WindSat Ocean Surface Emissivity Dependence on Wind Speed in Tropical Cyclones

Radiometers are adept at retrieving near surface ocean wind vectors. The Naval Research Laboratory (NRL) launched the WindSat polarimetric radiometer with five frequency channels to observe the sea surface and test its ability to accurately measure the wind in various weather conditions. This study focuses on its performance in extreme conditions – the high wind and heavy precipitation of a tropical cyclone. Storms Dennis, Katrina and Rita from the 2005 hurricane season are used to evaluate WindSat’s retrievals using an atmospheric clearing algorithm to extract emissivity measurements from the brightness temperature observations. The emissivity values are collocated with an independent surface wind field provided by H*Wind. Results show a well-behaved relationship between emissivity and wind speed that is also monotonic. The study indicates that microwave radiometers are capable of retrieving ocean surface wind speed during extreme conditions into early hurricane categories.