Transportation of Water Vapor into the Tibetan Plateau in the Case of a Passing Synoptic-Scale Trough

Abstract

The passing of a synoptic trough is expected to contribute strongly to water vapor (WV) transport processes from the Indian Ocean to the Tibetan Plateau (TP) during the monsoon season. However, the pathway of the WV into the plateau associated with passing a trough and its contribution to the WV budget over the TP are not clear. In this study, the processes of WV transport in the case of a passing trough in 1998 were analyzed using GAME reanalysis data and the numerical model, focusing especially on the WV transportation pattern in the Indian Monsoon region and the diurnal variation of WV intrusion from south of the Himalayas to the TP.
WV advection into the TP was larger in the case of a passing synoptic trough than in the case of a prevailing Tibetan High. Sub-continental scale circulation for the two synoptic types corresponded with active/break phase of the Indian monsoon. In case of prevailing Tibetan High, a cyclonic circulation with a low-pressure area over India, associated with active Indian monsoon phase, prevented the WV intrusion into the TP in the middle troposphere. However, in the case of the trough that corresponded with break phase of Indian Monsoon, WV was transported directly from Arabian Sea to the southern foot of the Himalayas with a northward shift of the low-level monsoon westerly and it intruded into the southeastern TP.
Numerical experiments showed that the WV transport process was composed of multiple steps in the case of the passing trough. WV was transported by the monsoon westerly to the southern foot of the Himalayas at 1500 m above sea level (a.s.l.). The moist air mass reached south of the TP with at about 5500 m a.s.l. during the noon to evening because of the development of a mixing layer and the enhancement of an upslope wind in southern slope of the Himalayas. A moist southwesterly flow was converged latitudinally in the southeastern TP because of the dry northwesterly flow prevailing in the rear of the passing trough over the TP. This convergence area expanded northward from mid-afternoon into the night and disappeared early next morning.