Abstract:Using observation data from automatic weather station (AWS), ERA5 reanalysis data, Himawari-8 satellite data and multi-source observation data at Xiang’an Station, we analyze the circulation situation, evolution characteristics and microphysical structure of a sea fog event that occurred in southern Fujian Coastal Area (SFCA) on 1 April 2021. The results show that this was a typical advection fog process. Multiple weather conditions provided stable circulation situation and abundant water vapor conditions for the occurrence and development of sea fog. In other words, when the fog was formed, there was west-southwest airflow at 500 hPa, the lower troposphere was controlled by consistent southwest airflow and anticyclonic sinking airflow, and the temperature inversion layer and wet layer existed near surface. Low-level cloud and sea fog conversed into each other during the whole fog process. Daytime was dominated by low-level cloud, but after sunset, the low-level cloud changes into fog with dropping temperature and slowing wind. Next morning, fog changed into low-level cloud under the effect of west wind. The fog height is deduced by aerosol lidar. The fog thickness was relatively low and fluctuated greatly in the initial and developing stages, and the fog top height was about 100 m in the mature stage. Microphysical analysis shows that average fog droplet number concentration (N) was 52.4 cm-3, the average fog droplet liquid water content (LWC) was 0.084 g·m-3, and the mean diameter (MD) was 9.4 μm during this event. The maxima of the average N per minute and the average LWC per minute were 132.6 cm-3 and 0.7321 g·m-3, respectively. Spectrums of N and LWC at different stages exhibited quite different characteristics, of which the size distribution of N was mainly a unimodal structure with the peak diameter at 4-6 μm in the preliminary, development and dissipation stages, but it was a bimodal structure with the main peak at 4-5 μm and the secondary peak at 24-26 μm in the mature stage. The LWC also had a bimodal structure, with the main peak at 24-26 μm and the secondary peak at 5-6 μm. Thus, the N should be dominantly influenced by small particles, but the greatest influence to LWC is fog droplets of 20-30 μm. Comparative analysis of size distribution at the development and mature stages shows that the main reason for the further deterioration of visibility should be the significant increase in LWC which resulted from the increase in the particles of 20-30 μm.