| The sea and land terrain in the north of Shanghai is complex,sea land breeze often occur,and it is an area with heavy air pollution.Based on the finite volume method in CFD,a two-dimensional wind field flow and heat transfer model of sea land breeze is established,and the influence of sea land breeze on pollutant diffusion in this area is studied by numerical simulation method.The study area includes Shanghai suburb Chongming Island Qidong urban area and the North Branch waterway and South Branch waterway between the three areas.In this paper,the equivalent method is used to characterize the near ground air heat change caused by solar radiation and long wave radiation.The Boussinesq approximation is used for the density change of air fluid,and K is used for turbulence calculation-? Two equation model equation.The simulation results are in good agreement with ECMWF reanalysis data.The simulation results of sea land breeze field show that in the sea breeze stage,with the continuous heating of the near ground by solar radiation,there will be a free plume phenomenon.At the same time,the temperature of wide sea(South Branch Channel)and narrow sea(North Branch Channel)is significantly lower than that of land area,which meets the formation conditions of sea breeze.The occurrence time of wide sea breeze is about 1 hour earlier than that of narrow sea breeze.The wind speed of Kuanhai Shanghai land sea breeze is large,the maximum front velocity exceeds 5 m/ s,and the narrow sea Chongming sea breeze is weak,which gradually disappears after meeting with Kuanhai Chongming sea breeze at 17:00;In the land breeze stage,only the temperature in the wide sea area is slightly higher than that in other areas,and there is a temperature difference of about 1K with the near ground temperature of Shanghai land and Chongming area.There is land breeze on both sides of the wide sea,in which the land breeze from Shanghai land can cross the wide sea to Chongming,narrow sea and Qidong.There is no obvious temperature difference between the narrow sea area and the land on both sides,so there is no obvious land breeze image.The coupling results of sea land breeze and pollutants show that the effect of sea breeze on the diffusion and dilution of air pollutants depends on the temporal and spatial relationship between the sea breeze front and pollutants.When the pollutants are located in front of the front,they will be entrained by the sea breeze front and brought back to the upper air,and the sea breeze has a cleaning effect;When the pollutants are located behind the sea breeze front,the sea breeze behind the front will push the pollutants inland,resulting in persistent pollution;Land breeze can always blow pollutants away from the land area and play a certain cleaning role,but its final effect is related to the release period of pollutants.For the pollutants released in the early morning,the gradually declining land breeze can only blow the pollutants to the offshore area,and then if there is a sea breeze,it will blow the pollutants back to the land,causing secondary pollution.For the pollutants released in the early morning,the land breeze can blow most of the pollutants away from the land,and the land air quality is optimized.However,in the northern part of Shanghai,because the land breeze from Shanghai can cross the wide sea to Chongming,the land breeze will not only optimize the air quality of Shanghai land area,but also deteriorate the air quality of Chongming area. |
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