Numerical Simulation And Diagnostic Analysis On The Rainstorm Event Occurring Over Eastern Fujian

Based on the 6 hourly conventional surface observations and radiosonde soundings, a case of heavy rain occurring over Eastern Fujian on 10-11 August 2002 was analyzed. The synoptic analyses show that the cold trough at 500hPa as well as low vortex and shear in the lower-mid troposphere were mainly responsible for the heavy rain. It is also revealed that in the vicinity of heavy rain, the diagnostic fields such as vorticity and divergence at low levels was coordinated to that at high levels of atmosphere, contributing to the sustaining strong updraft, one of the favorable conditions for occurrence and persistence of the rain. Moreover, the low-level jet and the atmospheric state of high energy, saturation and potential instability in the rainfall region supplied plentiful moisture and unstable energy for the heavy rain respectively.Further study on the case of heavy rain was based on a successful numerical simulation by a meso-scale NWP model MM5, with the NCEP reanalysis data as initial conditions. By using the high resolution outputs of MM5, the spatial-temporal distribution and variation of convective available potential energy (CAPE), helicity and moist potential vorticity (PV) was diagnosed to reveal possible formation causes and developing mechanism of the event. The results are as follows. The main weather system brought the heavy rain was a mesoscale vortex at low-mid levels, whose evolution was accorded with the variations in shift and intensity of the rainfall center. The heavy rain occurred over the region where there were a positive helicity center at low levels and a negative one at high levels. The rain intensified as the positive helicity layer extending up to 500hPa with a maximum at the altitude of 700hPa, and the decline in rain was associated with the altitude of the layer top falling and the helicity distribution shifting to the opposite gradually. The variations in CAPE also reflected the evolution of the heavy rain. The precipitation center was always accompanied by the high CAPE and its intensity varied with CAPE. In addition, the rainfall was consistent with Q vector convergences in the aspects of extent and intensity variation. The rainfall center was located under the positive PV and north of the negative PV center in the upper troposphere. The evolution of the rain was closely linked to that of PV. The rain intensified when the negative moist PV at low levels formed and extended upwards, and the converse was also true.Sensitivity experiments further reveal the amplifying effect of terrain and the impact of vertical wind shear on the heavy rain.