Macro and Micro Characteristics of a Fog Process in Changbai Mountain in Summer

In the summer of 2021, the fog droplet spectrum observation is carried out on the main peak of Changbai Mountain for the first time. From 31 July to 1 August, there is a fog process that lasts for 19 hours, and the minimum visibility in the extremely dense fog stage is less than 100 m. Using the observations of laser fog droplet spectrometer, combined with the ground automatic weather station, GPS balloon sounding, Himawari-8 satellite and ERA5 data, the macro and micro physical characteristics of the fog are studied, the causes of the fog are discussed, and the microphysical characteristics evolutions of the extremely dense fog period are analyzed.The results show that the fog process lasts for a long duration with occasional short dissipation, and during the process the ambient wind speed is high, the visibility is low, the number concentration of the droplets is low, with small effective diameter and low liquid water content. The wind speed is always high in the period of extremely dense fog, which is significantly different from that of plain fog. In the early stage the fog is arisen from windward slope, which is a typical topographical cloud and fog on the main peak of Changbai Mountain in summer. It is formed by the continuous southwest warm and humid airflow climbing along the terrain under the condition of stable temperature inversion stratification. While the latter process of the fog is generated by the advection to the main peak of Changbai Mountain. The temporary dissipation of fog is related to the intensity and movement of the jet core at 700 hPa. The average effective diameter of fog droplets is 5.7 μm, the average number concentration is 246.4 cm^-3, and the average liquid water content is 0.05 g·cm^-3. The microphysical characteristics are similar to those of sea fog.For the extremely dense fog, the minimum visibility is less than 100 m. The extremely dense fog is characterized by explosive enhancement. Due to the rapid expansion of the droplets through the turbulent collision process, a single peak structure is formed. The peak diameter of the droplet particles is 6.0 μm, which has a significant contribution to the formation of the summer fog on the main peak of Changbai Mountain. In the formation, development and weakening stages of the extremely dense fog, the changes of droplet number concentration, liquid water content and effective diameter have a good corresponding relationship, but it is not obvious in the mature stage.