THE DAYTIME PBL HEATING PROCESS OVER COMPLEX TERRAIN IN CENTRAL JAPAN UNDER FAIR AND CALM WEATHER CONDITIONS .3. DAYTIME THERMAL LOW AND NOCTURNAL THERMAL HIGH

The daytime thermal low and nocturnal thermal high over central Japan are analyzed for fair weather and weak synoptic wind conditions during the spring. The thermal low and high were usually analyzed making use of the sea level pressure, calculated by extrapolating temperature profiles to sea level with a lapse rate of GAMMA = 0.0065 K m-1. However, such analysis overestimates the strength of the low/high by a factor of 2 to 3. In this study, overestimation was avoided by estimating the meso-scale pressure field for given constant levels with the application of a new method. During the daytime, pressure in the atmospheric planetary boundary layer (PBL) decreases with time due to PBL heating, with the daytime thermal low developing in the late afternoon. The strength and depth of the low are about -2 mb and 2000 m ASL, respectively. The thermal low is caused by the horizontal difference in the PBL heating rate over a complex terrain, this difference being attributed to a combination of three effects: the basin heating effect, the plain-plateau effect, and the sea breeze cooling effect. The first two effects are especially important. The nocturnal thermal high develops around sunrise. The strength and depth of the high are about 1.6 mb and 2000 m ASL, respectively. The thermal high is caused by the horizontal difference in the PBL cooling rate over a complex terrain, and this difference is attributed to a combination of the basin cooling effect and the plain-plateau effect, which have the opposite effects of those in the daytime.