Using the interim reanalysis data of European Centre for Medium-Range Weather Forecasts (ERA-Interim), the tropical cyclone best track data from Shanghai Typhoon Institute of China Meteorological Administration, the NOAA daily optimum sea surface temperature (OISST), Himawari-8 satellite data, and observations collected at automatic weather stations (AWS) in China, the difficulties of the intensity forecasting of typhoon Mekkhala in the southern Taiwan Strait are analyzed. The environmental factors and the asymmetric distribution of the convective burst during the rapid intensification of Mekkhala in the southern Taiwan Strait are further studied. Results are as follows: (1) The rapid intensification of typhoon Mekkhala in the southern Taiwan Strait under the strong 200—850 hPa vertical wind shear and its landing in Fujian province at peak intensity are very rare, which resulted in a short warning time and great difficulties in intensity prediction. (2) The favorable ocean heat condition and large-scale environmental conditions such as the abnormal warm sea surface temperature in the northern South China Sea, the strong upper-level outflow caused by the easterly jet to the south of the South Asian High, and the abundant and stable southwesterly monsoon water vapor transport all play an important role in the rapid intensification of typhoon Mekkhala in the southern Taiwan Strait. (3) The traditional environmental vertical wind shear (200—850 hPa) is very strong, but the vertical distribution of the environmental wind shows that the environmental vertical wind shear is mainly concentrated in the middle and upper levels, while the shear in the middle and lower levels is relatively weak. However, the vertical wind shear in upper and middle levels has relatively little inhibition on the typhoon intensification. (4) During the rapid intensification of typhoon Mekkhala, the distribution of deep convection presented obvious asymmetric distribution characteristics. The convection was mainly concentrated in the down shear side and the left side of the environmental vertical wind shear (200—850 hPa), accompanied by cyclonic propagation of convective burst from the down shear side to the up shear side, and the tilt of typhoon significantly reduced. Further studies on the impact mechanism of environmental vertical wind shear at different levels on typhoon intensity change and characteristics of asymmetric convective bust during typhoon intensification under strong environmental vertical wind shear will be conducted in the future based on more cases.

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.