Improving the Simulation of Typhoon Mujigae (2015) Based on Radar Data Assimilation

Typhoon intensity and precise structure are hardly to predict by all kinds of numerical model, and one key problem is the lack of precise initialization data. Through a WRF-based ensemble Kalman filtering (EnKF) data assimilation system, impacts of assimilating China's coastal Doppler radar velocity observations for track, intensity and structure of Typhoon Mujigae (2015) is examined.Furthermore, assimilating sensitivity of observations in relative regions are also explored. The experimental results show that mean track error and max track error is reduced by 15 km and 38 km, respectively. The track error of the EnKF analysis becomes smaller with more cycles of assimilating data, and so do the deterministic forecast driven by EnKF analysis field. Through data assimilation, offshore enhancement process in Mujigae is well simulated. Intensity error in both EnKF analysis and prediction are smaller than 25 hPa after assimilation. After 9 h cycling radar velocity data assimilation, the deterministic forecast shows the typhoon continue to strengthen before landfall, and the typhoon eye is contracted much after data assimilation. The diameter of typhoon eye is reduced by about 70 km, and the eye wall convection asymmetric structure is closer to observation.The sensitivity of radar observation assimilation is tested by different radial distance area. Numerical sensitivity experiments show that radar observations within 100 km of the typhoon's inner core play a dominate role to assimilation results. Typhoon track, intensity and structure are all closer to observation by assimilating radar data within 100 km from typhoon center (about 20% of total observation) showing equivalent effects as assimilating all data. Typhoon is somewhat modified by cycling assimilating observations within 100-200 km from typhoon center. There is no obvious enhancement in typhoon track, intensity and structure after assimilating data 200 km away from inner core. Therefore, radar observation located in typhoon kernel is the key to determine assimilation effects. Because of less data assimilated, the strategy of only assimilating inner core radar data can reduce computing time to 1/3 of all data with somewhat same assimilation result. Efficiency of radar assimilation can be much improved by this radar assimilating strategy, and it can give reference to official typhoon real-time data assimilation and prediction work.