摘要:城市化对高温热浪的频次和强度具有重要影响，但目前对于城市化影响高温热浪过程的机理了解还不充分。本文利用WRF模式，对2010年7月2～6日（北京时）北京一次高温过程进行了模拟，分析了城市化对此次高温过程的影响机理。采用优化后的WRF模式，能够模拟出北京连续5日高温的特征和城市热岛强度的变化。城市下垫面的不透水性决定了城区2 m高度处相对湿度低于乡村，削弱了城区通过潜热调节城市气温的能力。日落后，城市感热通量下降缓慢，城区降温速率小于乡村，夜间边界层稳定、高度低，风速小，抑制了城乡之间能量的传输，形成了夜间强的城市热岛强度，造成夜间城市气温明显高于乡村。日出后城乡地面感热通量、潜热通量迅速上升，边界层稳定性下降。午后，城市下垫面分别为地表感热通量和潜热通量的高、低值中心，通过潜热调节气温的能力被削弱；边界层稳定性降低，有利于能量的垂直扩散；此时，城市热岛强度小于夜间。因此，北京城市下垫面形成了明显的城市热岛效应，加重了城区极端高温事件的强度。此外，在这次高温热浪期间，中国东部大部分地区受到大陆暖高压控制，晴空少云，西北气流越山后形成焚风效应，是北京地区高温热浪形成的天气背景。
关键词:极端高温/
城市热岛/
数值模拟/
WRF（Weather Research and Forecasting）/
北京
Abstract:Urbanization has a significant influence on the frequency and intensity of heat waves, but the mechanism of the effect of urbanization on the high-temperature process is not fully understood. In this study, the authors used the Weather Research and Forecasting (WRF) model to simulate a summer high-temperature process on 2–6 July 2010 in Beijing. This paper reports the main results obtained regarding the urbanization effect on the surface air temperature of urban areas during the heat-wave process. The optimized WRF model was able to simulate the temporal characteristics of the five consecutive days of high temperature and the variation in the urban-heat-island intensity (IUHI) in Beijing. The impermeability of the underlying urban surface lowers the 2-m relative humidity of urban areas with respect to that of rural areas, which weakens the ability of urban areas to regulate the surface air temperature via latent heat. After sunset, the urban-sensible-heat flux decreases slowly, and the cooling rate in urban areas is slower than that in rural areas. At night, the structure of the boundary layer is stable, and its height is low, as is the wind speed. In this case, the energy transmitted between urban and rural areas is constrained, and the strong urban heat island is formed, resulting in the temperature in urban area is significantly higher than that in rural area at night. After sunrise, both the sensible and latent heat fluxes of urban and rural land surfaces increase rapidly, and the stability of the boundary layer decreases. In the afternoon, the underlying urban surface favors high and low value centers in the sensible and latent heat fluxes, respectively, with a weakened ability to regulate temperature via latent heat. This is conducive to vertical exchange of energy, which decreases the stability of the boundary layer. The IUHI is lower in the afternoon than in the evening. Therefore, the obvious urban-heat-island effect created by the underlying urban surface in Beijing increases the strength of extreme-high-temperature events. Furthermore, in this heat-wave process, most of the eastern part of China is controlled by continental warm high pressure with clear skies and few clouds, and the northwesterly winds flowing over the Taihang Mountains generate a Fohn effect, which is the synoptic situations of the heat-wave formation in Beijing.
Key words:Extreme high temperature/
Urban heat island/
Numerical simulation/
WRF (Weather Research and Forecasting)/
Beijing
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