A Strong Mesoscale Convective Process in Landfalling Typhoon

The fifth typhoon named Haitang landfalls in Fujian in 2005, and then moved northwestward. During this moving, there is a mesoscale convective cloud increasing markedly, within the periphery cloud cinctures of typhoon connecting with tropic convergence cinctures. Moreover, the mesoscale convective cloud passes the east and the north of Wenzhou, and causes heavy rainfalls in these places. This case indicates clearly that a strong mesoscale convective system can be developed under appropriate circumstance, even if the landing typhoon is abating. Severe weather can be resulted from the strong mesoscale convective system, even more dangerous than typhoon. Therefore, principles of strong mesoscale convective system in landing typhoon should be discussed on the structure and evolution of MCS to forecast rainstorm caused by the landfalling typhoon better in the future.The evolution of MCS in landfalling typhoon can be seen through the nephogram and radar reflecting data. Research on the structure of MCS by the NCEP data shows that the MCS along landing typhoon is gradient in maturation period.The synoptic situation, occurring two hours before MCS formed, could be analyzed by NCEP data. Then the formation of MCS is found attributing to two factors. One factor is the convective instability. The profound southeast current of typhoon transports the warm and wet air into the cold area in the north of Wenzhou. In this situation, the cold air is meeting the warm and wet air. So the convective instability is caused and increasing gradually. Meanwhile, the convergence of air is coming out in the influence of the special landform of the north of Wenzhou. However, the air will be moving upward vertically, which is caused by this convergence. So the vertical upward of air is the other factor.Using the theory of slantwise vorticity development and moist potential vorticity, the evaluative theory of this strong mesoscale convective system is obtained. The evolution of MCS could be shown by looking into the situation of MCS in maturation. MCS begins to develop from the area where the cold air pile meets the warm air. In this region, there is slantwise moist isentropic surface. The northwest cold pile forces warm air to climb up along the slantwise moist isentropic surface, so the cyclonic vorticity develops. Because of the interaction between the condition instability at low level and the condition-symmetric instability at middle layer, the convective-symmetric instability develops. In this circumstance, the stream slantwise and ascend at middle layer is widely formed by the vorticity caused by the slantwise isentropic surface. On the other hand, the increasing in vertical shear of horizontal wind or enhancing in moist baroclinity, because of the slantwise of moist isentropic surface, results in intensifying the development of vertical vorticity and stretching towards northwest of MCS.