Transition of the rainfall characteristics related to the moistening of land surface over the central Tibetan Plateau during GAME-Tibet IOP

The Tibetan Plateau is characterized by deep convective activity and its marked diurnal variation during the summer monsoon season. It is expected that the conditions of the plateau surface, such as soil moisture and vegetation, have a large impact for the development of the convective clouds and associated rainfall. Over the plateau, land surface is heated in the daytime by strong solar irradiation resulted from little attenuation in the optimally thin atmosphere. Since the ratio of the surface-level sensible heat flux to latent one (i.e., Bowen ratio) depends on the surface conditions, this may influence the boundary-layer conditions, and may affect the development of convective clouds and rainfall characteristics.

To demonstrate the sensitivity of convective precipitation for the land-surface conditions, simultaneous observations on the land surface, precipitation clouds, and rainfall, were conducted over the central Tibetan Plateau during the summer season in 1998, as a part of the GAME-Tibet project (GAME: GEWEX Asian Monsoon Experiment). An X-band Doppler radar was placed on the grassland (31.4N, 91.9E, 4590 m above sea level), and was operated from 27 May to 19 September. Using the observational data (from a Doppler radar, raingauges, etc.), this paper describes the characteristics of monsoon rainfall and focuses on their significant transition that occurred three weeks after the onset of deep convective activity. The point of this study is that this transition occurred in early July when the increase in soil moisture content and activation of vegetation (short grasses) were observed within the radar observational area.

This study first classified all of rain events in the monsoon period into three types according to the synoptic-scale conditions because not only thermal instability but also large-scale frontal disturbances can cause monsoon rainfall. The separation was conducted using the geopotential height and horizontal wind components of the reanalysis dataset. One of the three type, characterized by no frontal disturbance but the presence of a surface heat low and a Tibetan upper high, was the subject of detailed investigation, since the daily rainfall amount of this type increased from 2.8 mm in June to 5.7 mm in August, while there was no such marked increase in the other two types.

The analysis of rainfall amount measured by the nine raingauges and radar echoes 3.0 km above each raingauge, showed that smaller amount of rainfall in June was related to the mitigation of rainfall by evaporation of precipitation particles within sub-cloud layer. The drier and deeper conditions of sub-cloud layer in June than July were confirmed using upper-air sounding data. The sub-cloud layer conditions favorable for the evaporation were demonstrated by examining the frequency of temperature drop larger than 5 Kelvin in an hour related to the passage of convective clouds. The frequency was higher in June than July, suggesting the dry environment in June.

According to the results, a conceptual model on the transition of rainfall characteristics in early July can be illustrated as Fig. 1. Both the moistening of the ground and the activation of grass cause the decrease in sensible heat and increase in latent heat, which affect to the depth and wetness of the sub-cloud layer. The sub-cloud layer becoming shallow and wet provides the unfavorable condition for the evaporation of precipitation particles, and yields the transition of rainfall characteristics. These results, therefore, indicate the sensitivity of convective precipitation for the land-surface conditions over the Tibetan Plateau during the summer.

Fig. 1 Schematic illustration showing the transition of the rainfall characteristics due to the moistening of the plateau surface.