Mesoscale characteristics and structures of stratiform precipitation on the Tibetan Plateau

During the GAME-Tibet Intensive Observation Period (IOP), the precipitation radar (PR) of the Tropical Rainfall Measuring Mission (TRMM) satellite detected a diurnal cycle of rainfall in Tibet. Much rainfall was brought both by convection in the daytime and by stratiform precipitation over a wide rain area in the evening and night. Two case studies were conducted to clarify the structure of stratiform rainfall in the nighttime. In Case 1 (7 and 8 July 1998), stratiform precipitation was observed in the evening and night. On 7 July, a synoptic convergence area developed in the southern part of the Plateau in the evening. Southwesterly wind dominated above the 6 km ASL, but the wind direction below 6 km ASL was variable during the rainfall. When the mesoscale convergence strengthened, the rainfall amount became large. On 8 July, a convergence zone between southwesterly wind and northerly wind was detected and passed over the radar site in the evening. A sudden wind direction change from westerly to northwesterly was observed below 8 km ASL from the vertical profiles at the radar site. The boundary between the two wind directions may correspond to the cold frontal surface. However, the precipitation with the frontal passage was stratiform and had a low echo top. The variation of rainfall on the two days was consistent with the diurnal variation revealed by TRMM PR. In Case 2 (1 and 2 Angust 1998), there was no remarkable disturbance in the synoptic field, but much rainfall (exceeding 4 mm/h) was observed in the midnight. A 90 km-diameter stratiform echo stayed over the radar site for seven hours. Below 6 km ASL, the wind direction varied from northerly to northeasterly, then to easterly. When northeasterly wind dominated, the convergence in the lower layer strengthened and stratiform precipitation was intensified. Mesoscale convergence of moist air in the laser layer effectively contributes to development and maintenance of stratiform rainfall.