The Evaluation of Modified YSU Scheme and Parameter Adjustment on the Simulation of Fog over the North China Plain

Recent studies showed that the modified YSU scheme, which added a fog top-down diffusion mechanism (ysu_topdown_pblmix), facilitated the bottom lifting of valley fog and sea fog by taking into account the “top-down” turbulent mixing and entrainment enhancement due to radiative cooling at the fog top, thus eliminating false fog areas. The applicability of this scheme to fog processes over the North China Plain is of interest. In this paper, we compared and analyzed the simulated effects of the modified YSU scheme on three fog events over the North China Plain with conventional ground-based data, 15-layer gradient and 5-layer eddy-related observations from atmospheric boundary tower. It was found that, unlike the results of existing studies, considering the fog-top turbulent diffusion mechanism resulted in lower TS scores for the fog area in the North China Plain, especially for the deep fog processes. A specific analysis of the simulated performance of a frontal fog on 17-20 December, 2016 showed that although the modified YSU scheme improved the simulation of nighttime near-surface temperature, humidity and vertical development of the fog layer, the simulated fog-top turbulence was too strong compared with the actual observations, resulting in a shorter fog duration and a significantly reduced fog area. By adjusting the parameter to reduce the intensity of the fog top turbulence entrainment, this operation can effectively reduce the under-reporting of the fog area and improve the simulation of fog over the North China Plain.