2016年冬季江苏省一次大范围强浓雾天气过程成因分析

Causes analysis on a large-range dense fog event in the winter of 2016 over Jiangsu province

基于2016年2月11日夜间至12日白天江苏省一次全省范围的强浓雾天气过程,利用地面常规气象观测资料、NCEP再分析资料和NCEP通量资料对此次强浓雾天气过程发生前后的能见度、风场、温度、湿度、探空数据及感热通量等进行了详细分析。结果表明:2016年2月11日白天江苏地区的降水为强雾天气过程的发生提供了水汽条件,逆温层的建立和合适的近地面风速及垂直风切变均有利于雾天气的形成;空气相对湿度增大先于雾天气的发生,对雾的预报具有一定的指示作用,可将相对湿度≥95.0%作为冬季雾形成的阈值。此次雾天气持续过程中江苏地区部分气象站的温度在夜间小幅上升,可能与水汽凝结时释放潜热有关;地气温差负值和地面感热通量负值均表明夜间地面对空气的冷却作用。从雾的成因来说,夜间晴空辐射降温是江苏地区此次大范围强浓雾天气过程形成的直接原因,而由于平流作用形成的逆温层和逆湿层是雾提前加强与维持的有利因素。对江苏省大部地区而言,此次强浓雾天气过程应为辐射雾;从雾预报的角度考虑,需密切关注逆温层是否建立并维持,当白天有降水时,还需注意夜间低云量的变化。

Conventional ground-level meteorological observation data,NCEP （ National Centers for Environmental Prediction） reanalysis and flux data were used to analyze the variations of visibility,wind,temperature,relative hu-midity,sounding and sensible heat flux during the whole process of a large-range dense fog during February 11-12, 2016 over Jiangsu province. The results show that the precipitation on February 11 over Jiangsu area supplied suffi-cient vapor for the fog formation,and the temperature inversion layers,suitable surface wind speeds,and vertical wind shears favor the fog formation as well. Increase in humidity before the fog formation had a certain indication for the fog forecast,with a threshold of above 95% in winter. The temperatures at some stations had a slight in-crease at night in the event,which may be related to latent heat release due to vapor condensation. Ground-air tem-perature differences and sensible heat flux both appear negative,suggesting the ground cooling effect on the air. Clear sky radiative cooling at night was a direct cause of the event,whereas the temperature and humidity inversion layers caused by advection favored the advanced formation and maintaining of the fog. As far as most areas of Jiangsu province is concerned,this dense fog event was of radiation fog. From a forecast point of view,it is neces-sary to pay close attention to the establishment and maintaining of temperature inversion layer,and variations of the lower-level cloud cover at night ought to be considered as well if it rains during the daytime.