The Weather Research Forecasting (WRF)numerical model and its coupled single-layer Urban Canopy Model (UCM)are used to simulate the Urban Heat Island (UHI)effect over Lanzhou during 25-28 January 2005.In order to evalu-ate the impacts of urbanization over Lanzhou,the land use data that inversed from the MODIS 1 km resolution data is applied. A sensitivity experiment without urban underlying surface is designed to investigate the effects of urban underlying surface on urban boundary layer structures.The main results are as follows:The urban underlying surface increases the surface air tem-perature and decreases the wind speed,especially in the night.The contribution rate of the urban underlying surface to Lanzhou UHI intensity is 44%.The urban surface air keeps the characteristic of neutral stratification at nighttime.The upward move-ment of UHI circulation promots the mountain breeze circulation,and reaches the height of 600 m.Due to the heating effects of mountain peak,there is an inversion layer existing at the height of 400-600 m above the city at daytime.The UHI circulation restrain the formation and development of the valley breeze during 11:00-15:00 BT.The low albedo characteristics of urban underlying surface and the multiple reflection of buildings lead the net radiation in urban areas to being more than that in non-urban areas.Because of materials waterproofness of buildings,the latent heat flux is far less than sensible heat flux with the heat storage capacity enhanced obviously.%利用中尺度数值模式 WRF耦合单层城市冠层模块UCM，引入2005年 MODIS土地利用类型资料，在对2005年1月25-28日兰州市热岛现象进行高分辨率数值模拟的基础上，设计了去除城市下垫面敏感性试验，探讨了城市下垫面对城市边界层的影响程度。结果表明，城市下垫面能使近地层大气温度升高而风速减小，并且，在夜间表现更明显。由城市热岛强度日变化分析可知，城市下垫面对兰州市热岛强度的贡献率为44％。夜间，城市上空200 m以下的近地层大气保持了白天的混合层特征，热岛环流的上升运动促进了山风环流，使得上升气流到达地面以上600 m 左右；白天，由于山峰加热效应，城市上空400-600 m存在一个脱地逆温层，城市热岛环流使得11-15时（北京时）市区近地层出现弱上升气流，抑制了谷风环流的形成及发展。城市下垫面的低反照率特性和建筑物的多次反射作用导致城市下垫面的净辐射通量大于非城市下垫面；城市下垫面由于建筑材料的不透水性，导致潜热通量远小于感热通量，而储热项所占比重明显增大。
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