中文摘要：

地面倒槽、华北地形槽和地面弱高压是天津冬季雾日多见的地面气压系统。为了解雾事件在上述三种天气系统下近地面层气象要素的演变规律,利用天津市250m气象铁塔梯度观测和常规气象观测资料,分别选取2002、2003和2004年相应气压场下的雾个例,比较分析了冬季雾天近地面层结构及低层水汽分布特征。结果表明：（1）三种天气系统条件下,均存在近地层逆位温层结和增湿现象,近地面40m以下高度为弱风。（2）地面倒槽形势下的平流雾过程中,逆温层结稳定且厚达千米,近地层呈多层逆温或弱逆位温层结;80m以上,雾前风力较强,雾中风力较弱;低空各层水汽显著上升时间提前于起雾时间约15h,且日夜增速持续均匀,雾中呈现出逆湿特征,雾顶超过250m。（3）华北地形槽和地面弱高压下的辐射雾过程中,日落后近地面浅薄逆温层结生成并于05时（北京时间,下同）左右达最强,日出后减弱,于11时左右消散;仅夜间近地层水汽显著增加,且塔层250m逆温强度达到3.0℃时才开始出现,距起雾时间约2～9h;雾形成后,逆温层底抬升,雾体中逐渐演变呈不稳定层结;雾中呈现下湿上干特征,雾厚分别为80m和60m左右。（4）华北地形槽和地面弱高压下的风廓线演变规律有显著差异,即前者80m以上6m.s-1左右南风和北风呈规律性日变化转换特征,而后者250m低层大气恒为弱风控制。

英文摘要：

Based on the observational data obtained from the 250-m meteorological tower in the urban area of Tianjin, together with the conventional observational data, the authors analyzed the structures of the surface layer and the characteristics of the specific humidity distribution in the lower atmosphere layer during fog events, under three different surface pressure patterns （surface inverted trough, topographic trough in North China, and weak surface high pressure pattern）in the winters of 2002, 2003,and 2004. The results show that light winds prevail in the surface layer beneath 40 m, and the surface layer is featured by that temperature inversion and specific humidity increase in fog days under the three different patterns. However, some specific characteristics of the meteorological variables distribution are revealed as follows：（1）In the process of advective fog, under the surface inverted trough pattern, a thick and steady temperature inversion layer up to 1000 m exists in the boundary layer, while multilayer or weak inversion layer in the surface layer, strong wind prevails before fog formation and slows down in the fog episodes above 80-m height. The increase of specific humidity starts 15 h before fog formation under the surface inverted trough pattern, with a constant increasing during night and the daytime. A water vapor inversion appears during the fog episode. The height of fog top exceeds 250 m. （2）During radiative fog events, a diurnal variation of the inversion is obvious, with nighttime formation and daytime dissipation, under the other patterns. The vapor inversion is strong with shallow thickness at night. The vapor increase is only remarkable in the surface layer, and vapor usually starts to increase, when the intensity of temperature inversion reaches 3.0 ℃ at 250-m height, which is about 2-9 h prior to the fog formation. An unstable layer develops after the fog formation, with humid in the lower layer while dry in the upper layer. The values of fog thickness are 80 m and 60 m, re