Mesoscale Disturbances and Related Rainfall Cells Embedded in the "Baiu Front", with a Proposal on the Role of Convective Momentum Transfer

Abstract

The frequency distribution of the wave length of mesoscale disturbances is studied for a 3-day period from July 8 to July 11, 1968, when an active "Baiu front" was situated over the Kyushu district. The predominant wave length of mesoscale disturbances is shown to be about 150∼200km.
A series of mesoscale disturbances accompanied by heavy precipitation are studied in detail particularly for the case of July 9. Significant rainfall amounts were limited to the southern part of Kyushu where a low-level jet stream was located, while the configuration of the associated mesoscale pressure system was elongated in the north-south direction to a large extent. Stages of precipitation intensity depend on the spatial phase relationship between the pressure and the divergence fields.
The synoptic-scale divergence and vorticity of the surface wind are found to fluctuate with a very short period of the order of 1.5∼3 hours. The phase difference between the fluctuation of divergence and that of vorticity changes from "in phase" to gout of phase" when a cyclone passes over Kyushu. This seems to be due to the convective exchange process in the area of convergence.