重庆城市热岛环流结构和湍流特征的数值模拟

利用中尺度模式WRF(V3.9)对2016年8月17~18日重庆一次城市热岛环流个例进行了数值模拟,探讨了山地城市热岛环流的三维结构和演变特征,分析了热岛环流期间湍流动能和各项湍流通量的特征。结果表明:15:00(北京时,下同)乡村风开始出现,随着热岛强度增强乡村风增大,18:00热岛环流结构最显著,次日02:00热岛环流结构被破坏,仅低层存在微弱的乡村风。其中,重庆市城市热岛环流最强时,水平尺度约城市尺度的1.5~2倍,垂直厚度约1.3 km,水平风速约2~4 m s-1,最大上升速度约0.5 m s-1。受地形、河流以及背景风的影响,环流呈现非对称的结构,且强度较弱。湍流特征分析结果表明,城市区域的湍流动能明显大于其它区域。此外,城市热岛环流通过湍流运动将郊区的水汽输向城市;高层湍流动量补充边界层中因热岛环流发展而造成的动量耗散。

A Numerical Simulation of Urban Breeze Circulation Structure and Turbulence in Chongqing

To investigate the structure and evolution of urban breeze circulation in a mountain city, we used the Weather Research and Forecasting model(V3.9) to simulate the typical urban breeze circulation from August 17 to 18, 2016 in Chongqing. We also analyzed the turbulent kinetic energy and turbulent fluxes during this period. The results show that the rural wind begins at 1500 BT(Beijing time) and increases as the heat island strengthens. The circulation reaches its maximum at 1800 BT and subsides at 0200 BT the next day. At 1800 BT, the horizontal scale of the rural wind circulation is about 1.5–2 times that of the urban area, with a vertical scale of about 1.3 km, a horizontal wind speed of about 2–4 m s-1,and a maximum rate of increase of about 0.5 m s-1. Due to the influence of the topography, rivers, and background wind,the circulation is weak and has an asymmetrical structure. We also found the turbulent kinetic energy in urban areas to be obviously larger than that in nonurban areas, which means that urban areas experience stronger transports of heat and water vapor by turbulence. With respect to the relationship between turbulent fluxes and urban breeze circulation, we found that the circulation of urban breezes transports water vapor from the suburbs to the city via turbulent motion, with the turbulence supplying momentum for dissipation of the urban breeze circulation.