Marine boundary layer structure for the sea fog formation off the west coast of the Korean Peninsula

Marine boundary layer (MBL) structure for the formation of sea fogs off the west coast of the Korean Peninsula are examined for the investigation period from January 2002 to August 2006, using the meteorological data measured at a buoy and the vertical sounding data measured at an island in this region. There is the total of 3,294 vertical soundings during the investigation period. Based on these vertical soundings, the MBL structure is classified as convective boundary layer (CBL; when inversion exists aloft but at altitudes lower than 3 km, 1,618 soundings), stable boundary layer (SBL; when inversion base is at the surface, 655 soundings) or near-neutral boundary layer (NNBL; when there is no inversion or inversion base is higher than 3 km altitude, 1,021 soundings). Under the CBL condition, the most frequently formed lower level cloud is stratocumulus but fogs do form in spring and summer months mostly as warm sea fogs [TSST (=T-SST)<0]. Under the SBL condition, stratus and cold sea fogs (TSST>0) are the most frequently found lower level clouds. The effects of turbulence, advection and radiation on sea fog formation vary with turbulence strength, represented by bulk Richardson number, R _b. For cold sea fog cases, in the highly turbulent regime (R _b<0.03), strong turbulent cooling and drying are canceled out by equally strong or even stronger warm and moist advection, and thus the additional radiative cooling turns out to be critical in the successful formation of fog. In the weak turbulent and non-turbulent (R _b>0.30) regimes, the effects of turbulence decrease dramatically and so do the advection effects but radiative cooling is still strong, again making it the crucial reason for the successful formation of cold sea fogs. On the other hand, the turbulent moisture supply from the warmer sea surface is the crucial factor for the formation of warm sea fogs while turbulent warming and radiative cooling largely cancel each other out and the advection effects are negligibly small.