The Composite Structure of Mesolows Accompanying Heavy Rainfall in the Taiwan Mei-Yu Season

Abstract

Five cases of mesolows accompanying heavy rainfall in the Mei-Yu season of 1985-1987 were studied. Data used consisted of the 3-h surface observations and the 3-h infrared digital data of the cloud top temperature observed by the Geostationary Meteorological Satellite of Japan. A composite technique was used to obtain the structure of each individual mesolow, the structure at the different life stages, and the structure for the different space scales. The composite fields included pressure, temperature, dew-point depression, streamline and isotach, vorticity, divergence, mean cloud top temperature, and frequency of deep convections of different cloud top temperature.
It was found that similarities and differences existed among all of the 5 cases. The mesolow which formed and migrated along the Mei-Yu front over the Taiwan Strait had a larger horizontal scale, a longer time duration, and a deeper center pressure as compared to that formed locally to the northwest and southwest of Taiwan. It had heavy rainfall occurrence to the south of the low center where the maximum convergence, the stronger southwesterly flows, and deep convections prevailed, as compared to the locally formed low which had heavy rainfall occurrence over the mesolow area. The bl king effect of the Central Mountain Range (CMR) on the low-level northeasterly cold air appeared to be important in the formation of mesolow to the west of the CMR. The shear vorticity due to the land-sea differential friction was suggested to be important for the formation and maintenance of the mesocyclone over the same area.
The composite structure at different life stages showed that the deep convections and thus the heavy rain ll to the south of the low center were closely associated with the formation and intensification of the mesolow through the enhanced southwesterly flows and the boundary layer convergence. The close relationship tween the deep convections/heavy rainfall and maximum convergence suggested the important role of the surface boundary layer (frictional) convergence in regulating the behavior of con­vections. The surface friction also possibly counteracted the vortex stretching process in the evolution of the mesocyclone. Finally, the scale dependent structure was obtained in the composite of the meso-a and meso-b lows, similar to that observed in the individual case.

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