Characterization of fog microphysics and their relationships with visibility at a mountain site in China

Abstract. Enhancing the understanding of fog microphysical processes is essential for reducing uncertainty in fog forecasts, particularly in predicting fog visibility and duration. To investigate the complex interactions between aerosols and fog microphysics and their impacts on visibility degradation, simultaneous measurements of aerosol and fog microphysical characteristics were conducted from April to May 2023 at a mountain site (1483 m a.s.l.) in the Yangtze River Delta (YRD) region, China. In this study, eight fog events were investigated during the campaign, revealing significantly higher fog droplet number concentrations (Nd) compared to those observed in clean areas. A strong correlation was found between pre-fog aerosol number concentration (Na) and the peak Nd of each fog event, indicating the substantial influence of pre-existing aerosol levels on fog microphysics. Water vapor supersaturation ratio (SS) within fogs was estimated to 0.07 % ± 0.02 %, slightly higher than previous estimates in urban and suburban areas. The broadening of the droplet size distribution (DSD) at formation, development, and mature stages was dominantly driven by activation, condensation, and collision–coalescence mechanisms, respectively. This evolution process often led DSD to a shift from a unimodal to a trimodal distribution, with peaks around 6, 12, and 23 µm. For fog events occurring under high Na background, a notable decrease in temperature during the mature stage promoted a secondary activation-dominated process, resulting in the formation of numerous small fog droplets and a reduction in the large droplet size. The evolution of DSD can significantly influence visibility (VIS) in fogs. Detailed comparison of several visibility calculation methods suggests that estimating visibility based on the extinction of fog droplets only led to considerable overprediction when 100 m <VIS≤1000 m. The results highlight the necessity of incorporating both fog droplets and aerosol extinction in fog visibility forecasts, particularly in anthropogenically polluted regions.