The boundary layer structure and the evolution mechanisms of a deep dense fog event

The field experiments were conducted during a deep dense fog event that occurred in Nanjing on 13-14 December 2007. This fog event persisted for 14 hours, including a 4hour super dense fog stage. The related boundary layer structure and physical mechanisms for the fog event were analyzed based on the field observations from tethersonde system, the eddy covariance system, the fog droplet spectrometer, and the automatic weather station, etc. Formed through radiative cooling, the surface fog layer was followed by a cloud layer caused by lowlevel cold advection. The thickness of the surface fog layer increased and the cloud layer descended to lower altitudes in the development stage. With the influence of weak cold advection just above ground, the surface fog layer entered its burst evolution with some microphysical parameters such as number concentration, liquid water content, mean and maximum diameter increased significantly in 15 min. The rising surface fog combined with descending lowlevel clouds to form the deep dense fog with the visibility less than 15 m and the thickness of the combined fog layer reached 600 m. Both the vertical momentum and downward longwave radiation fluxes increased significantly at the beginning of the burst stage, and the net radiation fluxes approached zero at the same time. A strong surface inversion layer persisted throughout the successive stages of fog evolution because of the continuous weak cold advection in the ground layer and the radiative cooling of the lower fog layer restrained by the upper fog layer.