Analyses on Boundary Layer Structure of a Frontal Heavy Fog Process in Nanling Mountain Area

Using the field observational data in Nanling mountain area in the first nine-day periods of March 2001,the structural characteristics of wind,temperature and humidity and weather pattern in the periods of fog-free and heavy fog have been analysed.The analyses of sounding reveal that the surface fog is essential the saturation area extending into the surface during the cold air outbreak,and the fog form when the cloud base reach the surface ground.The dissipation process of surface fog is virtual a process of the lifting cloud base from the surface ground.The construction of boundary layer is controlled by the influencing weather systems.The construction of frontal inversion play a major role in the sustaining of cloud system or fog,the mono-layer strong inversion construction is favourable to the development of fog,while the multi-or dual-layer weak inversion constructions may lead to the dissipation of fog.The occurrence of fog closely relate to strong south wind at low-level,southwest or west southwest wind at upper air and the warming before the occurrence of fog,resulting in the high humidity of the whole layer.The observation also show that when there is light to moderate precipitation during the fog process,the visibility get better with the phase of the peak precipitation rate,the oscillations of weak and strong rain result in the oscillations of bad and better visibility,showing the close relationship between microphysics processes and visibility inside the body of fog.