A drastic increase in precipitable water vapor (PWV) in the evenings was repeatedly confirmed in October 2000, by an analysis of GPS observational data at Lhasa in the southeastern Tibetan Plateau, China. In order to investigate its mechanism, a numerical simulation was performed for a typical day using a regional atmospheric model. During the daytime, moisture is transported toward the summits of the Himalayas from the Hindustan plain along the southern slope of the mountain range by thermallyinduced upslope winds. The moist air mass penetrates the Tibetan Plateau, through some cols of the Himalayas. Moisture accumulates over the northern foot of the Himalayas. Then a horizontal gradient of moisture increases north of the Himalayas, and a stationary moisture front forms between the moist air mass and dry air on the Tibetan Plateau. In the evening, the frontal structure begins to gradually decay until the midnight, and a large amount of moisture bursts toward the inner Tibetan Plateau in the lower atmosphere as a gravity current, causing a rapid increase in PWV. It is speculated, therefore, that the drastic evening increase in PWV, extending widely along the Himalayas, is generated by the plateau scale diurnal wind induced by the thermal effects between the Tibetan Plateau and the Hindustan plain. During the following day, the PWV level decreases over a wide area of the inner Tibetan Plateau, since dry air advection is intensified in the lower atmosphere by the synoptic scale westerly winds, because of the growth of the mixed layer accompanied by the vertical transportation of momentum.
2003 by Meteorological Society of Japan