| As a mesoscale circulation,the sea-land wind circulation significantly influences the weather conditions in coastal areas and has an important impact on the transport of atmospheric pollutants.The frequent occurrence of sea-land breezes in Shanghai and the severe local air pollution in the eastern coastal region make the study of sea-land breezes in this region particularly important.In recent years,the continuous development of mesoscale numerical simulation studies has greatly facilitated the study of sea-land breezes.However,due to the inability of conventional methods to decouple the various factors affecting meteorological data and the low grid resolution,it is not possible to study the influence of a single factor by means of controlled variables and to conduct microscale studies of the near-surface air environment,which affects the respiratory environment of people.In this paper,we use the Fluent programme of the Computational Fluid Dynamics(CFD)method to develop a two-dimensional model of wind flow and heat transfer in the sea-land wind field and to study the effect of sea-land breezes on pollutant transport by setting up a pulsating pollution source.This paper uses the equivalent method to characterise the effects of solar radiation and longwave radiation on near-surface air heat,and the simulation results are in good agreement with the fifth-generation reanalysis data from the European Centre for Medium-Range Weather Forecasts.This paper sets up three different solar radiation intensities for summer and autumn,producing different patterns of temperature change and thus creating three different characteristics of the sealand breeze circulation.From the simulation results,the spatial and temporal variation characteristics of air temperature under different solar radiation intensities in summer and autumn,the variation characteristics of air velocity of sea-land breeze,the spatial and temporal relationship between sea-land breeze and atmospheric pollutants,and the comparison of sea-land breeze characteristics between different seasons were analysed.The simulation results show that there is no significant correlation between the sea-land temperature difference on sea-land breeze days,the cooling effect of the sea breeze on coastal areas and the onset and duration of the sea-land breeze with the solar radiation intensity.In contrast,the maximum extension distance,the mean and maximum wind speed and the maximum thickness of the sea and land breeze in different seasons are all positively correlated with the intensity of solar radiation.The thickness of the sea-land breeze has the same characteristics of variation from season to season.If pollutants are released before the sea breeze front reaches the source,the near surface area remains clean due to the combined action of the thermal plume and the sea breeze front;if pollutants are released after the sea breeze front reaches the source,the near surface area is heavily polluted.Pollutants that would otherwise disperse to higher altitudes may also be brought back to near-surface areas by sea breeze fronts and thermal plumes,causing secondary pollution.Combining the dispersion effects of pollutants released at different times of the summer and autumn shows that even though some of the pollutants released during the land breeze period may be blown back to the land side the following day,they cause less impact near the ground compared to when they are released during the sea breeze period.Near-surface air pollution levels are negatively correlated with the intensity of solar radiation and distance from the coastline.Pollutants are more concentrated near the coastline and in the height range of the near-surface layer in autumn than in summer.Near-surface air pollution levels are negatively correlated with the intensity of solar radiation and distance from the coastline.Pollutants are more concentrated in areas near the coastline and in the height range of the near-surface layer in autumn than in summer. |
