| Rapid intensification（RI）of Tropical Cyclone（TC）is usually associated with large-scale conditions such as high sea surface temperature,weak vertical wind shear（VWS）,relatively high humidity in the low-to-middle troposphere.Recent studies,however,have shown that TC can still occur in some moderate to large VWS environments(VWS200-850 h Pa>10 m s-1).As RI has made landfalling TC intensity forecasts more challenging,this paper focus on the distribution and environmental characteristics of the offshore RI in the South China Sea（SCS）.The large-scale environmental fields associated with the shear-type RI events have been thoroughly studied to provide a reference for the offshore RI forecasting.Additionally,Super-typhoon Rammasun（2014）has been selected as a representative case to investigate the structural characteristics of the asymmetric RI under a relatively large sheared environment.By studying the spatial distribution of TCs that experienced offshore RI during landfall in the China’s mainland,this study shows that up to 80%of all offshore RI events are found in the SCS during the period between 1973 and 2017.Further examination of the large-scale environmental fields suggests that VWS,mid-level water vapor,and monsoon gyre have the most significant impact on the RI process.Targeting at the RI events under a relatively large VWS(>10 m s-1)environment in the SCS,this study implies that most events occurred during the large circulation adjustment just before the termination of the Meiyu season.The strengthened southwest monsoon,westward retreat of the South Asian High,and the northward movement of the Subtropical High have jointly promoted the occurrence of offshore RI.These results provide a reference for the RI forecast of landfalling TC in the SCS.To better understand the dynamic process of the asymmetric RI in the SCS,Rammasun was selected as an example of RI events under sheared environments.Experiments of numerical simulation were conducted to obtain high-resolution（667 m）data that was verified by various observations.In a relatively large VWS(11-15 m s-1)environment,Rammasun underwent a 36-hour continuous asymmetrical offshore RI before landfalling in Wenchang,Hainan Province at7:30 on July 18,with the following four unique characteristics.First,convections of the eyewall on the west and south sides of Rammasun is closely related to the transportation channel of the southwest monsoon,which provides sufficient water vapor to the lower-to-middle layers of Rammasun and helps stimulate the convection of the eyewall on the south side.Second,the highly asymmetric core structure has been kept through the entire RI process.The low-layer inflow only happened at west and south sides（downshear）while the most middle-layer subsidence infllow occurs at east side（upshear）,resulting in an unclosed eyewall structure.Third,the distributions of tangential winds and warm cores are relatively symmetric.The convective burst activity of the upper layer of the eyewall on the south side corresponds well with the warm core near the RMW,with a tendency to strengthen the warm core of Rammasun center.Fourth,during the asymmetric RI process,the vertical tilt of the Rammasun center to the southwest side rotates counterclockwise and decreases.When the RI started developing,the tilt is so large（20 km）that suggests the vertical alignment is not a necessary factor for RI development.Based on the aforementioned asymmetrical dynamic structural,this study proposes a hypothesis of the physical mechanism for the asymmetric RI in the SCS:warm core with horizontal symmetry and vertical alignment is important for the asymmetrical RI.First,the southwest monsoon flow helps TC to stimulate convective bursts on its south side（downshear-left）.These convective bursts are closely related to the warm core in eyewall region in the middle-to-upper levels,transporting the warm core to the eye region through the upper-layer airflow.As a result,the warm core of the middle-to-upper levels is enhanced.Additionally,the counterclockwise rotating adjustment of the vertical tilt makes the warm core more symmetric and upright,which greatly decreases the surface pressure,and thus contributing to RI. |
