Assimilation of QuikSCAT Observations for Typhoon Mindulle (2004)

High-resolution QuikSCAT satellite wind observations retrieved from several flyovers of Typhoon Mindulle (2004) were assimilated using the Local Ensemble Transform Kalman Filter (LETKF) algorithm implemented on the Regional Spectral Model at a moderate resolution. The QuikSCAT observations provide u and v wind components at a height of 10 m with 4 km spatial resolution. In this study, we test different approaches for the definition of the observation operator to show that its careful definition is important for the gainful assimilation of the QuikSCAT data in the area of tropical cyclones (TCs). The approaches we test range from a very simplistic method of assimilation to a much more sophisticated method. The most simplistic method assimilates the u and v components of the 10 m wind observations as they were taken at the first model sigma level, which is at about 40 m in our implementation of the model. A more sophisticated approach interpolates the wind in the model to 10 m assuming that the boundary layer is well mixed. Other approaches include assimilating the wind speed or surface stress, instead of the retrieved wind components. These methods are based on the idea that the wind speed is often better characterized by the QuikSCAT observations than wind direction. We test the effects of each approach using both single-observation experiments and fully cycled experiments, the latter of which assimilate all conventional observations in addition to the high-resolution QuikSCAT observations. We show that implementing the more advanced operation operators with high-quality QuikSCAT observations benefits the analysis and forecast of TC track and intensity.