湘潭市大雾的时空分布特征及其发生的天气形势分类

利用1959—2012年湘潭3个观测站资料,采用线性倾向估计、普查分析等方法,研究了湘潭市大雾的时空分布特征并将大雾发生的天气形势进行分类。结果表明:全市大雾年平均日数为18.9天,辖区内湘乡最多(25.7天),韶山最少(11.8天)。所辖3站的大雾日数整体均呈逐年增加的趋势,变化速率在1.13天/10a~2.42天/10a,1977—2002年为大雾多发时期;20世纪90年代以前每年代以4~6天的趋势递增,进入21世纪后年大雾日数呈下降趋势。湘潭连续大雾天气过程最长持续时间为12 d,80%的连续大雾为2 d。低空有下沉逆温、地面有辐射逆温,是连续大雾产生的重要条件。形成大雾的500 h Pa形势场特征主要有NW型(槽后西北气流型),SW—NW型(低槽过境型),W型(持续平直气流型),SW型(槽前西南气流型),NW—W型(由脊转槽型)等5种;地面形势主要有两种类型:一种是地面上受小高压控制或处于均匀场中,另一种为地面高压底后部、低压倒槽中或弱冷锋前。

Temporal and Spatial Distribution and Weather Situation Typing of Heavy Fog in Xiangtan City

Base on the observation data in 3 stations from 1959 to 2012 in Xiangtan city, the temporal and spatial distributions of the heavy fog were analyzed by methods of linear trend estimate and survey analysis. The results show that the annual average days number of heavy fog is 18.9 d in Xiangtan. The most heavy fog days number in Xiangxiang station is 25.7 d, the least in Shaoshan station is 11.8 d. The heavy fog days number shows an increasing trend with the change rate between 1.13 day per decade and 2.42 day per decade, and concentrated period of the heavy fog is from 1977 to 2002. The heavy fog days number is in increasing trend with 46 days per year before the 1990s, while the heavy fog days number is in decreasing trend after the 21st century. The longest duration of sustained fog is 12 days in Xiangtan city and the 80% is about 2 days. Low subsidence inversion and ground radiation inversion are important conditions of sustained heavy fog. According to the 500 hPa circulation pattern, the circulation patterns of heavy fog are categorized into 5 types such as NW type, SW-NW type, W type, SW type, NW-W type. The surface synoptic system are mainly two types. One is controlled by the small high pressure or in a uniform pressure field, the other is the bottom back of the surface high pressure and the depression inverted through or the front of the weak cold.