Transition from Fog to Stratus over the Northwest Pacific Ocean: Large-Eddy Simulation

Over the midlatitude northwest Pacific Ocean, summer fog frequents the Kuroshio-Oyashio front as a result of the warm advection by the prevailing southerly to southwesterly winds, and stratus clouds are prevalent downstream of the fog regime in the subpolar northwest Pacific. The present study tracks a boundary layer air column along a typical northeastward trajectory along which fog on the sea surface temperature (SST) front makes its transition to stratus clouds. A turbulence-closure large-eddy simulation model can capture the evolution of the air column forced by the time-varying SST along the trajectory. Results show that the surface cooling effects across the SST front and the longwave radiative cooling (LRC) at the cloud top dominate the evolution of the boundary layer and the related turbulent processes. The sharp SST decrease across the SST front cools the surface layer, leading to condensation through shear-induced turbulence. Once the fog forms, the LRC at the fog top cools the boundary layer strongly through thermal turbulent mixing. The buoyancyinduced turbulence near the fog top entrains the warm and dry air from the free atmosphere into the boundary layer, reducing surface humidity and ultimately lifting the cloud base away from the sea surface to form stratus clouds. Sensitivity simulations also suggest that neither the latent heat flux from ocean nor and the diurnal solar variation is essential for the summer fog-to-stratus transition over the northwestern Pacific.