Numerical Simulation And Diagnostic Analysis Of Fog Process Along Shanghai-Nanjing Expressway Based On Wrf Model

Great fog weather events have brought a lot of imperiling to the transportation of Shanghai-Nanjing Expressway. Especially, the local dumpling fog events occurred along the Zhenjiang section of the Expressway have been endangering on the traffic safety of the Shanghai-Nanjing Expressway. In order to predict precisely the engendering and disappearing of the regional fog weathers and the local dumpling fog events on the Expressway and ensure the unblocked passage of highway transportation, it is essential to understand scientifically, comprehensively, systemically the emerging and maintaining mechanisms of fog weather.Combined with the vertification of the observed data from an automatic monitoring weather system (AMWS) on the Shanghai-Nanjing Expressway, WRF (Weather Research and Forecast for Version2.2), a new generation of mesoscale numerical forecasting model, and the reanalyzed data (1°×1°) from NCEP (National Center for Environmental Predicting in USA) were utilized to simulate and diagnose a regional dense fog process appeared along the Shanghai-Nanjing Expressway and coverd the southern part of Jiangsu province on October26th in2007and a dumpling fog weather event took place in the Zhenjiang section of the Expressway on November24th in2007. The results of the simulations and diagnostic analysis showed that the WRF model displayed objectively the engendering and disappearing processes of these two fog events. As a dumpling fog event usually appears in the height of a few or tens meters, its engendering and disappearing processes have close relate to the landforms condition. In order to explore the influence of terrain and landforms on the dumpling weather on the Expressway, some sensitivity tests about the scheme of land surface physical process, the landform and the distribution of the water system were carried out and the digital elevation model (DEM) in ArcGIS9.2, a software of geographical information system were used to analyze the effect of terrain and water system on the engendering, developing, maintaining of the fog events.The results of the regional dense fog simulation showed as followed:(1) the diagnostic analysis displayed that this typical fog event was caused by mixed effect of radiation and advection and it resulted from the specific backgrounds of atmospheric circulation and the suitable moisture, dynamical and thermal conditions.(2) One of the main reasons accounted for the heavy fog event was a rainfall process before the fog formation. The rainfall brought the plenty water vapor and made the maximum value of relative humidity pass over90%. At the same time, it showed that the fog region was a weak convergence zone of air stream.(3) The vorticity field with the minor and negative value was advantageous to the sinking of the air and it ensured the moist air not to spill-over.(4)The strong ground radiation cooling and weak cold advection were the important factors of the engendering, developing and maintaining of the fog weather process. The ground radiation cooling was a dominant factor in the thermal conditions and the effect of cold advection was far less than the ground radiation cooling.The observed data from AMWS and the numerical simulations of the dumpling fog based on WRF model indicated that:(1) The Zhenjiang section of the Shanghai-Nanjing Expressway has been easy to appearing some dumpling fog weathers and the endengering conditions of the dumpling fog were the stable atmospheric environment, the ground surface radiation cooling at night, and the cooling of the warm and wet flow from the southern regions moved through the cooler ground surface.(2) When two branches of airflows from the different directions of the north and south sides converged in the western part of the Expressway, it was advantageous to the collecting of wet air mass and aerosol particles and it also was one of the main reasons for the developing and maintaining of dumpling fog weather.(3) The dumpling fog dissipated rapidly when the short-wave radiation from the sun warmed the underlying surface and the air close to the ground after the sunrise.(4) The topographical and geomorphological sensitivity experiments appeared that the two factors were sensitive to the formation of dumpling fog. So exploring the topographical and geomorphological characteristics of the Zhenjiang region around the Expressway will be significant to explain the mechanism of the dumpling fog occurrence.Considering the impact of topographic characteristic of underlying surface, the sensitivity tests of coupling different land surface (SLAB, NOAH and RUC) schemes were conducted. The results were described as followed:(1)The dumpling fog processes simulated by the WRF model was sensitive to the selection of different land surface processes parameterization schemes and the integrated simulations coupling into different schemes was closer to the reality.(2)When the SLAB scheme coupled into the WRF model, the simulation on the intensity change and lasting time of the dumpling fog event was more effective than the NOAH scheme or the RUC scheme and there was little difference between the NOAH scheme and the RUC scheme.(3) In the simulations on the heat flux, there was some difference between each other for the three schemes. While in the simulation of the long-wave radiation, the NOAH scheme had more ideal effect than the SLAB scheme or the RUC scheme.(4) Results of the simulating the high-value areas of the vorticity field displayed that the NOAH scheme had a best effect in the three schemes.