污染天气下成都城市热岛环流结构的数值模拟

Numerical simulation of urban breeze circulation in a heavy pollution event in Chengdu city

本文利用中尺度模式WRF V3.9对2016年7月16日成都一次污染天气下的城市热岛环流个例进行了数值模拟,分析了城市热岛环流的三维结构及演变特征.并通过设计减少气溶胶光学厚度的敏感性试验,研究了污染对城市热岛环流的可能影响.结果表明:当地时间17∶00城市热岛环流开始形成.随着热岛强度增强,环流增强,城乡边界处的城市风锋不断向城市中心推进.19∶00热岛环流结构最显著.21∶00环流结构被破坏,仅低层存在微弱的乡村风.在热岛环流结构最显著时刻,近地面风场由郊区向城市辐合,地面以上2.0 km处风场由城市向乡村辐散,辐散中心与辐合中心位置大致对应.此时城市风锋在城市处合并,环流的水平尺度约为城市尺度的2~2.5倍.当气溶胶光学厚度减小后,城市热岛环流尺度和强度以减小为主.特别是在热岛环流最显著时刻,低层乡村风风速减小,城市风锋最大上升速度降低,环流的水平尺度在西、南、北三个方向均减小,且高空回流高度降低.可能的影响机制是,气溶胶光学厚度减小后,净辐射通量增大,城乡地表能量通量差异增大,城市边界层高度升高.但城市边界层高度升高对城市热岛的抑制作用超过了城乡地表能量通量差异增加对城市热岛的增强作用,最终造成城市热岛减弱,环流减弱.

To investigate the structure and evolution of urban breeze circulation,WRF V3.9 is used to simulate a typical urban breeze circulation in a heavy pollution event on July 16,2016 in Chengdu city.In addition,the influence of atmospheric pollution on urban breeze circulation is quantified by a sensitivity test.The results show that from 17∶00 to 21∶00,the circulation develops as the intensity of the heat island increases and reaches the maximum at 19∶00.It is destroyed at 21∶00 only with weak country wind in the lower layer.At 19∶00,the wind at the surface converges to the city center and diverges from city to country at 2.0 km above ground level.The horizontal scale of the circulation is about 2~2.5 times that of the urban scale.When the aerosol optical depth is reduced,the intensity and scale of urban heat island circulation decrease,especially at 19∶00.The possible mechanism is that the reduction of the aerosol optical depth can change the radiation and energy balance at the surface.Then the net radiation flux increases,mainly resulting in the increase of the difference in the surface energy balance between urban and rural areas and the height of the urban boundary layer.However,the suppression of less stable city atmosphere on urban heat island exceeds the enhancement of increased city country energy difference on it.Finally,the intensity of the urban heat island is reduced,and the urban heat island circulation intensity is weakened.