Influence of Turbulent Diffusion Driven by Fog-Top Radiative Cooling on Heavy Fog in the North China Plain

Radiative cooling at the top of a fog is a crucial physical process during fog evolution, driving a “top-down” turbulent mixing. The YSU (Yonsei University) scheme incorporates a module (ysu_topdown_pblmix) to represent this process. However, the influence of such “top-down” turbulent mixing on heavy fog events in the North China Plain remains unclear. Therefore, this study evaluates the suitability of ysu_topdown_pblmix in simulating fog events in the North China Plain using data from Himawari-8 satellite retrievals, automatic station observations, 15-layer atmospheric boundary layer gradient measurements, and 5-layer turbulence-related observations. Large-eddy simulations are also employed to supplement the analysis. The investigation reveals that, while ysu_topdown_pblmix overall decreases the fog area simulation scores in the North China Plain, it significantly improves the simulation of the near-surface temperature, humidity, and fog boundary layer structure. This enhancement is attributed to the “top-down” turbulent mixing driven by radiative cooling at the fog top. However, the current version of ysu_topdown_pblmix exhibits an excessive entrainment effect, resulting in the intrusion of excessive warm air above the fog top and causing the rapid dissipation of fog. Sensitivity tests demonstrate that reducing the evaporative enhancement coefficient in ysu_topdown_pblmix can mitigate the strength of turbulent entrainment at the fog top, thereby improving the simulation of fog events. These findings suggest that incorporating ysu_topdown_pblmix is essential for accurately simulating heavy fog in the North China Plain, however, further refinement of relevant parameters is required.