城市化对珠江三角洲强雷暴天气的可能影响

通过对2004年8月11日午后发生在珠江三角洲地区的一次强雷暴天气的高分辨数值模拟，研究了城市化发展可能对雷暴活动的影响问题，主要考察了与城市土地利用类型改变相关的“城市热岛”的形成和演变特征，城区粗糙度增大可能引起的低层辐合的增强过程，及其与雷暴发展强度变化的关系。结果表明:模拟的雷暴发展和演变过程与这一地区城市化的发展有密切的联系。引进了更加真实的关于珠江三角洲地区的土地利用类型资料之后，耦合了陆面模式Noah LSM的MM5模式可以更加成功地模拟出强雷暴天气的发展和演变过程。雷暴系统移经主要城市区后在珠江口西岸的增强过程与这一地区“城市热岛”的效应有关。中午时热岛开始形成于广州城区的上空，之后向南移动，范围扩大。另外，城区粗糙度增大引起的低层辐合增强可能在雷暴发展和演变过程中也起到了作用。模拟的与城市影响有关的低层辐合主要位于500 m以下的近地面层，开始时形成于城市的上风方向，并在下风方向增强， 由此引起的强烈上升运动有利于新的对流的启动和发展，促使雷暴强度增强。

Possible Impact of Urbanization on Severe Thunderstorms over Pearl River Delta

During the late afternoon hours of 11 August 2004, convective storms moved through the urban areas of the Pearl River Delta in South China. The convective system included severe thunderstorms with wind gusts (>30 m/s) and caused extensive damage in the urban and suburban areas. To investigate the impact of urban land use changes on the evolution of the severe thunderstorms in the vicinity of Pearl River Delta, a high-resolution numerical simulation is conducted using the MM5 (V3.6) modeling system. In the current investigation, two comparison experiments are performed. The first experiment (Expt 1) is carried out with the MM5 original surface characteristics and a simple five|layer soil model. Then, the second experiment (Expt 2) is designed to use a new land use dataset derived from local GIS to identify the Pearl River Delta urban land use changes and couple the MM5 model with the advanced NOAH LSM (land surface model), in which the urban effects has been represented and enhanced by increasing the roughness length to 0.8 m and reducing the surface albedo to 0.15. The two experiments employ three fixed grids with spacing of 36, 12, 2 km respectively. The outer two grids are set as a two-way run, and the 2 km grid is nested within the 12 km domain by using a one-way run. The initial and boundary conditions are provided by the NCEP FNL analysis and include assimilation of the upper air and surface observations. Both simulations are initialized at 0800 LST on 11 August 2004 and run for 24 h through the course of the convective event. Model results from the 2 km domain are compared with observations.The comparison experiments show that the development and evolution of the simulated thunderstorm are sensitive to local urban land use changes. By using the new land use dataset and coupling with Noah LSM, the MM5 modeling systems more successfully capture the characteristics of storm evolution. The simulation results show that Expt 2 works reasonably well in developing an urban heat island (UHI) over Guangzhou city and convection that is enhancing on the west bank of Pearl River estuary, the downwind side of the main urban areas. The UHI firstly appears over Guangzhou city at noon and then intensifies when it advects downwind of the main city areas. It is the enhanced low-level heating that made the urban atmosphere more unstable and favorable for the convection. Surface convergence enhanced by urban roughness also plays an important role in initiating the severe storm. Analysis reveals that except for the low-level outflows from the former convection storm, convergent flows produced by the urban roughness also contribute to the enhancement of convergence around the urban areas. The urban induced convergence firstly forms on the upwind side of the Guangzhou city, mainly located below 500 m, and then intensifies on the urban downwind side, which produces the sufficient vertical motion to initiate new convections in front of the original storm. As a result, convections on the downwind side of the urban areas are enhanced.