复杂地形条件下城市热岛及局地环流特征的数值模拟

Numerical simulation of urban heat island and local circulation characteristics under complex terrain conditions

应用基于多层城市冠层方案BEP(Building Environment Parameterization)增加室内空调系统影响的建筑物能量模式BEM(Building Energy Model)方案的WRF模式,模拟研究重庆热岛的特征、成因以及局地环流对热岛形成的影响。文中共有两个算例,一为重庆真实下垫面算例,称之为URBAN算例,二为将城市下垫面替换为耕地下垫面的对比算例,称之为NOURBAN算例。结果表明:1)WRF方案模拟结果与观测2 m气温的对比吻合较好,误差主要出现在正午温度峰值和凌晨温度谷值处,由城市下垫面特性及城市内建筑分布误差引起。2)BEP+BEM方案较好地模拟出了重庆地区的热岛分布的空间和时间特征。重庆市温度的分布受地形和城市下垫面的双重影响,越靠近城区,温度的分布受城市化影响就越大,在海拔低处,温度就越高。3)城区立体三维表面对辐射的陷阱作用导致城市表面总体反射率小,向上短波辐射小于郊区约20 W/m^2。城市表面以感热排放为主,而郊区则表现为潜热的作用占主导。夜间城市地表储热以及空调废热向大气释放,是城市热岛形成的重要原因。4)模拟区域背景风场主要为东南风,局地环流呈现出越靠近山区风速越大、城市区域风速较小的特性,体现了城市密集的建筑群对低层大气流场的空气动力学效应,以及复杂山谷地形的山谷风环流特性。在市区的西侧和东南侧均有高大山脉阻挡,山脉对城市出流的阻碍作用、气流越山与绕流运动对城市热岛的形成有一定影响。

Using WRF model coupled with multilayer urban canopy scheme BEP(Building Environment Parameterization) and BEM(Building Energy Model) scheme,a simulation was conducted to explore characteristics and causes of Chongqing urban heat island and impact of local circulation on it. There were two simulation cases conducted,one was URBAN case that utilized real Chongqing land use data,another was NOURBAN case that replaced urban category with crop in order to understand impact of urban on Chongqing heat island. Results show that:(1)WRF model produces good results compared to observed 2 m air temperature. Errors mainly occur at noon temperature peak and morning temperature valley,which are caused by the characteristics of urban land use and unreal building parameters.(2)BEP+BEM scheme can simulate well spatial and temporal features of urban heat island in Chongqing. Spatial distribution of temperature in Chongqing is influenced by topography and urban underlying surface. When closer to the city,greater temperature is affected by the urbanization,and higher temperature locates at low elevation.(3)Urban 3D surface leads to trap effect in urban surface albedo(Total reflectivity of urban surface is low),and the urban upward shortwave radiation is less than about 20 in suburbs. Sensible heat is a major factor in urban energy balance however latent heat in suburbs. The larger urban surface heat storage and the waste heat of air-conditioner released to the atmosphere at night are important reasons for urban heat island conformation.(4)The background wind field is mainly southeast wind in the simulated area. The wind speed is higher in mountain area and lower in urban area,which reflects the aerodynamic effects of dense urban buildings on the low-level atmospheric flow field,as well as the characteristics of valley wind circulation over complex valley terrain. There are high mountains in the western and southeastern sides of the city,which block the outflow from the city,let the background wind to climb or circle around the mountains,and contribute to the enhancement of urban heat island.