Narrow Results

Based on NCEP/ NCAP reanalysis data, the formation and development mechanisms of two different types of landfall cyclone rainstorms occurred on the north slope of Dabie Mountains were researched by the diagnosis of the typical cases. The first kind of rainstorm was formed by the interaction between the landfall tropical cyclone and the westerly trough. The cold air guided by westerly flow into landfall tropical cyclone on the trough, promoted the warm frontogenesis. The MCSs occurred and developed in the θse frontal zone which located between low pressure of the landfall tropical cyclone and the cold air in rear of westerly trough. The ground cold front was the trigger mechanism of MCS. The other kind of rainstorm was caused by the circulation of the tropical cyclone. The MCSs occurred and developed in the θse high-energy shaft which located in the Tropical cyclone trough. The surface convergence line was the trigger mechanism of MCSs. All the water vapors of these two kinds of rainstorm came from the Bay of Bengal, South China Sea and East China Sea. The composition of the water vapor transport belts was also the same. The water vapor transport belts were mainly composed of partial eastward flow formed by the tropical cyclone and the subtropical high. There were two water vapor transport paths in the 850 hPa and 950 hPa layers. One conveyor belt was consistent with the direction of the southwest monsoon. The other conveyor belt was in the southwest of the tropical cyclone and the subtropical high. In these two kinds of rainstorm, the function of the tropical cyclone was mainly manifested in two aspects. Under the condition of moist potential vorticity conservation, due to the tilt of the wet and other entropy surface, the vertical shear of the atmospheric horizontal wind or the increase of the wet oblique pressure, which lead to the significant development of the vertical vorticity and the strengthening of the ascending motions.

Numerical simulation and diagnostic analysis on the heavy rainfall in Hainan Island during 14-15 October, 2000 were conducted using the WRF model and NCEP re-analysis data. The observed 48 h accumulative rainfall amount is successfully simulated in the 4km experiment, with the similar result that the rainfall distributed more in the east than in the west and more in the north than in the south. The accumulative rainfall of the simulation center is about 100 mm more than the observations (651 mm), the center of simulation is about 50 km south of observation. The results of diagnosis show that: the trigger mechanisms of rainfall are cold air infiltration in low-level and dry intrusion in middle-level. The tropical depression in south of Hainan Island not only brought plenty of water vapor for rainfall, but also enhanced the lower-level jet stream (LLJ) because of its strong pressure gradient. The structure characteristic shows that LLJ have both of the shear of wind direction and strong wind speed. Two belts of water vapor convergence from ocean were confluent in north of Hainan Island. Divergence vertical flux and water vapor flux divergence indicate the location of rainfall, which is valuable for rainfall forecasting using model.