The Characteristics Of Winter Fog In Nanjing And The Impact Of Turbulence On Fog

A general situation of fog in December has been introduced based on the data collected from a fog observation experiment during the winter of2007at Nanjing University of Information Science and Technology. Besides, the formation and dispersion mechanisms, micro-physics structure, turbulence structure and the characters of the temperature, humidity, wind and dynamic structure of the boundary layer has been analyzed via a radiation fog (December18,2007) and a advection-radiation fog (December19to20,2007) respectively. The result shows that:Vapor was mainly supplied by Evaporation of soil moisture in the radiation fog and the transportation of southeast warm advection in the first half of advection-radiation fog. In the second half of advection-radiation fog, surface evaporation became the main source of water vapor due to advection weakens. Burst reinforcements in this two fog progresses were caused by different reasons. It was the combined effect of enhanced evaporation of soil moisture after sunrise, fall of temperature and the increase of turbulence intensity in radiation fog. But in the advection-radiation fog, burst reinforcement was caused by the increase of cooling rate due to the influence of northerly air.Wind shear has a significant effect on the Temperature inversion structure:the wind shear reduce is conducive to the formation of temperature inversion structure. On the contrary, when the wind shear increased, which can cause the development of turbulence, the inversion layer will be destroyed.Micro-physics structure:the formative stage of the fogs shows the minimum micro-physical parameters and the narrowest fog droplets spectrum, while the mature stage of the fogs shows the maximum micro-physical parameters and the widest fog droplets spectrum. In the burst reinforcements of fogs, fog droplet number concentration in every scale increased markedly and more larger droplets formed constantly. advection-radiation fog has more large droplets and a wider spectrum than radiation fog. Turbulence structure shows different characters in different stages:the atmosphere is usually in a stable state in the formative stage of fog, which means the turbulence intensity is relatively weak. In the development stage, turbulence shows varying degrees of fluctuation, liquid water content and droplet number concentration in fog increase accordingly. Strong turbulence intensity often appears in the mature stage of the fog. The sensible heat flux is negative and heat is transferred downward due to the effect of turbulence before sunrise. After sunrise, the sensible heat flux turns positive and the water vapor flux increased significantly. There are many factors can affect the turbulence intensity. For example, low-frequency turbulence would increase due to the wind direction changes wildly, which can make the abnormal increase of turbulence intensity in the case of wind speed decreases. On the contrary, if the wind direction trend to stabilized, turbulence intensity may reduce while wind speed increase. Turbulence intensity amplify after sunrise is normally caused by the enhancement of ground radiation and the development of thermal turbulence.Turbulence shows a very important influence to development of fog. Turbulence can broaden the droplets spectrum by increasing the collision efficiency of small droplets, while water vapor and heat are transferred upward to maintain the vapor super-saturation and promote the nucleation growth, which can cause the burst reinforcements or the significant increase of microphysical and keep the strong fog last.