Assimilation of Oceansat-2-Scatterometer-Derived Surface Winds in the Weather Research and Forecasting Model

Abstract

This paper describes, for the first time, the impact of Oceansat-2 scatterometer (OSCAT) surface winds in the Weather Research and Forecasting (WRF) 3-D variational (3-D-Var) assimilation system. Before using OSCAT winds into WRF assimilation system, we compared OSCAT surface wind retrievals against National Centers for Environmental Prediction analyzed winds, Advanced Scatterometer retrievals, and buoy-measured winds. After the initial assessment of the quality of the OSCAT winds, the control (CNT) (without OSCAT) as well as experimental (which assimilated OSCAT surface winds) runs were made for 48 h starting daily at 0600 Universal Time Coordinated (UTC) during July 2010. The assimilation experiments demonstrated positive impact of OSCAT winds on both the analysis state as well as subsequent short-term forecasts of surface winds. Compared to CNT run, the assimilation of the OSCAT winds improved the surface wind analysis as large as 25%, when compared with Advanced Microwave Scanning Radiometer (AMSR-E) measured winds. The assimilation of OSCAT winds also showed small, but positive, impact on the forecast (particularly later hours of forecast) of midtropospheric moisture, temperature, and upper tropospheric winds. Compared to the CNT run, the assimilation of OSCAT winds improved precipitation forecast for moderate to heavy rainfall thresholds when validated against Tropical Rainfall Measuring Mission precipitation.