Physical characteristics of persistent dense fog based on ground-based remote sensing observations

A rare and intense fog event occurred in East China region at the end of November 2018, lasting for a long period and affecting a wide area. However, notable variations in the fog"s formation and dissipation characteristics were observed at different stations within a 50-kilometer radius of Nanjing during this event. This study used high-precision observational data from five stations in Nanjing (Luhe, Pukou, Jiangning, Lishui, and Gaochun) to investigate these variations in fog characteristics and evolution across different stations. The study provides a detailed analysis of the circulation characteristics and atmospheric boundary layer features associated with this unusual prolonged fog event, and extracts forecast indicators for the explosive development of fog. The results indicate that the circulation pattern during this rare persistent heavy fog was characterized by straight flow in the mid to high latitudes, situated in a southwest airflow ahead of a trough, with a dominant surface isobaric field and three instances of weak cold air infiltration. The atmospheric stratification was stable, with weak turbulent movement and convergence-divergence. High humidity and significant inversion structures were observed, often with multiple layers of inversion. The average height of the inversion top ranged from 400 to 800 meters. Pukou station is located on the southeast side of Laoshan, at a higher elevation, resulting in significantly fewer periods of low visibility compared to other areas, which indicating that terrain forcing and the properties of the underlying surface have a regulatory effect on the phase change of fog. Below the top of fog, the liquid water content is high and the water vapor is saturated, while above the top of fog, the liquid water content sharply decreases and is accompanied by a rapid drop in water vapor density to its minimum value before quickly rising again, indicating a significant water vapor density mutation phenomenon in the vertical structure of the top of fog. A fog height discrimination index based on water vapor density gradient was established, and the average height of fog was measured to be 200-500 m. In most heavy fog processes, the height of the fog layer is lower than the inversion height, which providing more physical parameters for the remote sensing inversion of the vertical structure of boundary layer fog. The explosive enhancement of fog is caused by radiative cooling, while near-surface gentle wind, warm moist advection, long-term maintenance of inversion, weak cold air infiltration, and a brief super-low-altitude easterly jet in the inversion layer are all conducive to the maintenance and enhancement of dense fog. The dynamic-thermal coupling effect formed by the weak cold air infiltration and the brief super-low-altitude easterly jet (wind speed > 8 m/s) within the inversion layer promotes the rapid generation and vertical expansion of fog droplets. The explosive enhancement of fog does not necessarily accompany an increase in inversion, and the indication effect of inversion is limited. However, the synchronous jump increase of liquid water content (>0.1 g/m3) and water vapor density (>8 g/m3) can advance the forecast time of fog enhancement by 0.5-2.5 hours, providing a quantitative forecast indicator for the fine-scale forecasting of fog.