Numerical Simulation and Diagnostic Analysis of a Heavy Fog Process in Shanghai-Nanjing Expressway

Using the WRF2.2,a new generation of mesoscale numerical forecasting model,and NCEP reanalysis data with 1°×1° grid,a heavy fog process appeared in the southern part of Jiangsu Province which cover the Shanghai-Nanjing Expressway on 26 October 2007 was simulated.The effect of the simulations was verified by the comparison between the simulated and observed data from the Auto-meteorological Monitoring System(AMW)(one time per minute) on the Expressway.It is found that the simulation in this fog weather process is successful.The results are as follows:(1) One of the main reasons accounted for the heavy fog event occurs a rainfall process before the fog formation.The rain bring abundant water vapor and make the maximum value of relative humidity reachs over 90% in the region.At same time,it shows that the fog region is a weak convergence zone of air stream.(2) The vorticity field with the minor and negative value is advantageous to the sinking of the air and it ensures the moist air not to spill over.(3) The cold advection and ground radiation cooling are the important factors of the engendering,developing and maintaining of this fog weather process.The ground radiation cooling is a dominant factor in the thermal conditions and the effect of cold advection is far less than the ground radiation cooling.