Analysis of Meteorological Elements in Rain/Snow-Mixed Fogs

Data analysis from the multi-channel microwave radiometer, boundary layer wind profiler, and auto/artificial weather station observation in Beijing region, displays those changes of surface and upper-level meteorological elements accompanied with raining or snowing processes during the formation and maintenance of two rain/snow mixed events that occurred in the autumn and the winter of 2007, respectively. The results indicate that high humidity and light wind are the main meteorological conditions in favor of fog occurrence and causing low visibility too. During the fog formation and maintenance there is a distinct inverse correlation between the horizontal visibility and surface relative humidity. The lower the visibility is, the more abundant liquid water the fog contains. Lighter rain or snow could diminish the water content and then increase the visibility within the fog. On the other hand, rain or snow evaporates and subsequently the humidification will further benefit the maintenance of the fog. In the rain-mixed fog, a significant increase/decline of the upper air temperature during the rain process is related to the released latent heat of water vapor condensation and substantial increase of water content besides the possible influence of weather systems. While in the snow-mixed fog, the upper air temperature generally declines with the time in a quite small change range. A vapor-saturated layer during the raining or snowing processes is always observed in the low layer in both the fog cases. As precipitating gets stronger, the top of the saturated layer rises higher and the layer also gets thicker, with surface horizontal visibility reduced. In regard to alike mechanism between fog dispersal and precipitation stimulation, these cases results reveal that weaker rain or snow processes have only limited effects on the dissipation of warm fogs or cold fogs, in another word, it is less conducive to increasing the surface horizontal visibility. The analysis offers some enlightenment on the effects and techniques of fog dispersal with seeding agent.