A Numerical Study Of Urban Breeze Circulation In Chengdu

Urban breeze circulation is a local circulation which is caused by urban heat island in the urban boundary layer.It can affect the diffusion of pollutants in urban areas and urban precipitation,so it is the key of urban meteorology research.In this paper,the mesoscale numerical model WRF V3.9 is used to simulate a tipical urban breeze circulation case in Chengdu on July 16,2016.The horizontal,vertical structure and evolution characteristics of urban breeze circulation are discussed in detail.Besides,the possible reasons for the generation and evolution of urban breeze circulation are discussed from the aspects of surface energy balance and stability.Finally,a sensitivity test on reducing aerosol optical depth is designed to quantify the influence of atmospheric pollution on the circulation,and the possible mechanism that why atmospheric pollution affects the circulation intensity and scale is discussed.The main conclusions are as follows:?1?In this case,the urban breeze circulation mainly occurs from 17:00 to 21:00 and reaches the strongest at 19:00.At 19:00,the wind near the surface converges to?104oE,30.68oN?,with a wind speed of 4⁵ m·s^-1 and diverges from city to country at 2.0 km above ground level with a wind speed of 3⁴ m·s^-1.At this time,the urban breeze front merges in city with a maximum rise speed of 1.0 m·s^-1.The horizontal scale of the circulation is about0.4⁰.5 latitude,which is about 2².5 times of that of the city and the vertical thickness is up to 2 km.?2?The urban breeze circulation is mainly determined by the urban heat island intensity,urban atmospheric stability and background wind intensity.17:00¹9:00,it is mainly affected by urban heat island,and 19:00²1:00,it is mainly affected by atmospheric stability.?3?When the aerosol optical depth decreases,the intensity of urban heat island circulation decreases as a whole,and the difference is greatest at 19:00.At this time,the wind speed at low level decreases by about 1 m·s^-1,which causes convergence center change from?104.0oE,30.68oN?to?103.98oE,30.68oN?.The horizontal scale of the circulation decreased by about 6 km,11 km,3 km,and the height of the upper circulation decreased by0.1 km,0.2 km,0.3 km,respectively,in the west branch,the south branch and the north branch.The maximum rising speed of urban breeze front on the west side and south side decreases by about 0.1⁰.2 m·s^-1.?4?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 result 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 beyond the enhancement of increased city country energy difference on it.Then,the intensity of the urban heat island is reduced by 0.3 degrees at 19:00.Finally,the intensity of the urban heat island is reduced,the urban heat island circulation intensity is weakened.