Statistical Analysis of Micro-Physical Features of Mountain Fog and Its Parameterization Scheme in Southern Fujian

Abstract

This study analyzed the circulation patterns and micro-physical features of mountain fog in Southern Fujian using fog droplet spectrum data from meteorological stations, sounding data, and ERA5 reanalysis. Results suggested that both the convergence of cold and warm air in spring and the presence of southwestern warm moist airflow can lead to the formation of mountain fog in Southern Fujian. The former featured lower temperatures and denser isotherms in low levels compared to the latter. This resulted in an increase of supersaturation in the coastal atmosphere, thereby accelerating particle nucleation and condensation growth, forming larger droplets or even precipitation particles. Mountain fog in Southern Fujian has an average total particle number concentration of 314 cm⁻³ and an average total liquid water content of 0.1721 g·m⁻³. Average fog droplet spectrum features an unimodal distribution, with a peak at 5–6 μm. However, the average liquid water content spectrum showed a bimodal distribution, with the main peak at 8–9 μm interval and a secondary peak at 22–24 μm, indicating that total particle number concentration in fog was mainly controlled by small particles, but particles smaller than 10 μm and those in the 20–30 μm intervals both contributed significantly to the total liquid water content. Four parameterization schemes were used to fit visibility. Results showed that fitted coefficients differ significantly from those in other regions; hence, establishing local parameterization schemes for visibility was very important. In the evaluation results, fitting using the total particle number concentration as a factor showed the best performance, with a determination coefficient of up to 0.7. Mean absolute errors were significantly higher between 200 and 1000 m, especially in the 200–500 m interval. This was attributed to the larger ratio of standard deviation to the average value of particle concentration and liquid water content in this interval, indicating more uneven distributions of micro-physical parameters.