Impact of Turbulent Mixing Driven by Fog-Top Cooling on the Development of Sea Fog

Turbulent mixing driven by cloud/fog-top (radiative and evaporative) cooling (top-driven mixing), including top-down mixing and top-cooling entrainment, is critical for the development of cloud/fog. Previous work mainly focused on impacts of top-driven mixing on stratocumulus-topped planetary boundary layers (PBLs) and radiation fogs over land. However, its exact role in sea fog processes is yet unclear. Using the Weather Research and Forecasting (WRF) model with the updated Yonsei University (YSU) PBL scheme, in which the ysu_topdown_pblmix option is responsible for the parameterization of top-driven mixing, the present study investigates the respective mechanisms of top-cooling entrainment and top-down mixing in an advection fog event over the Yellow Sea that occurred on 20–21 May 2014. Evaluations confirm that top-driven mixing significantly improves the sea fog simulation. The top-cooling entrainment allows more relatively quiescent air above the fog top penetrate into the sea fog layer, leading to a higher mixed layer with increased temperature and decreased humidity. Under the enhanced buoyant turbulence, the warming and dehumidifying moderately lift the sea fog bottom and diminish the false fog area. The top-down mixing performs quite well in reproducing a stable structure near the fog top, which results from the enhanced turbulent mixing determined by a revised profile. Note that such performance of top-driven mixing shows high sensitivities to the shortwave radiation and vertical resolution. Besides the fog-top cooling, the fog droplet sedimentation is necessary to be included in the top-cooling entrainment.