Numerical Study Of A Long-lasting Wide Spread Dense Fog Event

Fog presents a serious hazard in areas of intense traffic, such as airports, highways. Moreover, because air pollutant is not easy to disperse in foggy days, it is also harmful to the human health and agricultural production. Its prediction is essential for public safety and has a high economic value. To precisely predict foggy weather, it is essential to first understand the mechanisms responsible for fog formation and maintenance.In this study, the NCAR/PSU MM5v37 was utilized to simulate and diagnose a dense fog event occurred in Jiangsu province and the surrounding areas during 24-27, December, 2006. In the control simulation, the Gayno-Seaman parameterization of the boundary layer was used. The deep convection parameterization of Kain-Fritsch2 was adopted, and the explicit treatment of warm rain was employed. To account for the important role that longwave and shortwane radiation play in fog formation, the cloud-radiative scheme was employed. The center of the simulation area was set in (32°N , 117°E). Two computational grids with horizontal grid distances of 30 and 10 km and domain sizes of 70×70 and 76×76 grid points were considered, respectively, in a two-way nesting way. Thirty-six vertical levels, stretched monotonically from the surface to 100 hPa, provide high resolution in the boundary layer (another 13 levels joins below 200m). Sensitivity experiments were performed for understanding the role of radiation and terrain. In the sensitivity tests, a series of simulations were performed with different radiative schemes, while keeping the others physic schemes fixed. The results show that longwave radiative cooling from the Earth's surface and the atmosphere were the most important factors for the formation and development of this fog event. Meanwhile, the steady atmospheric stratification and plenty of moisture supply also played an important role. The fog dissipation was influenced mostly by the shortwave radiative heating and turbulent heat transfer after sunrise. During the development and maintenance stages of the fog, the surface layer was basically weak convergence region of water vapor, while in the weakening and dissipating period, the majority of the fog area was weak divergence region of water vapor. It indicates that the wide area of divergence descending accompanied with surface cooling and cold advection were the favorable conditions for formation of a temperature inverse layer near the ground which is responsible for the maintenance of this prolonged fog event. While the lower resolution whole world terrain and land surface data were used, the location, formation and dissipation of sensitive simulated fog are in accordance with the control experiment, but the intense of cloud liquid water is high than the result of the real situation.Finally, by using the water bodies of under surface type replace with the urban landuse type, the influence of radiation on the fog is not obviously.