Microphysical structure and evolution of four-day persistent fogs around Nanjing in December 2006

Persistent heavy fogs occurred in Nanjing during 24-27 December 2006, and the super dense fog with a visibility below 50-meter persisted for more than 40 hours. Many studies have been done based on the observation data of the fog process from automatic weather station, ZQZ-DN visibility meter and FM-100 fog particle spectrometer. The microphysical characteristics of the persistent fogs were analyzed based on the continual fog droplet size, number concentration, liquid water content (LWC) data as well as the temperature, humidity, and visibility observations, and compare the results of the microphysical parameters with those of fogs in Nanjing in 29-30 December 1996. The average LWC and diameter of the fogs are equivalent to those of 1996, yet the maximum LWC is four times and the number concentration is smaller than that of 1996, respectively. It is the high number concentration and LWC in the first and second processes that led to the bad weather of the long-duration super dense fogs in Nanjing. Combining with the data of tethersonde, it was found that the continuous strong vapor advection resulted in the persistent heavy fogs, and therefore it possessed the characters of advection fog. According to the variations of the micro-physical parameters, the whole process was divided into four fog processes, and the average droplet spectrum distributions of the four processes were analyzed and compared, and the results indicate that the average droplet spectrum distributions of both the whole process and the four processes obeyed the Deirmendjian distribution, i.e. the number concentration declined exponentially with the increase of droplet diameter and droplets with a smaller diameter dominated. At last, the micro-physical parameters of fogs in the first process were analyzed in more detail, it was found that the fog formed after the radiation cooling in the clear night sky (December 24), reached its peak intensity in the midnight and then weakened gradually along with temperature rise after the sunrise next day (December 25), reflecting the diurnal variation character of radiation fog. In addition，it was also found that the fog did not change much after its formation, but along with further radiation cooling, the fog masses formed incessantly on the ground, then the fog explosively developed into a super dense fog.