QuikSCAT geophysical model function for hurricane wind and rain

The SeaWinds scatterometer on the QuikSCAT spacecraft has been operating since August 1999 to provide global mapping of ocean winds. The ocean surface winds from the QuikSCAT scatterometer have been shown to be accurate, except for precipitating and extreme high wind conditions. It is known that the QuikSCAT scatteroemter winds typically underestimate the strength of tropical cyclones and overestimate the wind speed for low to moderate wind speeds (3-10 m/s) under rainy conditions. We examined collocated QuikSCAT radar data and SSM/I rain rate to assess the effects of rain. It is shown that the QuikSCAT sigma0s increase with increasing rain rate for low and moderate wind speeds (<15 m/s) and has an opposite trend for hurricane force winds (>32 m/s). It is also shown that the QuikSCAT sigma0 modulation by the wind direction is reduced by the rain. The results are consistent with the existing QuikSCAT wind speed biases and characteristics of wind direction solutions at the presence of rain. Our results suggest that the rain rate can be introduced as an additional modeling parameter for the Ku-band scatterometer model function to reduce the wind retrieval bias resulting from the rain for adverse weather conditions.