Observation Study On Microphysical Processes And Causes Of Freezing Weather Over Yunnan-Guizhou Plateau

Freezing weather is an extremely disastrous weather in winter, the icing process which it results can cause huge socioeconomic loss and threaten human safety. To better understand the formation mechanism and study the various microphysical processes occur during freezing weather, the data of an optical disdrometer, a fog monitor, sounding and meteorological elements were collected during the freezing weather on January 2014 in Weining Guizhou. In this paper, we analyzed the synoptic background, meteorological conditions, sounding profile and microphysical characteristics of freezing precipitation and fog events, then compared with other regions of Southern China. The Results show that:(1) Freezing weather occurs with stationary front system, the activity of stationary front is a necessary condition for the mountain fog and freezing precipitation in the region.(2) During the freezing precipitation events, inversion appear in troposphere. The cloud thickness was thin and cloud base was low. The near-surface temperature was below 0℃, relative humidity was above 85%, and wind speed was low.(3) All freezing precipitations were freezing drizzle, average diameter was 0.37mm and average rain intensity was 0.02mm. The liquid water content was highly correlated with the number concentration. The fitting result of raindrop size distribution shows Г distribution is better than the M-P distribution. Compared to other regions of Southern China, Weining had smaller raindrops, weaker rain intensity and narrower spectra.(4) The freezing precipitation in Weining was formed through "supercooled warm rain" process, the Yunnan-Guizhou quasi-stationary front and the high altitude mountainous terrain were likely responsible for the droplets characteristics.(5) During two fog events a deep isothermal layer and inversion appear near and a moist layer(RH> 90%) exists near the surface. The air temperature rise slowly due to the prevailing southeast winds, relative humidity was above 95%, wind speed was low. Several fogs show diurnal variation. After analyzing the fog cases, the difference between air temperature and surface temperature was 3～6℃ while the temperature began to rise rapidly after precipitation. The differences indicates the evaporation process was affect fog formation.(6)The fog microphysical characteristics indicate:Precipitation fog parameters were significantly less than the mountain fog, precipitation inhibition process related. Compared to other regions of China Precipitation fog was considerable with same mountain area, but stronger than urban area, this might be due to the difference of pollutions。(7)During all fog events droplets spectra was narrow and dominate droplets were small droplets (D<10μm). Droplet diameter with peak concentration were less than 5μm, indicating that the formation of fog droplets should be mainly through coagulation activation growth.