Weak and Frequent Monsoon Precipitation over the Tibetan Plateau.

Abstract

This study analyzed the temporal and spatial scales, types, and intensity of precipitation with their temporal variability by using the precipitation data, intensively observed as part of the GAME-Tibet international project, at the Naqu basin (4500-5000 m) in the Tibetan Plateau from May to September, 1998. The monsoon period at the basin was defined from the middle of June to the beginning of September by the precipitation and GMS/IR convective index variation. Basin averaged monsoon precipitation was 336 mm, and most of the precipitation were provided as rain or hail. The lowest altitude for a 50 % possibility of snowfall was estimated at almost around 5000 m above sea level. Daily precipitation data records from June to August in the the Asian monsoon region highlighted frequent precipitation with weak intensity in the Tibetan Plateau. In the Naqu basin, the percentage of days with precipitation of more than 0.25 mm/d was 87 % during the monsoon season, and the average hourly precipitation intensity was 1.27 mm/h. Difference in the accumulated precipitation amount between daytime and nighttime was not obvious, but the clear diurnal change existed in the precipitation intensity, i.e., weak precipitation events below 1 mm/h in the daytime and rather heavy precipitation events above 3 mm/h at night. Most of the events are associated with the passing of synoptic (plateau) scale cloud area over the plateau. The daytime events consisted of isolated convective echoes with a small spatial scale, the nighttime events consisted of widespread stratiform echoes. According to these observational results, conceptual mechanisms underlying the daytime frequent precipitation with weak intensity are discussed.