Transition of the Rainfall Characteristics Related to the Moistening of the Land Surface over the Central Tibetan Plateau during the Summer of 1998

Abstract

This paper describes a transition of rainfall characteristics related to the moistening of the land surface over the central Tibetan Plateau. This transition was observed three weeks after the onset of the summer rainy season of 1998. The objective is to clarify the potential of the plateau surface to modify the characteristics of monsoon rainfall. Summer rain events were first separated according to large-scale conditions into three types: one with a near-surface heat low and a Tibetan upper high, one with a near-surface low associated with a midlatitude trough, and one without a near-surface low. The first type was studied in further detail because of its intraseasonal variability in the rainfall amount (from 2.8 mm day?1 in June to 5.7 mm day?1 in August). The smaller amounts of the diurnal rain in June than July are related to the evaporation of precipitation within a drier and deeper subcloud layer. The moistening of this layer was related to the increase in the soil moisture and activation of vegetation. These results suggest a significant impact of the plateau surface upon the modification of the rainfall characteristics. This impact is the largest under the condition with a near-surface heat low forming due to strong solar heating.