Abstract:

 Using surface observed data, upper sounding data, NCEP reanalysis data, wind profile radar data and meteorological station data along the motorway in Wuhu, the circulation and physical conditions about formation and dissipation of a fog process on March 6, 2012 along the eastern Yangtze river in Anhui province were analyzed. The results show that this spring fog process belongs to radiative fog. There are mainly westerly to southwesterly winds over the fog area when a heavy fog occurs, without significant cold air. It is controlled by the equal high pressure field in Anhui province, so it is beneficial to the formation and maintenance of fog. The physical conditions about formation and dissipation of the fog show that enough water vapor and moisture condensation caused by long wave radiative cooling are important reasons of the heavy fog formation. The inversion layer formed by radiative cooling near the ground benefits the maintenance of fog, and the fog becomes thicken and develops with rising of the inversion layer near the ground. Stability stratification formed by the inversion layer of low level prevents moisture transport upwards. Low wind speed near the surface, weak vertical shear of wind in middle and low atmosphere, turbulence in low layer, no obviously ascending motion in middle atmosphere, these all constitute the favorable dynamical conditions of fog formation, and they can make the wet layer thickened and prevent moisture transport upwards, which is also beneficial to formation and maintenance of fog. After sunrise, the solar radiation becomes stronger, and conditions beneficial to formation and maintenance of fog like radiation cooling, inversion layer and dynamical conditions disappear, so the fog dissipates.

Key words: Radiation fog, Radiation cooling, Inversion, Dynamical condition