南京地区城市下垫面特征对雷暴过程影响的数值模拟

本文选取2011年7月23日发生在南京的一次雷暴个例, 利用中尺度数值模式WRF(Weather Research and Forecasting model),耦合Noah/UCM,并采用NCEP FNL 1°×1°每日4次的全球分析场资料作为初始场及南京自动站观测数据等,对南京地区城市下垫面特征对雷暴过程的影响进行了数值模拟。结果表明:模拟的雷暴发生发展过程与该地区城市下垫面有着密切的联系。首先,雷暴发生前期,南京地区热岛效应明显。其次,城市上空的感热通量较高,结合城郊下垫面热力差异造成的城市热岛环流,加强了城区的辐合上升,为雷暴的形成提供了重要的抬升作用。城市下垫面扩张,使其上空边界层高度相应提升,垂直混合高度增加,有助于对流云的发展。此外,城市下垫面加强了大气低层的扰动位温,为雷暴提供了不稳定的层结条件。最后,城市地表较大的粗糙度使雷暴降水在城区低层的迎风面一侧明显增强。

Numerical Simulation of the Impact of Urban Underlying Surface Characteristics on Thunderstorm in Nanjing

To study the impacts of urban underlying surface characteristics on thunderstorms in Nanjing, a mesoscale numerical model is used to study a specific case occurring on July 23, 2011. The Weather Research and Forecasting (WRF) model, coupled with the Noah/Urban Canopy Model (UCM), used 1°×1° four times daily global analysis data from the National Centers for Environmental Prediction Global Forecast System (NCEP FNL) and observation data from the Nanjing automatic weather stations. The simulated results suggest that the occurrence and the development process of the thunderstorm are closely related to urban underlying surface in this region. In particular, the urban heat island effect in Nanjing is evident before the thunderstorm. Moreover, the sensible heat flux over the urban region is significantly higher than that over the surrounding regions. This, combined with the effect of the urban heat island circulation, caused surface thermal differences between urban and rural areas, which enhanced the convergence over the urban areas and provided the uplift for the formation of the thunderstorm. The expansion of the urban underlying surface is attributed to a larger boundary layer thickness and vertical mixing height, which contributed to cloud development. In addition, the perturbations in the potential temperature of the lower atmosphere were also enhanced by the urban underlying surfaces, offering unstable stratification conditions for the thunderstorm. Finally, larger roughness over the urban underlying surface caused precipitation of the thunderstorm in the windward side of the city of the low-level surface to increase significantly.