Microphysical Structure and Evolution of a Four-Day Persistent Fog Event in Nanjing in December 2006

A persistent thick fog event occurred in Nanjing during 24-27 December 2006, and the bad visibility below 50 m lasted for more than 40 h. Microphysical characters and evolution of the fog event have been analyzed based on the continually observed data of drop-size distribution, number concentration, liquid water content (LWC), etc., by an FM-100 fog particle spectrometer, as well as routine observations by an auto-weather station and a ZQZ-DN visibility meter during the fog episode. The results were compared with those from another persistent fog event in Nanjing in 1996. The average LWC and droplet diameter in this fog event are equivalent to those in the 1996 fog, but the maximum LWC is four times greater and the droplet number concentration is smaller than that in the 1996 case. According to the evolutional characters of microphysical parameters (MPPs), the entire fog process was divided into four sub-processes. It is believed that the high number concentration and LWC in the first and second sub-processes led to the bad weather with a visibility of less than 50 m for such a long-duration. In combination with the planetary boundary layer sounding data observed with a DigiCORA III tethersonde system, it is inferred that the reason for the thick fog is the existence of strong moisture advection in the surface layer, and therefore the fog possessed the characters of typical advection fogs. The drop-size distributions in each of the four sub-processes and for the whole fog episode all obeyed the Deirmendjian distribution. The number concentration declined exponentially with the increase in droplet diameter, and fog droplets mainly concentrated in the section of smaller diameter. At last, the fog MPPs in the first sub-process were analyzed in detail. It is found that the fog formed after radiation cooling under clear sky at the night of December 24, reached its peak intensity at mid night, and weakened gradually along with a temperature rise after the sunrise the next day (December 25), which reflects the diurnal cycle character of radiation fogs. In addition, it is also found that the fog did not change greatly soon after its formation, but along with further radiation cooling, the fog masses successively formed on the ground surface, then the fog explosively developed.