An analysis of summer rain showers over central Japan and its relation with the thermally induced circulation

Abstract

The summer rain shower which develop in central Japan under otherwise fair-weather conditions during the summer of 1985 were investigated, making use of routine observational data from weather stations. According to the statistical analysis, the diurnal cycle of precipitation exhibited a distinct peak around 1800 LST, while little rainfall occurred from 0000 to 1200 LST. This distinct peak was associated with the summer rain shower developed over the heated land surface during the afternoon. The rain shower activity increased as the atmospheric static stability for dry (or moist) convection decreased, under conditions that the atmospheric precipitable water exceeded 40 mm. The rain shower was concentrated in the mountainous regions which consist of several mountain ranges having the horizontal scale of about 100 km. The areas where the rain shower was concentrated largely did not move with time. The degree of spatial concentration of the rain showers however was weaker under higher activity conditions of the rain showers. A thermally induced local circulation developed over central Japan during the daytime under fair weather and weak synoptic wind conditions from the spring to summer seasons. This circulation was strongly dependent on the topography, and converged in the mountainous regions. According to the previous study, the daytime thermally induced circulation contributes to an increase in the water vapor content over the mountainous regions through the moist air advection from the plain and basin regions, and the greatest amounts of water vapor are accumulated over the mountainous regions in the late afternoon when the horizontal scale of topography is close to 100 km. Specific humidity measured in the mountainous area displayed an afternoon maximum, exceeding that at the basin bottom in the afternoon. The increase in water vapor over the mountainous regions is expected to contribute to the development of cumulus clouds, which was confirmed by decreased sunshine duration during the late afternoon over the mountainous regions. These results suggest that summer rain showers are triggered over the mountainous regions by the thermally-induced local circulation.