A study on the transition mechanism of a stratus cloud into a warm sea fog using a single column model PAFOG coupled with WRF

The transition mechanism of stratus cloud into warm sea fog over the Yellow Sea near the western coastal area of the Korean Peninsula is investigated using numerical simulation with a 1D turbulence model, PAFOG, coupled with a 3D regional model, WRF. The coupled model system was run in the two approaches, Eulerian and Lagrangian. For the selected warm sea fog case, the model results in the Eulerian approach showed that the bottom of the stratus cloud was lowered by cooling of the air just below the cloud base by turbulent heat loss. The Lagrangian approach showed the lowering of the stratus cloud top, owing to the evaporation of cloud droplets in this region by the entrainment of warm and dry air above the cloud top. The sensitivity test to SST indicated that the timing of water vapor saturation just below the cloud base depended on the magnitude of the turbulent heat flux from the sea surface. The subsidence rate was found to be important: when the subsidence rate was set to be half of the prescribed value, neither the lowering of the stratus cloud top nor the bottom occurred and the model could not produce a fog.