南岭大瑶山高速公路浓雾的宏微观结构与能见度研究

A STUDY ON MACRO-AND MICRO-STRUCTURES OF HEAVY FOG AND VISIBILITY AT FREEWAY IN THE NANLING DAYAOSHAN MOUNTAIN

南岭山地地处南亚热带湿润型季风气候区,每年9月至次年5月有华南准静止锋活动时均会有浓雾发生,每月浓雾日可高达15—18 d,尤其是中国目前最长的京珠高速公路通过南岭主脉大瑶山的乐昌—乳源段,路面海拔高度从200 m增至800多米,山地的抬升使雾害更加严重。在南岭大瑶山高速公路开展的两次多学科综合野外观测,内容包括目测能见度、器测能见度、雾滴谱、雾含水量、系留探空、双参数低空探空、雾层湍流扩散、气溶胶粒子谱、气溶胶成分谱、雾水样品成分、雨水样品成分。对典型个例进行了天气学分析,雾的宏观结构特征分析,雾的微物理特征分析。认识到南岭山地浓雾发生频率高,雾害十分严重,是典型的平流雾和上坡雾,实质上是出现在相对较高海拔高度上的低云,与华南锋面活动尤其是华南准静止峰的活动密切相关,局地地形的作用也非常重要。其特点是浓雾持续时间长、能见度极其恶劣、团块结构明显、雾滴尺度大、浓度不高、含水量较大、雾层内的湍流扩散能力比晴空区强,与中国过去研究较多的辐射雾差别较大。发现雾含水量与能见度呈明显的反相关关系,含水量较大时能见度较小;南岭山地雾含水量等微结构特征量的起伏变化,除与雾体本身的结构不均匀有关外,一个重要的原因是平流因素的影响,南岭山地下垫面的不均匀性,雾体随环境风的平移过程中,不规则的爬坡、翻越山坡的运动是造成雾体结构不均匀、振荡起伏变化的另一个重要原因。该地气溶胶粒子谱是呈单调下降的幂函数谱,次微米粒子浓度甚高,南岭山地气溶胶中含有高浓度的硫酸盐粒子,是优质凝结核,有利于雾的形成。雾的存在可以清除大气中的微量成分,雾滴可以包含浓度很高的污染物成分。同时,较之云滴而言,雾滴也很容易被地表物体(如植被、建筑物等)的垂直表面所截获,构成另一类清除过程。在南岭这样的大面积森林地区,这类清除过程可能是很重要的。研究本地区雾的特征变化,对建立本地区雾的预警预报系统有很大的现实意义,并为开展消雾试验提供了基本资料。

Belonging to the southern subtropical moist type of monsoon climate, the Nanling mountainous area experiences heavy fogs whenever quasi-stationary fronts appear from September to next May, with heavy fog-day as many as 15 - 18 days per month. In particular, Fogs become more severe in the Lechang-Ruyuan section of the Beijing-Zhuhai Expressway （the longest of its kind in China） which passes through the main part of the Nanling Mrs, where the road elevates from 200 m to more than 800 m a. s. 1. Two multidisciplinary integrated field observations were carried out at the freeway located in the Nanling Dayaoshan Mt to measure visibility by both the bare eye and instrument, spectrum of fog droplet, water content of fog, T and H profiles using tethered sounding and dual-parameter low-level sounding respectively, turbulence dispersion parameters within fog layers, aerosol size and component spectra, fog water compositions, and rainwater compositions. Typical cases of fog processes were analyzed for their synoptic, macro structures and microphysics. Analyses results show that heavy fogs occur at high frequency in the freeway and bring about serious consequences to the traffic. Being typical advection and upslope fogs, they are in essence low clouds appearing at high altitude area, and closely related to the activity of fronts, especially, of quasi-stationary fronts over South China, as well as the local terrain. The heavy fogs are characteristic of long duration, extremely low visibility, well-organized lump structure, large droplet size, medium number concentration, high water content and stronger turbulent dissipation within the fog layers than outside. Those characteristics are much different from radiation fogs that have been previously investigated more in China. It is found that moisture content of fog is significantly inversely proportional to visibility so that large moisture content is always associated with small visual range. It is also noted that one of the reasons for the fluctuation of characteristic quantities of micro structure such as the moisture content of fog in the area is, in addition to the inhomogeneous structure of the fog itself, the effect of fog advection on inhomogeneous underlying surface; during the translation of fog with the ambient wind, irregular uphill and cross-over movement is another reason for the inhomogeneous structure and oscillation of fog. The spectrum of the aerosol particles displays itself as a power function of monotonous descent. The concentration of submicrometer particles is even higher. The high-concentration sulfate particles found in the aerosols of the Nanling Mts are actually good nuclei for condensation, which are favorable for the formation of fog. The presence of fog can help cleanse the trace compositions in the atmosphere since fog droplets contain high levels of polluting elements. In the meantime, compared to cloud droplets, fog droplets are easier to be captured by the vertical surfaces of objects on the land surface, such as vegetation and buildings, which constitutes another kind of cleansing process. In vast stretches of forests like the Nanling Mts, this kind of cleansing may be quite important. Studying the characteristic variation of fogs in the area actually assists in setting up a pre-warning system for local fogs and provides basic information for fog dispersal experiments.