Early Spring Dry Spell in the Southeastern Margin of the Tibetan Plateau

Abstract

The climatic features of the distinctive early-spring dry spell in the southeastern margin of the Tibetan Plateau and the relationship between this dry spell and topographic forcing are investigated. The results show that the early-spring dry spell is characterized by the lowest relative humidity and highest evaporation in the year and by an extremely low amount of rainfall in March and April. The dry spell is a regional phenomenon, as centers of high relative humidity and large rainfall amounts are located to both the east and west of the dry region. In the major dry region, more than 80% of the days in March and April are “dry” days, during which the daily precipitation amounts to no more than 0.1 mm. The mean duration of a dry period between two consecutive rainfall events is found to be 7 days. Further analyses indicate that the relationship between the prevailing westerly flow and the topographic conditions plays an important role in the formation of the dry spell. Strong orographic uplift occurs as the westerly wind flows over the north-south-oriented mountain ranges to the west of the dry region; this in turn causes heavy rainfall over the windward side. After flowing over the mountains, the air subsides over the dry region on the leeward side, suppressing precipitation and increasing the surface air temperature. On the basis of sensitivity experiments carried out using a regional climate model, the role of the topographic conditions in modulating the local climate is confirmed. Reduction in the height of the western mountains as well as lowering of the elevation of the eastern highlands can lead to significant increases in the early-spring rainfall over the dry region.