Occurrence Pattern Of Dumpling Fog Events And Numerical Simulations Of A Great Complex Fog Event On Shanghai-Nanjing Expressway

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 the emerging and maintaining mechanisms of fog weather scientifically, comprehensively, systemically. In this paper, the observed data from the automatic weather monitoring system (AWMS) on the Shanghai-Nanjing Expressway was used to conduct a statistics analysis of the regional great fog events and the dumpling fog events during2006to2009.The charcteristics difference between the dumpling fogs occurred in the eastern lakeside section and the dumpling fogs occurred in the central-western riverside section on the expressway was compared in detail. The occurrence pattern and engendering meteorological conditions of the dumpling fog events was discussed. Also a typical complex fog event that occurred over the expressway on7th, November in2009was selected from a set of fog weather processes. Based on the analysis of the weather actuality during the fog event occurred, the Weather Research and Forecasting Model for Version3.1(WRF3.1), an non-hydrostatic mesoscale numerical forecasting model with the high spatial and temporal resolutions was used to simulate a typical fog event combined with NCEP0.5°×0.5°meteorological reanalysis data. The numerical simulations of some physical variables were verified from the observed data of the fog event in AWMS. The engendering moisture, dynamic and thermodynamic conditions of this complex fog event were analyzed. The main conclusions were concluded as follows: The results of the occurrence pattern and local characteristics of dumpling fog events on the Shanghai-Nanjing Expressway:(1) The seasonal variations of dumpling fog and regional fog was similar, with the higher occurrence frequencies in autmunl, winter and spring but the lower occurrence frequencies in the summer.(2) The occurrence frequencies and spatial distribution of the dumpling fog and regional fog along the expressway was quite different due to the different sections and different stations in AWMS. There were more regional fog events occurred in the eastern section than in the western section. There are4stations with the higher occurrence frequencies of dumpling fog in the eastern section but the other stations with the lower occurrence frequencies. The occurrence frequencies of the dumpling fog were generally higher in the western riverside section than in the other sections.(3)Usually, it was the meteorological conditions of the dumpling fog engendering that the air temperature lied in the range from-4to30℃, the relative humidity changed from85%to95%, wind speed was3m·s-1or less. The occuring time bucket of the dumpling fog standed in hours between00:00am to6:00am in the later night and they disappeared in4hours after the engendering.(4) The lakeside environment of the eastern section on the expressway provided a favorable situation for the engendering of dumpling fog and the central-western riverside section of the expressway was the highest frequency area of the dumpling fog due to its special topography and water vapor transport channels.The results of the Numerical simulations of a great complex fog event on Shanghai-Nanjing Expressway were showed as follows:(1) The atmospheric circulation situation in Jiangsu Province and its contiguous regions was stable before and during the great fog event occurred. These regions were affected by the southeastern stream at the southwestern side of anti-cyclone system over the Yellow sea and the surface wind force has been weak in the whole fog process and it provided a favorable dynamic condition for the engendering of heavy fog.(2) The simulated distribution areas of the fog bodies depicted by the liquid content water (LWC) was agree with the observed fog scopes.(3)The simulated visibilities were very close to the observed visibilities from AWMS.(4) The mixed fog event occurred during18:00(BST),7th to08:00(BST),8th of November in2009.The embryo of this heavy fog event was a local dumpling fog. It converted into a radiation fog under the influence of the radiation cooling. Then, a warm and humid air advection from the surface of the East Sea in the southeastern direction entered into the southern part of Jiangsu Province and an advection fog body came into being. It combined the earlier radiation fog body into a complex fog from the dual effect of radiation cooling and advection cooling and expanded to a greater scope.(5)The warming of the surface caused by short-wave radiation after sunrise was the main reason for the quick dissipation of the fog body.