Study Of Macro And Micro Structures Of Spring Sea Fogs In Xiamen

The macro and micro processes of sea fogs during a comprehensive fog experiment in Xiamen from 23 March to 22 April 2013 were studied, using visibility, fog droplet spectra, automatic weather station observations, meteorological elements, and NCEP Final Operational Global Analysis (FNL) data. The effect of synoptic system and meteorological characteristics on the sea fog formation/dissipation of eight cases were explored, the microphysical characteristics were discussed, and the microphysical processes (nucleation, condensation, collision-coalescence, etc.) were analyzed, which revealed correlations among main features. The main conclusions are as follows:(1) When Xiamen was located at the forepart of a cold front or a depression, or at the rear of a high-pressure system, the frequency of fog occurrence was high. The main synoptic weather pattern was cold front, which lasted the longest and caused the lowest visibility.(2) During the processes of sea fogs, a rapid increase in the southwesterly flow was important to the formation of sea fog. The warm-moist air flowed to a colder sea, which provided plenty of vapor for the development of sea fog. Air-sea temperature difference was important to the formation of sea fog in Xiamen.(3) Inversion or isothermal layer existed in sea fog. The stable stratification prevented water vapor from flowing into the upper layer, and the accumulation of water vapor on the ground was important to fog formation and maintainatence.(4) Compared to the other regions of China, the fog droplet number concentration in Xiamen was high, but the liquid water content (LWC) and fog droplet mean radius were small. The reason for the lower LWC was the observation site that was located near the coast; so sea fogs would be affected by different surfaces of surrounding environment by mixing with unsaturated air. The droplets evaporated and the LWC decreased. Neaerly 26.8 percent of sea fogs were mist, the droplets were small, so the LWC was low. The droplets bigger than 25μm were unable to be detected by the instrument, whose rang is 1μm<r<25μm.(5) The correlation of the fog droplet number concentration, LWC, fog droplet mean radius, and automatic threshold were analyzed. The results indicated that the dominant physical processes were condensational growth of nuclei with subsequent reversible evaporation. The average fog droplet size distribution was in a good agreement with the Junge distribution, mostly monotonically decreasing type. The fog droplet number concentration concentrated in the 2-7μm diameter range.