Macro-and Micro-Structures of Heavy Fogs and Visibility in the Dayaoshan Expressway

Belonging to the southern subtropical moist type of monsoon climate, the Nanling mountainous area experiences heavy fogs whenever quasi-stationary fronts appear there from September to May. There can be as many as 15-18 days of heavy fogs per month. Fogs have more serious consequences in the Lechang-Ruyuan section of the Beijing-Zhuhai Expressway （the longest expressway in China） that passes through the main part of the Nanling Mts., where the road rises from 200 m to more than 800 m above sea level （ASL）. For a major motorway in the mountainous areas of Nanling Mts., two multidisciplinary integrated field observations were carried out, which measured visibility by the naked eyes, visibility by instrument, spectrum of fogdrops, liquid water content （LWC） of fog, tethered sounding, dual-parameter low-level sounding, turbulence diffusion within fog layers, aerosol spectra of size and composition, sampled fog water compositions, and sampled rainwater compositions. Typical cases were probed for their analyses of synoptics, micro- and macro-structures and microphysics. It is understood that heavy fogs take place with high frequency in the area and bring about serious consequences. Being typical advection and upslope fogs, they are in essence low-lying clouds appearing at high altitudes, which are closely related with the activity of South China frontal processes, especially the South China quasi-stationary fronts, and reflect on the role of local terrain as well. The heavy fogs are characteristic of long duration, extremely low visibility, well-organized lumpshaped structure, large-size fog-drops, moderate concentration, high LWC, and stronger turbulent diffusion within the fog layers than in fine sky. They differ much from radiation fogs, which are better documented in previous study in China. It is found that fog LWC is in significant anti-correlation with visibility so that large LWC is 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 LWC of fog in the area is, in addition to the inhomogeneous structure of the fog itself, the effect of advection and inhomogeneous underlying surface; during the translation of fog with the ambient wind, irregular upslope and cross-over movement is another reason for the inhomogeneous structure and fluctuation of fog. The spectrum of the aerosol size displays itself as the power function of monotonous descent. The concentration of submicrometer particles is even higher. The high-concentration sulfate particles found in the aerosols of 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 so that 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 to constitute another kind of cleansing process. In vast stretches of forest like the Nanling Mts., this kind of cleansing may be quite important. Studying the characteristic variation of fogs in the area realistically assists in setting up a forecast and warning system for local fogs and provides basic information for fog dispersal experiments.