Research On The Evolution Mechanism Of Mesoscale Convective System Associated With A Heavy Rainfall Event In Southern China

Southern China is located in the lower latitude, where not only affected by the westerly wind belt, but also influenced by the tropical weather systems. Additionally, with the complex terrain and huge land-sea thermal contrast, it can be considered a cradle of Mesoscale Convective Systems (MCSs), the weather forecasting there is correspondingly a burden. Based on NCEP/NCAR global objective analysis data, the FY-2E TBB data and the mesoscale model WRFV3.2, a heavy rain case occurred in Southern China during the period from18th to19th June2010and the developmental mechanism of MCSs which caused the rainstorm are investigated in this paper.It is found that the formation and maintenance of southwest vortex and shear line at850hPa are the mesoscale systems. The low-vortex heavy rain mainly happened in the development stage of MCC (Mesoscale Convective Complex), while the shear line rainfall occurred in later stage of the round MCC turning into banded MCSs. The coupling mechanism between upper-level and low-level jet near the rain belt provides a great background field for the generation of horizontal vorticity. In addition, the vapor from Bay of Bengal and South China Sea supplies with a good condition for the development of MCS and rainstorm.Through the analysis for the wind vertical shear in the precipitation process, we furthermore find that the vertical shear forms strong horizontal vorticity near the rain area. The rotation caused by horizontal vorticity corresponding to the ascending branch of vertical circulation can trigger rainstorm, and the different distribution of u, v field may produce different circulation. The horizontal vorticity near the low vortex and shear line has obvious differences which may lead to different reasons of the formation of rainstorm. The vertical shear of v field mainly causes the low-vortex heavy rain, while the shear line rainfall forms owing to the vertical shear of both u and v field. The secondary circulation caused by the vertical shear of v field leads to east-west oriented rain belt along the shear line. Meanwhile, the vertical motion aroused by nonuniformity of the vertical shear of u field has close relationship not only with the generation and development of MCSs, but also with the formation of multi-rain cluster. Researching to the equation of vorticity budget also confirms the factor mentioned above. The positive twisting term near the rainfall center has obvious differences between the low vortex and the shear line, whose magnitude is much larger than other items. The convertion in the low vortex is up to (?)v/(?)p<0, while the convertion in the shear line is up to-(?)u/(?)p>0. The influence of the horizontal vorticity transforming to vertical vorticity and the circulation triggered by the horizontal vorticity can be not ignored. Moreover, the horizontal vorticity of x-direction gradually develops into the configuration that is positive in southern rain area and minus in the north. This configuration is beneficial to vertical motion and the formation of MCSs.By using the Q vector, equation of continuity and the simulation data, the results show that the horizontal gradient of horizontal vorticity has a close relationship with vertical motion and it is of obvious significance for precipitation forecasting and the diagnosis to vertical velocity.The isentropic potential vorticity (IPV) appears as dispersing to pooling and weak to strong under the cyclonic disturbance when in the developmental stage of MCC. The disturbance area of the strongest IPV presents the near-circular outlines and forms the typical near-circular cloud shield in the process of MCC turning to mature. The moving toward east of high potential vorticity areas is not only the early response of MCC turning to MCSs, but also the physical mechanism of conversion.Outside of the strong convective center of mature MCC, the rise at mid-upper level and subsidence at low level is conducive to the formation of a wide range of cold cloud field. Compared with MCC, the subsidence at mid-upper level which is in the north of mature MCSs inhibits the formation of the cold cloud field.