长江中上游平原地区冬季雾观测分析

长江中上游年平均雾日数达到20~106 d,是我国主要雾区之一。利用2010年12月在江汉平原观测获得的边界层廓线和雾滴谱资料,重点分析了该地区冬季雾过程的边界层结构及其生消过程。结果表明:荆州冬季雾多出现在寒潮过境1~2 d后,多为平流辐射雾;雾顶发展是水汽在上层逆温下积累,并伴随200~300 m高度冷平流降温引起;近地层冷平流降温导致饱和水汽压减小,同时上层系统性下沉增温引起逆温增强,水汽积累促使强浓雾过程产生;低空急流促使外界偏干气流与雾体混合后雾滴蒸发,是该地区雾顶迅速下降的主要原因;平均数浓度为150~406个·cm-3,极大值达到1 983个·cm-3,平均液水含量为0.014~0.118 g·m-3,极值达到0.786 g·m-3,与南京和重庆强浓雾观测值相似,超过其他地区观测值。城市地区高气溶胶浓度,配合充足的水汽条件,使得荆州雾过程微物理参量数值较大,易出现能见度小于50 m,持续时间4~9 h的强浓雾过程。

Analysis on winter fog over the upper and middle reaches of the Yangtze River

The upper and middle reaches of the Yangtze River are one of the fog-prone areas in China, with 20-106 fog days each year. By using the data of boundary layer profiles and fog droplet spectra over Jianghan Plain in December, 2012, the boundary structure and generation and disappearance process of fog case in winter of the area were studied emphatically. Results show that the fog in Jinzhou in winter usually appeared 1-2 days after a cold wave passage and most of them are advection-radiation fog. The development of the fog top was due to the accumulation of moisture under upper-layer inversion as well as substantial temperature decrease at 200-300 m; the sharp strengthening of ground fog was caused mainly because of lower saturation vapor pressure in the near-surface layer under the influence of cold advection and because of the inversion enhancement owing to upper-layer systematic sinking motion and combined by the moisture accumulation under inversion. During the fog-top declining, the fog layer evaporated via mixing with dry air outside, which was caused by a low-level jet. The mean number concentration was 150-406 (cm³),and its maximum value was up to 1983 (cm³). The mean liquid water content was 0.014-0.118 g/m³, with a maximum value of 0.786 g/m³, similar to the observation values in Nanjing and Chongqing but larger than those in many other regions. High aerosol concentration and sufficient supply of moisture led to extremely dense fog (visibility less than 50 m) in this region.