Evolution of environmental circulation and dynamic and thermodynamic conditions before and after the onset of typhoon rapid intensification

Using the tropical cyclone best track data from Shanghai Typhoon Institute of China Meteorological Administration for the period 1949—2020 and the reanalysis interim data of the European Centre for Medium-Range Weather Forecasts (ERA-Interim) for the period 1991—2020, the EOF combination analysis of u and v components of the wind field on the 200 and 850 hPa is conducted to summarize characteristics of larger-scale environmental circulation at the onset of typhoon rapid intensification and the evolutions of environmental dynamic and thermal conditions before and after the onset are further analyzed. The results indicate that in lower levels, the main environmental circulation of EOF decomposition is the confluence pattern of monsoon trough at the onset of typhoon irapid ntensification, and the circulation is conducive to low-level water vapor transport. The upper-level circulation shows obvious typhoon outflow channels, and this characteristic can be used as a typical circulation pattern for the rapid intensification forecast. The thermal condition (such as sea surface temperature, water vapor and convective instability) and the dynamic condition (such as environmental vertical wind shear and the strength of upper-level outflow) can generally reach the fitness range of conditions that are favorable for typhoon intensification. However, the values of the above environmental factors have not changed significantly or suddenly during the transition from slow intensification process to rapid intensification process. Some extreme cases even show that some of environmental factors change towards unfavorable conditions for typhoon intensification. These research results provide a reference for the prediction of typhoon rapid intensification and further typhoon studies in the future. As for the unfavorable conditions shown in some RI cases, further studied are necessary to determine whether there are other favorable factors that offset the negative effects of these conditions and what are the corresponding physical processes.