Turbulent mixing driven by cloud/fog-top radiative and evaporative cooling (hereafter top-driven diffusion), 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 diffusion on stratocumulus-topped planetary boundary layer (PBL) and radiation fog over land. However, its exact role in sea fog process 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 the top-driven diffusion, the present study investigates respective effects of the top-cooling entrainment and top-down mixing on an advection fog event over the Yellow Sea that occurred on 20-21 May 2014. Evaluation of the modeling results confirms that the top-driven diffusion significantly improves the sea fog simulation. The top-cooling entrainment allows more relatively quiescent air above the fog top to mix into the sea fog layer, leading to a higher mixed layer with increased temperature and decreased humidity. With the enhancement of buoyant turbulence, the warming and dehumidifying moderately lift the sea fog bottom and reduce the false fog area. The top-down mixing performs quite well in reproducing a stable structure near the fog top, which results from the added turbulent mixing determined by a revised K profile. Note that the effect of top-driven diffusion shows high sensitivities to shortwave radiation and vertical resolution. Besides the fog-top cooling, the fog-droplet sedimentation is necessary to be included in the top-cooling entrainment.
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