Combined active/passive hurricane wind retrieval algorithm for the Seawinds scatterometer

Because of their high wind gradient structures, tropical cyclones (TCs) present a major challenge to space-borne scatterometer measurements of ocean surface wind vectors. Frequently spiral bands of strong rains accompany the high winds, and this precipitation attenuates the ocean backscatter measured by the scatterometer. Furthermore, traditional geophysical model functions (GMF), which relate wind speed and direction with radar backscatter (sigma-0), have not been tuned for the high wind conditions of TC´s. The SeaWinds scatterometer has the ability to measure simultaneously the ocean backscatter and brightness temperature. By using this combined active/passive approach, simultaneous wind and rain estimates are made in TC´s. Rain rate, determined passively, is used to model both the attenuative and scattering effects of rain. These parameters are used to correct the measured ocean sigma-0 at 12.5 km resolution. Wind speed retrievals are performed using a special TC-GMF developed using airborne scatterometer measurements in hurricanes. SeaWinds wind speed results for several hurricanes occurring between 1999 and 2002 compare well with high-resolution surface wind fields available from NOAA´s Hurricane Research Division aircraft flights.