Assimilation of satellite and radar observations in studying the impact of upper-level atmospheric processes on tropical cyclone genesis and rapid intensification

Understanding the processes that contribute to tropical cyclone (TC) formation, intensity and structure changes is essential for improving the predictability of TCs. Previous studies have focused mostly on the low- to mid-level processes leading to TC formation and rapid intensity and structure changes. The influence of upper-level atmospheric processes, especially the evolution of the outflow layer (the layer between 100 and 300 hPa) has not received much attention until recently. In this study, we use advanced data assimilation methods and high-resolution numerical simulations to study the impact of upper-level atmospheric processes on tropical cyclone genesis and rapid intensification.

First, high-resolution numerical simulations of Typhoon Nuri (2008) over the Western Pacific Ocean were conducted with assimilations of satellite retrieved temperature profiles and airborne Doppler radar observations in two recent studies. The mesoscale community Weather Research and Forecasting (WRF) model and variational data assimilation systems (WRF 4DVAR and GSI) were used. In the first study, the 4D-VAR radar data assimilation significantly improves the numerical simulation of Nuri's genesis. Simulations using radar data assimilation predict Nuri's genesis, while the control experiment (without radar data assimilation) fails to predict Nuri's genesis. Specifically, the improved initial conditions and forecasts from the data assimilation imply that the enhanced midlevel vortex and moisture conditions are favorable for the development of deep convection in the center of the pouch and eventually contribute to Nuri's genesis. More importantly, the improved simulation of the convection and associated environmental conditions produce enhanced upper-level warming in the core region and lead to the drop in sea-level pressure. In the second study, the impact of AIRS derived atmospheric temperature profiles on numerical simulation of Typhoon Nuri (2008) was examined. Results indicated that assimilation of AIRS derived temperature profiles results in a more accurate prediction of Nuri's genesis. Meanwhile, data assimilation results also imply that the upper level warming contributes to Nuri's genesis. Assimilation of AIRS data enhances the representation of upper level warming in the WRF model. The outcomes from both studies are further diagnosed to confirm the role of upper level processes, especially the contribution of outflow layers on the genesis of Typhoon Nuri. An additional experiment was also performed with a higher attitude for the model top in the numerical simulations of Nuri's genesis with assimilation of AIRS temperature profiles. It demonstrates further improvements in predicting Nuri's genesis and confirms the benefit of both the use of a higher model top and assimilation of more data into the model.

Finally, some results with the cases during recent ONR TCI-14 field program will be presented.