南京冬季浓雾的演变特征及爆发性增强研究

2007年12月18—19日，南京地区出现了一次持续20h的浓雾过程，其中能见度低于50m的强浓雾几乎占到整个雾过程的1／3。利用同期在南京市北郊的外场观测数据，结合NCEP再分析资料，分析了该次雾的演变过程、微物理结构及边界层特征，探讨了地面雾爆发性增强的成因。结果表明：本次雾在西南平流的增湿作用下触发生成；日出后，平流输送和地表蒸发提供了充足水汽来源，贴地层逆温因高空下沉增温而向上抬升且稳定存在，因此大雾得以维持；整个雾过程中雾滴数浓度、平均直径、含水量随时间的变化趋势基本一致，平均谱曲线均呈指数下降分布，雾滴集中在小滴端；两次地面雾爆发性增强均发生在夜间，其特征为各微物理参量明显增大，滴谱上抬拓宽；爆发性增强的原因是地表气温陡降、贴地层逆温增强及可充当雾滴凝结核的气溶胶大粒子数增多。

Evolution characteristics and burst reinforcement of winter dense fog in Nanjing

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.