Numerical Simulation Of The Urbanizations Effect On The Formation And Dissipation Of The Radiation Fog

A heavy radiation fog occurred on February8,2010in Baoshan, Shanghai.In this paper, a preliminary numerical simulation of this case was conducted by using a one-dimensional numerical model, NCEP (National Centers for Environmental Prediction)/CFSv2(Climate Forecaset System Verison2) reanalysis data and routine observation data. The roles of dynamical factor, thermal conditions, microphysical processes and urbanization effect in the formation and dissipation of a radiation fog were inverstigated by means of diagnostic analysis.The results showed that longwave radiation cooling at the surface in the night led to the formation of the fog, and the relatively stable atmospheric stratification and abundant vapor conditions was prerequisite for the formation of the fog. The main cooling factor of the fog was longwave radiation cooling. The heating effect of turbulence increasing rapidly after sunrise was the main heat factor which lead to the dissipation of the fog. Shortwave radiation impacted the disspation of the fog indirectly by heating the surface, changing the surface heat balance, enhancing the turbulence exchange process between the surface and nearsurface atmosphere. The dissipation of the fog was the results of the interaction of evaporation, turbulent exchange and gravitational sedimentation.The characteristics that high thermal conductivity, high roughness, low albedo and others of the urban surface affeced the surface energy balance, the sensible heat transfer played the leading role in the process of urban surface heating atmosphere. Surface sensible heating rate increases with the increasing solar elevation angle, and was up to1.2℃/h, latent heating rate changed small, with an average of about0.15℃/h. Turbulent exchange in the urban surface is stronger than in the country, the maximum turbulent exchange coefficients near the ground was up to2m2/s. The height of the urban radiation fog formed was higher than the rural,the fog formed first at the32meters height. The strong turbulance exchange of the urban surface caused the top of the fog developed to150meters. The main reason of the formation of the upper part of the fog was condensation caused by longwave radiative cooling, while the reason for the height of8meters or less fog formation was mainly caused by the accumulation of gravitational settling of liquid water content and droplet.