Analysis for the rapid intensification of typhoon Mekkhala in southern Taiwan Strait

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.