Interaction Among Macro-and Microphysical Characteristics Of Fog,Turbulence,Radiation,and Aernsol

Data collected in the surface layer in Nanjing University of Information Science&Technology from2006to2009and in Zhanjiang from15March to18April2010were analyzed to investigate the interaction among macro-and microphysical characteristics of fog, turbulence, radiation, and aerosol. Macro-and microphysical characteristics of sea fog on the east coast of Leizhou Peninsula were obtained, and we discussed the influence of synoptic weather pattern on the macro-and microphysical properties of the sea fogs, as well as the dominant microphysical process during the sea fog events. This dissertation stressed on the impact of turbulence intensity, radiation flux and aerosol to the macro-and microphysical structure of fogs, and typical fog cases were selected to investigate the effect of fog on aerosol. Furthermore, the critical turbulence exchange coefficient was used to analyze the influence of turbulence on the formation, development, mature, and dissipation of fogs. The effect of turbulence, radiation, and aerosol on macro-and microphysical structure of fogs presented significant threshold characteristics, in turn, the development of fog would affect the turbulence, radiation, and aerosol in the boundary layer. Through the above research, we come to the following conclusions.1. The fog droplet size distribution of the sea fog events was mostly monotonically decreasing type, with the spectrum width being always larger than20μm. But in the different stage of fog events, the unimodal, bimodal or multimodal fog droplet size distribution were also observed. During the sea fog events, the dominant physical process was activation with subsequent condensational growth or reversible evaporation processes; turbulent mixing process played an important role in the sea fog events; the collection process occurred but not the vital process for the sea fogs.2. The influence of turbulence on fog is two sided, and there do exist a critical value of turbulence intensity. If the turbulence intensity is less than the critical value that turbulence may result in larger inhomogeneity of fog droplet spatial distribution and promotes the formation and development of fog; however, if the turbulence intensity is greater than the critical value, too strong turbulence intensity will restrain the development of macro-and microphysical process and makes fog microphysical structure distribute uniformly, which may diminish the fog and leads to fog dissipation. During the formation stage, because of the very thin fog layer, strong turbulence was easier to violate the critical value, which may lead to fog droplet evaporation and the decrease of fog droplet number concentration, liquid water content and droplet size. But in the development or mature stage, the turbulent critical value was relatively large and the strong turbulence strengthened the turbulent transportation of water vapor and fog droplets, making larger mean values and fluctuations of fog droplet number concentration, liquid water content and droplet size. Furthermore, turbulence embodied various effects on the distribution of fog droplets with different size. The smaller the fog droplet is, the greater the influence of turbulence on the spatial distribution of droplet is. Turbulence has little influence on the distribution of fog droplets with radius greater than5μm, and the inhomogeneity of fog droplet spatial distribution is mainly caused by fog droplets with radius less than5μm. With the increase of turbulence intensity, the maximum radius of fog droplets increased significantly through collision-coalescence process when the maximum radius of fog droplets is greater then10μm, but the maximum radius of fog droplets was little changed through condensational growth when the maximum radius is smaller than10μm. Strong collision-coalescence process appeared when fog droplet spectrum width was greater than10μm, which subsequently lead to sudden increase of droplet spectrum width. Compared with the presence of dense fog and the explosive development of fog microphysical structure, the increase of the difference (Kc-Km) between critical turbulent exchange coefficient (Kc) and turbulent exchange coefficient (Km) usually present20to50minutes earlier. This suggested that Kc is a good indicator for studying the life cycle of radiation fog, which can afford some indication to prediction of radiation fog.3. When the value of net radiation flux in the range of-50W m-2～+25W m-2, the fog droplet number concentration increased gradually and the fog droplet number density increased more significantly with decreasing fog droplet size. This indicated that the surface radiation cooling and very weak heating facilitated the activation and condensational growth of fog droplets. While the net radiation flux greater than+25W m-2, the fog droplet number concentration declined significantly and fog droplet number density reduced more rapidly with decreasing droplet size. Solar shortwave reflectivity was strongly influenced by fog droplet number concentration, mean radius, and liquid water content. During the fog events, for every100cm-3and0.001g m-3increase in fog droplet number concentration and liquid water content, solar shortwave reflectivity increased5.29×10-3and1.18×10-4respectively.4. In given atmospheric environment, there is a critical value of aerosol number concentration for the activation and condensational growth of aerosol. When the aerosol number concentration is smaller than critical value, fog droplet number concentration and liquid water content increase with the increasing aerosol number concentration. Otherwise, the increasing of aerosol number concentration will restrain the activation and condensation process, which may lead to the reduction of fog droplet number concentration, mean radius and liquid water content. The effect of aerosol number concentration on fog droplet number concentration is characterized by the variation of number density of fog droplet with radius less than2μm. This suggested that the influence of aerosol on fog microphysical structure is achieved by changing activation and condensation process. Under influence of oceanic environment, fog had insignificant wet scavenging effect on aerosol, but mainly change the size distribution of aerosol. The aerosol particle size distribution during the fog events is mostly unimodal, and the peak diameter is in the range of0.02μm～0.1μm. During the fog event, the particle number density of aerosol with diameter less than0.02μm decreased sharply during the fog event and spectrum peak became more steeply. After the fog dissipation, the peak of aerosol particle size distribution shifted towards small particle size with the water evaporation.