The processes associated with the diurnal variation of precipitable water over the “semi-basin” were studied using GPS and radiosonde data. Precipitable water over the semi-basin exhibited a pronounced diurnal variation, with a maximum around 18-20 JST and a diurnal range of 10 to 25 mm. The variation of precipitable water agreed well with the variation of ESE component of the surface wind, due to a thermally induced local circulation. The diurnal variation of water vapor was recognized in two different layers of Layer L (surface to 1.5 km), and Layer H (1.5 km to 3.0 km).
In the increasing period (09:00 to 20:00 JST), precipitable water increased simultaneously all over the south part of the mountains in the north Kanto District. The variation of precipitable water in the Layer L would be, in large part, controlled by convergence of surface wind. Furthermore, the amount of water vapor in Layer H over the semi-basin that is located on the lee side of mountains increased abruptly from evening to midnight. It was expected that the water vapor that had been transported into the mountains due to the thermally induced local circulation traveled into the semi-basin by ambient wind and/or return flow, and the moisture advection caused the increase of water vapor in Layer H over the semi-basin.
In the decreasing period (21:00 to 08:00 JST), precipitable water decreased simultaneouly all over the GPS network using the present study. The decrease in water vapor coincided with increase in potential temperature in the Layer H. The large scale subsidence due to the Pacific High seemed to be necessary for the prominent diurnal variation of precipitable water over the semi-basin.