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

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 similar to 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.