Observational study of factors influencing the dispersion of warm fog droplet spectrum in Xishuangbanna, China

The microphysical characteristics of fog and stratiform clouds are somewhat similar. The study of the microphysical characteristics of warm fog, fog droplet spectral relative dispersion, and their influencing factors can deepen our understanding of the variability and influencing factors of cloud droplet spectral relative dispersion, while also investigating the formation and maintenance mechanisms of fog. This is currently a scientific issue that still remains controversial in cloud physics research and climate prediction. In this paper, we analyzed three months of Xishuangbanna radiation fog observations to explore the microphysical characteristics of fog. The results show followings: (1) When the autoconversion threshold (T) increased to greater than 0.4, the positive correlation between the relative dispersion of fog droplet spectrum and the volume mean diameter or water content of fog droplet weakened, also the positive correlation between relative dispersion and number concentration increased where the main mechanism needed to be integrated considering the interaction of collision-coalescence, condensation, and nucleation processes. It is found that the strength of the collision-coalescence process has a certain influence on the variation rule of dispersion. (2) The number concentration of 2–12 µm droplets in the fog constrained the relationship between the T and relative dispersion, with the number of large droplets reflects the strength of the collision-coalescence process. (3) Supersaturation changed microphysical quantities by increasing the number concentration of small droplets in the fog, which affected the variations of relative dispersion. For supersaturation greater than 0.12 %, the number concentration of droplets larger than 30 µm may be decreased due to gravitational settling. In addition, there is no significant relationship between supersaturation and relative dispersion if the initial nucleated fog droplet spectrum is narrow.