南京城市下垫面对夏季暴雨云团特征的影响

Urban impacts on the structure and evolution properties of warm-season thunderstorms over Nanjing,China

为了理解城市下垫面对降雨的影响机制,选取南京地区2次不同天气背景的降雨过程,利用中尺度气象模式和暴雨云团追踪算法开展数值试验,揭示城市下垫面对暴雨云团形态、结构和演进特征的影响机制。结果表明:当天气强迫较弱时,城市热岛效应增强大气边界层对流活动,暴雨云团进入城市后快速坍塌,云团数量减少,空间覆盖增大,降雨在城区增加;当天气强迫较强时,城市冠层影响暴雨云团特征,城市地区出现大量面积小、移动缓慢且空间结构“尖瘦”的暴雨云团,降雨在城区和城市下风向增加。城市下垫面对暴雨云团特征的影响与降雨前期天气强迫有关,更新了城市对降雨影响机制的传统认识,为城市地区暴雨临近预报和设计暴雨方法提供了科学支撑。

Urban impacts on spatial and temporal rainfall variabilities present significant challenges for effective urban flood mitigation and adaptation strategies.However,the physical mechanisms underlying these impacts remain unclear.In this study,we conducted modeling analyses using the Weather Research and Forecast(WRF)model,combined with thunderstorm identification and tracking algorithms,to investigate the influence of urban areas on warm-season thunderstorms in Nanjing,China.Our findings reveal divergent urban impacts on the structure and evolution properties of thunderstorms based on different pre-storm synoptic conditions.When the synoptic conditions are weak,the urban heat island effect dominates,enhancing convective activities over urban areas.This leads to a reduction in the number of storm cells but an expansion in spatial coverage,ultimately resulting in increased rainfall over downtown areas.Conversely,when the synoptic conditions are strong,the urban canopy effect becomes prominent,slowing down storm movement and increasing the frequency of small storm cells over urban regions.These storm cells exhibit distinct“sharp”structures in terms of rainfall distribution and tend to intensify over downwind areas due to moist convergence.As a result,both downtown and downwind regions experience enhanced rainfall.This study improves our understanding of urban rainfall modification and offers valuable insights for storm nowcasting algorithms and the design of urban-specific rainfall events.