安徽省浓雾天气时空特征及边界层影响机制OA

Dense fog poses significant challenges to transportation safety and operational efficiency,particularly in Anhui Province,where complex topography,diverse climatic conditions,and frequent localized fog events con-tribute to a high occurrence of dense fog.However,the boundary-layer structure and physical mechanisms under-lying dense fog formation in this region remain insufficiently understood.Using observations from the national me-teorological station network in Anhui Province and ERA5 reanalysis data for the period 2016-2023,this study investigates the spatiotemporal characteristics of dense fog and examines the roles of surface meteorological condi-tions and boundary-layer processes in its formation and maintenance.Temporal analysis shows that dense fog oc-curs most frequently from late autumn to early spring,with the highest frequency in January and the lowest in Au-gust.On a diurnal scale,dense fog most commonly develops between 02:00 BST and 08:00 BST,reaching its highest occurrence frequency at 05:00 BST,while no dense fog events were observed at 15:00 BST.Among the four seasons,winter is characterized by the earliest onset,latest dissipation,and longest duration of dense fog,while summer exhibits the opposite characteristics,with spring and autumn showing intermediate behavior.Spatial analysis reveals pronounced regional heterogeneity.During spring,autumn,and winter,dense fog occurs most fre-quently in areas along and north of the Huaihe River and throughout much of the region south of the Yangtze.In summer,dense fog is largely confined to localized areas within the Dabie Mountains and the mountainous region of southern Anhui.During winter,the mean duration of dense fog reaches approximately 4.9 h in areas along and north of the Huaihe River,with minimum visibility failing below 150 m in some locations.In contrast,most areas experience dense fog lasting less than 3 h during summer,with minimum visibility generally exceeding 230 m.Spring and autumn exhibit intermediate characteristics.The key surface meteorological factors associated with win-ter dense fog are relative humidity,nocturnal temperature decrease,and surface wind speed at 05:00 BST.Regions characterized by greater nocturnal cooling and weaker surface winds generally correspond to areas with more fre-quent and long-lasting dense fog.A significant negative correlation is identified between the nocturnal temperature decrease and the initial relative humidity at 16:00 BST on the previous day.Specifically,when the initial relative humidity ranges from approximately 55%to 80%,nocturnal cooling of 5-11.5℃is typically required for dense fog formation.The vertical structure of the boundary layer structure is further examined using observations from four representative stations by comparing dense fog days with fog-free days.From a dynamic perspective,wind speeds throughout the boundary layer are consistently lower during dense fog events,indicating that weak horizon-tal ventilation favors fog formation.The largest differences in wind speed occur within the 950-900 hPa,with Huangshan station exhibiting the greatest reduction at 900 hPa,likely owing to terrain-induced effects.Local verti-cal wind shear(VWS)in both the upper(875-700 hPa)and lower(1 000-900 hPa)boundary layers is wea-ker on dense fog days,with the most pronounced differences occurring in the lower boundary layer.These results suggest that suppressed vertical mixing near the surface plays a direct role in promoting fog formation.Thermody-namically,weak vertical temperature gradients produce temperature inversion layers at 1 000-975 hPa over Suzhou,Shouxian,and Hefei stations and below 875 hPa over Huangshan station.These inversion layers inhibit vertical convection and suppress upward moisture transport.Additionally,dense fog events are characterized by a distinct"moist-below,dry-above"vertical humidity structure,with a transition near 800 hPa.The relatively dry upper layer enhances nocturnal radiative cooling of the surface,while the moist lower layer promotes water vapor condensation and fog droplet growth.These results demonstrate that the formation and persistence of dense fog in Anhui Province result from the combined effects of radiative cooling,weak horizontal and vertical turbulent mix-ing,and a favorable vertical humidity structure within the boundary layer.The identified physical mechanisms pro-vide a sound basis for understanding the pronounced regional and seasonal variability of dense fog occurrence and may contribute to improved forecasting and risk assessment of dense fog events.