Mechanism And Characterstics Of Diurnal Variations Of Warm-season Precipitation Over East Of The Tibetan Plateau And Northern China

This study explores the diurnal variations of the warm-season precipitation over the east of the Tibetan Plateau and northern China using the high-resolution CMORPH precipitation data and the NCEP GFS gridded analyses during mid-May to mid-August of2003-2009. Complementary to the past studies using surface observations, satellite data and radar data, it is found that there are strong diurnal variations in the summertime precipitation over the two focus domains. The diurnal cycles differ substantially from region to region and during the three different month-long periods, namely, the pre-Meiyu period (May15-June15), the Meiyu period (June15-July15), and the post-Meiyu period (July15-August15).To the east of the Tibetan Plateau, these diurnal precipitation cycles are strongly associated with three thermally driven regional Mountain-Plains Solenoids (MPS) circulation due to the differential heating between the Tibetan Plateau, the highlands, the plains, and the ocean. In particular, there is substantial difference in the propagation speed and eastward extent of the peak phase of the dominant diurnal precipitation cycle that is originated from the Tibetan Plateau during three different periods. This diurnal peak has a faster (slower) eastward propagation speed, the more (less) coherent propagation duration, and thus covers the longest (shortest) distance to the east during the pre-Meiyu (post-Meiyu) period than that during the Meiyu period. The differences in the mean midlatitude westerly flow and in the positioning and strength of the Western Pacific Subtropical High (WPSH) during different periods are the key factors in explaining the difference in the propagation speed and the eastward extent of this dominant diurnal precipitation cycle.Over northern China, the areas of focus are the Yanshan-Taihangshan Mountaion ranges and the adjacent North China Plains. During three different periods, it is found that the peak phase of the dominant diurnal precipitation cycle begins at early afternoon hours (with maximum solar heating) on or near the top of the mountain ranges and propagates downslope and southeastward. The diurnal peak reaches the central North China Plains around midnight and the early morning hours resulting in a broad area of nocturnal precipitation maxima over plains. The diurnal precipitation peak is closely collocated with the upward branch of a Mountain-Plains Solenoid (MPS) circulation. Both the upward branch of MPS and a low-level south westerly nocturnal jet are likely to be jointly responsible for the nighttime precipitation maxima over the plains. In particular, there is substantial difference in the characteristics of the peak phase of the dominant diurnal precipitation cycle that is originated from the mountain ranges during three different periods. This diurnal precipitation peak signal is vaguer and has a slower eastward propagation speed during the post-Meiyu period than that during the pre-Meiyu period and the Meiyu period, which are due to the regional weather regime has obvious difference between the post-Meiyu period and the other periods resulting from the Western Pacific Subtropical High (WPSH) moving northward and extending to the west.The diurnal precipitation cycle numerical experiments using the Weather Research and Forecast (WRF) model are performed to examine the impact of a thermally-driven Mountain-Plains Solenoid (MPS) circulation on the diurnal variations of warm-season precipitation over northern China. We choose17-24June2004on which to base our diurnal precipitation cycle simulation. The focus of the analyses is a15-day simulation that used the8-day average of the NCEP GFS gridded analyses at0000UTC as the initial condition and the8-day averages at0000UTC,0600UTC,1200UTC and1800UTC as lateral boundary conditions (with diurnal variation only). Despite differences in the rainfall intensity and location, the control experiment successfully simulated the diurnal variation of warm season precipitation and southeastward propagation of the peak phase of the diurnal precipitation from Yanshan-Taihangshan Mountaion ranges to the North China Plains. It is found that the main upward branch of the MPS is primarily responsible for the moving southeastward rainfall belt that is originated from the mountains. Fake-dry sensitivity test is performed to study the role of latent heating release and cooling in maintenance and development of the MPS. In the low-troposphere, this "cold pool" behind the main upward branch of the MPS resulting from vapor cooling when rain fall is responsible for the convection development and also pushes the main upward branch moving southeastward.