3B.3 Impact of the 4D-Var assimilation of Airborne Doppler Radar data on numerical simulations of the genesis of Typhoon Nuri (2008)

Li, Zhan, and Zhaoxia Pu, University of Utah

Initial conditions have a significant impact on forecasts of Tropical Cyclone (TC) genesis in the numerical weather prediction. In this study, data assimilation experiments are conducted using the Weather Research and Forecasting (WRF) model and its four-dimensional variational data assimilation (4D-Var) system in numerical simulations of the genesis of Typhoon Nuri (2008). The three-dimensional wind analyses, retrieved from the ELDORA airborne radar data, are assimilated into the WRF model for improving the atmospheric conditions in the pre-genesis phase (00 UTC 16 Aug 2008 - 05 UTC 16 Aug 2008) of Typhoon Nuri during the Office of Naval Research’s Tropical Cyclone Structure 2008 (TCS-08) field program. The WRF simulation is then extended to 18 UTC 16 Aug 2008 (Nuri’s genesis time) and 30 h beyond. Results show that the assimilation of radar wind data leads to the accurate forecasts of Nuri’s genesis. In contrast, without radar data assimilation, the WRF simulation fails to predict the genesis of Nuri. In addition, the track, intensity and precipitation simulations of Nuri are also improved by the radar data assimilation. Additional diagnoses show that the radar data assimilation results in the enhanced middle-level vortex and moist conditions in pre-Nuri environments, thus provide favorable conditions for the development of deep convection. Further experiments are performed to direct assimilate the radar radial velocity into the WRF model. The impact of this direct assimilation on predicting Nuri’s genesis will be evaluated and compared with that from assimilating the retrieved wind.