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 August 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 lower layer effectively contributes to development and maintenance of stratiform rainfall.
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机译：在GAME-西藏密集观测期（IOP）期间，热带降雨测量团（TRMM）卫星的降水雷达（PR）探测到了西藏的每日降水周期。白天的对流和傍晚和晚间的大雨区的层状降水都带来了很多降雨。进行了两个案例研究，以阐明夜间的层状降雨结构。在案例1（1998年7月7日至8日）中，在傍晚和晚上观测到层状降水。 7月7日晚上，高原南部出现了一个天气汇合区。在6 km ASL以上，西南风为主，但在6 km ASL以下，风向在降雨过程中是可变的。当中尺度辐合加强时，降雨量变大。 7月8日，检测到西南风和北风之间的会聚区，并在傍晚越过雷达站点。在距雷达站点垂直剖面8 km ASL以下观察到风向突然从西风向西北风变化。两个风向之间的边界可以对应于冷的前表面。但是，随着正面通道的降水呈层状，回波顶较低。这两天的降雨变化与TRMM PR揭示的日变化一致。在案例2（1998年8月1日和2日）中，天气场没有明显的扰动，但降雨很大（超过4 mm / h）在午夜被观察到。直径90公里的层状回波在雷达场上方停留了七个小时。在ASL低于6 km时，风向从北向东北，然后向东变化。当东北风为主时，下层的辐合加强，层状降水加剧。中层潮湿空气的中尺度收敛有效地促进了层状降雨的发展和维持。
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