Improved Simulation of Landfall Typhoon based on coastal Radar Data Assimilation

Typhoon intensity and structure are poorly predicted by numerical models, and one key point of this problem is a lack of precise typhoon initialization before model prediction. Based on a WRF-based ensemble Kalman filter (EnKF) data assimilation system, the impacts of assimilating Chinese coastal Doppler radar velocity observations on track, intensity and structure of typhoon Mujigae(1522), Rammasun(1409) and Meranti(1010) are examined in this study. Assimilating sensitivity of radar data in different region relative to typhoon centre is also explored. The results of EnKF experiments showed that both the positioning and deterministic track forecast of typhoons were improved with more time cycles of assimilation. Through data assimilation, offshore enhancement process in these three cases are well simulated. Typhoon intensity errors in both EnKF analysis and prediction are 25hPa less than experiment with no data assimilation，with more precise structure simulations in three landfall typhoons.

In order to test sensitivity of radar data in different typhoon radial distance area, radar data is divided into three parts to be assimilated individually. Result of typhoon Mujigae(1522) simulation shows that radar observations within 100km from the typhoon center are dominate in the assimilation of typhoon track, intensity and structure simulation, which just account for about 20% of the total observations. Moreover, it makes no obvious difference on the typhoons’ track, intensity and structure errors for all radar data assimilated or those only within 100km. The simulated typhoon structure is somewhat modified by cycling assimilating observations in 100-200 km or out of 200 km from typhoon center. Experiments for typhoon Rammasun and Meranti also show that radar observations located close to the typhoon center is dominant in typhoons intensity and structure. Since only assimilating inner core radar data can reduce computing time to 1/3 of all data with somewhat the same assimilation result. Efficiency of radar assimilation can be much improved by this radar assimilating strategy, and it can provide operational guidance for typhoon real-time data assimilation and prediction work.