The Micro-physical Characteristic Of Winter's Fog In Nanjing Area

An overall probing of fog was made in Nanjing Area in the winter of 2006 and 2007. Using the FM-100 Fog Monitor, which produced by American DMT corporation, we inspected fog spectrum, water content, droplet size distribution and other micro-physical parameters continuously. By use of fog droplet spectra data from 22 severe events of winter fog, summarized the fundamental characteristics of the fog micro-physics in Nanjing Area. Classed several types by the causations of fog formed and found out the average characteristics of different types, such as radiation fog, frontal fog, advection-radiation fog, Precipitation fog etc. , and they have obvious differences. Radiation fog have tow types of fog droplet spectrum, one type is broad-droplet spectrum fog and the other is narrow- droplet spectrum fog. The amounts of broad-droplet spectrum fog are more than that of narrow- droplet spectrum fog in small droplet segment as well as in large droplet segment, the line of broad- droplet spectrum is mostly above that of narrow-droplet spectrum.We found that: the visibilities are very low when in broad-droplet spectrum, and Sustained heavy fog which the visibilities below 15 meters occurred in all of these fogs, Sustained more than one hour. Narrow-droplet spectrum sustained in short times and the visibilities are mostly above 400 meters; The spectrum line of frontal fog and Precipitation fog are narrow; The broad-droplet spectrum fog and advection-radiation fog which continuance for long time are easily bring sustained heavy fog.We chose a typical advection-radiation fog occurred in Nanjing during 24-27 December 2006, and many studies have been done based on the data from auto-weather station, visibility monitor and FM-100 Fog Monitor, inspect fog spectrum, water content, droplet size distribution and other micro-physical parameters continuously. We contrast the result of the micro-physical parameters with that of 1996 : the average liquid water content and average diameter is correspond to that of 1996,yet the max- liquid water content is fourfold and concentration is smaller than that of 1996. It is believed that the high value of concentration and liquid water content in the first and second processes induced the long-time sustained heavy fog in Nanjing. Combining with the data of tethersonde , we considered that continuous subsisted vapor advection resulted in the sustained heavy fog, which has the characters of advection fog. According the varieties of the micro-physical parameters, we divided the whole process into four processes, analysed and compared the Averaged droplet spectra distribution of the four processes, Averaged droplet spectra distributions of the whole process and the four processes obey the Deirmendjian distributing ,which the lines are in exponential decline. At last, we analysed the micro-physical parameters of fog in the first process particularly, and found that the heavy fog formed after the radiation cooling in night, it was heavyest in the midnight and then weakened gradually alonging with temperature rise after sunrise, which reflected the day-variety of radiation fog. In addition, we also found that there was little variety after the fog formed, but according to the farther radiation cooling fog masses engendered incessantly on the ground, then the fog burst.Sustained heavy fog occurred in Nanjing during 24-27 December 2006, the data of fog droplet spectrum during this fog are analyzed by the Morlet wavelet transformation. It shows that there are not only daily variations,12.5 hours and 7 hours periodic variations, and 90-180minutes,40minutes and 10-20minutes quasi-periodic oscillation, which is related to fog microphysical process such as fog condensation,collection-sedimentation- condensation,collection and renlated to fog structure Asymmetry or advection, but 1～4 minutes,6seconds,10 seconds,14 seconds and 30 seconds micro-periodic oscillation, which is consider that evaporation and condensation are also important. It found that the LWC,average-diameter and concentration are in positive relationship, and think that the advection play an important fuction .