长江中上游冬季山地雾边界层特征及生消过程分析

Boundary layer characteristics and formation processes of winter valley fog in the upper and middle reaches of the Yangtze River

利用2010年12月在湖北宣恩观测山地雾获得的边界层廓线、雾滴谱、能见度资料,分析了该地雾过程的边界层特征及其生消过程。结果表明：1）宣恩山地雾主要由夜间辐射冷却引起,且能见度多在200 m以上;垂直发展深厚,成熟时厚度达到400~600 m。2）夜间风场主要由山风环流控制,风向多为东南风;入夜及雾生前期,地面风速不超过0.5 m/s,雾消前增大至2.0 m/s左右。3）雾生前观测到＂C＂字型温度层结,中下层气温降温率在0.3~1.0℃/（100 m）之间;结合该时段近地层露点温度逆温,离地200 m左右率先饱和成云;雾消时低空相对湿度依然保持较大值,重新变为空中雾层。4）雾前1~2 h地面及植被表明温度显著上升,个别升温率达1℃/h,对应时段地面相对湿度达到饱和,与其他地区有明显区别,对预报宣恩山地雾有积极意义。

Valley fog in China has barely been studied. To improve our understanding of its macro and microphysics in the upper and middle reaches of the Yangtze River,a comprehensive fog experiment was conducted in Xuanen during 6—31 December 2010 with a tethered balloon system,fog droplet spectrometer,and visibility meter. Based on the data of the boundary layer profile,fog droplet spectrum,and visibility over the Xuanen Mountains of Hubei Province,six fog events are discussed in terms of their boundary layer structures and development processes. The results showthat fog events in the Xuanen Mountains are mainly formed by radiation cooling at night and the visibility is usually more than 200 m. The fog-layer top is relatively uniform at 400- 600 m above ground level. Xuanen Station is located at the bottom of the valley near the south hill,and the surface wind at night in Xuanen is mainly controlled by downslope wind with a southeasterly direction. The surface wind speed is no more than 0. 5m ·s- 1in the early stage of fog formation,and increases to 2. 0 m ·s- 1in the dissipation stage. A C shape of the temperature profile in the boundary layer is observed before fog,and the rate of temperature decrease reaches 0. 3- 1. 0 ℃ /（ 100 m） in the middle and lowlevels. Upper-level inversion is formed due to the transportation of cold air from the surface by upward flowof the mountain wind cycle,but anticyclonic subsidence can also lead to its formation. With dew-point temperature inversion and an approaching isothermal temperature distribution at lowlevels,the air at a height of 200 m reaches saturation first. Dewdeposition is observed on the grass in the early evening on clear nights with lowwind speed,and this creates the low-level dew-point inversion. The bottom of low-level cloud is raised by upward flowin the early evening. When lowcloud appears,it will absorb the upward longwave radiation from the surface and launch downward longwave radiation,with upward soil heat flux and upward latent heat flux by dewdeposition,and the layer beneath the lowcloud develops an unstable lapse rate. The evaporation of large droplets falling from the lowcloud,and surface dew,which provides water vapor,causes fog droplets to form at ground level. The relative humidity remains high at lowlevels during the dissipation stage,which will form rising fog. Fog lifting is also found at midnight,when it cannot be caused by solar radiation. A steeper than moist adiabatic lapse rate beneath the persisting part of the fog is found during these fog events. Upward flowdue to the unstable layer near the surface and raised upslope wind（ northeasterly） may lead to fog aloft. Finally,it is important to note that,at 1- 2 h before fog occurrence,the surface temperature and vegetation temperature rise significantly,even 1. 0 ℃ h- 1in some cases,with saturated relative humidity at the surface,which are quite different conditions to those of other regions,and thus has positive implications for fog forecasting in this region.