAT foam fraction increases as the swell and wind directions become opposite for colder sst (below 279 K) and warmer sst (above 297 K). Between 279 and 297 K, the foam fractions derived from all four WindSAT frequency channels, are noticeably affected when the swell direction is perpendicular to the wind direction. WindSAT wind speed bias to ECMWF shows dependence to sst, where the change in bias can be as high as ∼1 m/s as the sst shifts from 273 to 303 K. For sst below 279 K, the bias increases as the swell and wind directions become opposite. Above 297 K, the bias generally decreases as the swell/wind direction difference increases. For sst between 279 and 297 K, the change in absolute bias is as high as ∼0.4 m/s when the swell direction is perpendicular to the wind direction. Reducing WindSAT wind speed dependence on both sst and swell will improve WindSAT speed product performance, and possibly decrease WindSAT derived foam fraction dependence on sst.