Abstract:A heavy fog event occurred in Nanjing area on 18—19 December 2007,lasting for 20 h,in which almost 1/3 time period was occupied by the super dense fog with visibility lower than 50 m.By using the field observational data in the north suburb of Nanjing during the fog event and NCEP reanalysis data,the evolution process,microphysical structure and boundary layer characteristics in the fog event are analyzed,and the causes of burst reinforcement of the ground fog are also discussed.Results show that the formation of fog is due to increasing moisture by southwest air current.Sufficient vapor is supplied by the advection transport and surface evaporation,and the inversion near bottom atmosphere uplifts and exists stably owing to warming effect of upper-layer air sinking,therefore the heavy fog can persist after sunrise.In the whole fog event,the temporal evolutions of fog droplet number concentration,average diameter and liquid water content are in agreement with each other,the average droplet spectrums all obey exponential decreased distribution,and fog droplets mainly concentrat in the section of smaller diameter.The two burst reinforcements of ground fog occur at nighttime,with all microphysical parameters increasing obviously and the droplet spectrum uplifting and broader.The burst reinforcements are mainly caused by sharp decline of temperature near surface,enhancement of inversion near bottom atmosphere,and increase of large aerosol particles which may act as condensation nuclei of fog droplets.