Diurnal Cycle Of Rainfall Over Yangtze And Huai River Basin During Meiyu And Analysis Of A Typical Convective System In The Morning

Under the influence of the East Asian summer monsoon, the most severe convections and heavy rainfalls are often observed over the Yangtze and Huai River (YHR) basin. Due to the unique background large-scale circulation and the complex underlying surfaces, the spatial distribution of precipitation over the region is extremely heterogeneous, with a significant rainfall center around the Dabie Mountains. It is worth noticing that a general bimodal distribution of rainfall is found over the YHR basin with one peak in the early morning and the second in the late afternoon. Previous studies have stated the importance of the multi-scale mountain-valley breezes as well as the large-scale atmospheric circulation on the bimodal distribution of rainfail, while the local effects have not been clarified. Based on data from automatic weather stations (AWSs), high resolution radar network, a convection-resolving simulations and abundant observation data collected during the OPACC 2014 field campaign, the role of the local terrain and frontal convection itself in the morning rainfall are investigated in the thesis.Firstly, high spatial and temporal precipitation data from AWSs and radar composite reflectivity dataset during summer,2013 are used to analyze the precipitation characteristics including the spatial distribution, meridional propagation and diurnal cycle during different periods. In line with findings from former studies, the precipitation and the convective activities exhibit distinct features at different periods. During the pre-Meiyu period, under the influence of the high-level westerly flow, rainfall systems exhibit meridional propagation with a wide extent and long duration. Occurrence frequency of precipitation events significantly increases during the Meiyu season. Most precipitation occurs east of 114°E, over the main topography of the Dabie Mountains. The bimodal distribution of rainfall diurnal cycle is most outstanding. During the post-Meiyu period, eastward propagation almost completely ceases and single rainfall peak in the afternoon dominates the diurnal cycle of precipitation. Consistent with the rainfall patterns, convective activities also show distinct diurnal variations, making an important contribution to the precipitation.Secondly, the diurnal variation of precipitation over the Dabie Mountains during the Meiyu season is investigated with simulation results from a regional convection-resolving model. The observed morning precipitation peak on the windward side (relative to the prevailing synoptic-scale wind) of the mountains is reproduced with good spatial and temporal accuracy. The interaction between the mountains and a nocturnal boundary layer low-level jet (LLJ) due to the inertial oscillation mechanism is shown to be responsible for this precipitation peak. When the LLJ, aligned with the lower-level southwesterly synoptic-scale flow that carries abundant moisture, impinges upon the windward side of the mountains in the early morning, mechanical lifting of the moist air leads to condensation and subsequent precipitation.Finally, based on the data collected during the OPACC 2014 field campaign, the effect of convective processes near the Dabie Mountain is investigated. The observations reveal back-building of the new sub-frontal convection which is continuously triggered to the south of the Meiyu front rain band and moves eastward, leading to strong morning rainfall. Further analysis from the dual-Doppler radars and the 4DVAR pointes out that the vertical shear maintains inclination of the meso-β-scale convection, avoiding raindrops falling into the convective updraft region. The cold pool left behind by the previous rainfall systems leads to surface outflow towards the south. Combined with the southwesterly flow due to the intensified nocturnal low-level jet and the meso-convective vortex, strong convergence results in the middle level. Thus new convective cells would be triggered and incorporated to the existing convection system and prolong its duration, causing the constant morning rainfall.