PBL triggering mechanism of convective rainfall viewed from Helicity during TC landfall

Based on the conception of multi-scale helicity and high-resolution simulation of tropical cyclone Fung-wong (2008) during landfall on the east coast of China, the PBL trigger mechanisms of TC convective rainfall were preliminary examined. It is found that the trigger of convection and subsequent convective heavy precipitation is companied by the energy cascading process between larger-scale TC environmental flow and TC inner-circulation convective scale process during the procedure of stabilization of the environmental flow. Quite different from the positive relationship between CAPE and large scale instability, it is interesting to note that there is negligible connection between CAPE and small scale convection, which imply that CAPE might not be a direct supplier of energy that fuels small scale convection. During the process of TC heavy rainfall, multi-scale moisture advection provides moisture on various scales and favors the development of convective systems on the associated scales. During TC landfall, the subsidence of mid-troposphere cold flow, mainly generated within the downdraft region of TC spiral band, is favorable and responsible for the genesis of a cold front within PBL. Convective instability is likely to be induced in the vicinity of this front in the case of PBL vertical wind shear that produced by topography. The growth of instability is then favored by the energy cascading and moisture advection, which also has a tendency of transition from large scale to small scale, resulting in the rapid development of small scale convection and its associated intensive rainfall.