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北京城市下垫面形态对强降水过程的影响及作用机制研究

Study on the Impact and Mechanism of underlying surface morphology on a local heavy rainfall event in Beijing urban area

  • 摘要: 城市化快速发展导致下垫面急剧演替,其通过改变动力传导、水力渗透等性质,对地表能量平衡及局地气象环境造成影响,并可能显著改变局地降水分布。为研究城市形态特征对降水的影响,基于局地气候分区(Local Climate Zone,LCZ)方案,首先分析了北京城区不同下垫面特征及网格分辨率适用性,然后将其引入WRF模式,对2023年8月20日北京市一次局地强降水过程开展数值模拟试验,最后研究下垫面形态对此次过程的作用机制。结果表明:(1) 北京城区建筑物下垫面约占总面积的52.0%,其中以开阔中层建筑、开阔高层建筑及密集低层建筑为主。采用500 m及更高的网格分辨率能更准确表征建筑物形态分布特征;(2) 北京城市建筑物形态对此次过程最大降雨量及小时雨强的影响幅度分别可达−16.1%~21.1%及−22.9%~15.0%,建筑物密度大、垂直尺度高的城市形态有利于降水增强;(3) 改变建筑物形态对降水的作用机制有多重效应,提升城市建筑物密度或垂直高度有利于增强低层热力辐合、积累对流不稳定能量,加强局地垂直上升运动。同时也会因地表粗糙度及不渗水下垫面比例增加进而削弱局地水汽供应,对降水有负面作用。研究结果可为精细化气象服务以及城市规划建设提供参考。

     

    Abstract: The rapid development of urbanization has led to a rapid succession of underlying surfaces, which affects surface energy balance and local meteorological environment by altering properties such as dynamic conduction and hydraulic infiltration, and may significantly alter local precipitation distribution. To investigate the potential impact of urban three-dimensional morphological characteristics on precipitation process in Beijing area, this study first analyzed the composition of Beijing’s underlying surface categories by local climate zone (LCZ) method and the applicability of LCZ grid resolution. After that, the LCZ classification data is diverted into the mesoscale numerical weather prediction model WRF, in order to conduct simulation experiments on a heavy rainfall event occurred in Beijing area on August 20, 2023. The results show that: (1) The underlying surface of buildings in the urban area of Beijing accounts for 52.0% of the total area, mainly consisting of open mid-rise buildings (LCZ5), open high-rise buildings (LCZ4), and dense low rise buildings (LCZ3). Using a grid resolution of 500 m or higher for the semivariogram model can more accurately characterize the distribution characteristics of building morphology; (2) The impact of Beijing's urban building morphology on the maximum precipitation amount and hourly rainfall intensity during this precipitation process can reach -16.1% to 21.1% and -22.9% to 15.0%, respectively. Overall, urban morphologies characterized by high building density and vertical scale are conducive to precipitation enhancement (3) Altering the underlying surface morphology has multifaceted effects on local precipitation process. Increasing urban building density or vertical height can enhance low-level thermal convergence, accumulate convective instability energy, and strengthen local vertical upward movement, which are beneficial for precipitation development and intensification. At the same time, it may also weaken the local water vapor supply due to the increase in surface roughness and the proportion of impermeable underlying surfaces, which is detrimental to precipitation development. These complex mechanisms may lead to uncertainty in the comprehensive impact of urban three-dimensional morphological characteristics on precipitation process. The findings of this study can provide reference for refined meteorological services and urban planning.

     

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