The variation of boundary layer circulation caused by the influence of complex underlying surface is one of the reasons why it is difficult to forecast hourly heavy rainfall (HHR) in the middle Yangtze River Valley (YRV). Based on the statistics of high-resolution observation data, it is found that the low resolution data underestimate the frequency of HHR in the mountain that are between the twain-lake basins in the middle YRV (TLB-YRV). The HHR frequency of mountainous area in the TLB-YRV is much higher than that of Dongting Lake on its left and is equivalent to the HHR frequency of Poyang Lake on its right. The hourly reanalysis data of ERA5 were used to study the variation of boundary layer circulation when HHR occurred. It can be found that the boundary layer circulation corresponding to different underlying surfaces changed under the influence of the weather system. Firstly, the strengthening of the weather system in the early morning resulted in the strengthening of the southwest low-level air flow, which intensified the uplift of the windward slope air flow on the west and south slopes of the mountainous areas in the TLB-YRV. As a result, the sunrise HHR gradually increases from the foot of the mountain. The high-frequency HHR period of sunrise occurs when the supergeostrophic effect is weakened, the low-level vorticity and frontal forcing are strengthened, and the water vapor flux convergence begins to weaken. Secondly, the high-frequency HHR period of the sunset is caused by stronger local uplift and more unstable atmospheric stratification, but the enhanced local uplift is caused by the coupling of the terrain forcing of the underlying surface and the enhanced northern subgeostrophic flow, which causes the HHR to start closer to the mountain top at sunset than at sunrise.

中文翻译：

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下垫面对长江中游逐时暴雨分布的影响

受复杂下垫面影响引起的边界层环流变化是长江中游逐时强降水（HHR）预报困难的原因之一。对高分辨率观测资料进行统计发现，低分辨率资料低估了长江中部双湖盆地之间山区HHR的频率（TLB-YRV）。TLB-YRV 山区HHR 频率远高于其左侧洞庭湖的HHR 频率，与右侧鄱阳湖HHR 频率相当。利用ERA5小时再分析资料研究HHR发生时边界层环流的变化。可以发现，不同下垫面对应的边界层环流在天气系统的影响下发生了变化。一是清晨天气系统加强导致西南低层气流加强，加剧了青藏高原-黄河流域山区西坡和南坡迎风坡气流的抬升. 于是，日出HHR从山脚下逐渐增加。超地转效应减弱，低层涡度和锋面强迫加强，水汽通量辐合开始减弱，出现日出高频HHR期。其次，日落的高频HHR周期是由更强的局部隆升和更不稳定的大气层结引起的，