The influence of the thermal effect of underlayin g surface on the spring sea fog over the Yellow Sea: Observations and numerical simulations.

A sea fog event occurred over the Yellow Sea on 2-3 May, 2008 is investigated in this paper through various observational data and a threedimensional mesoscale model WRF． The satellite images depict the fog area of almost seabasin scale at its peak phase, and the fog is a typical cooling advection one that often occurs in the Yellow Sea fog season (from April to July when sea surface is cooler than air temperature) according to the synoptic conditions and sea surface tempertures.The buoy observations near Qingdao show that the surface air temperature (SAT， 2 m above sea level) decreases obviously when fog starts to occur with visibility dropping sharply below 1000 m and that the differences between the SAT and the sea surface temperature (SST, 1 m under water) reduce to less than 0.5℃ during the sea fog maintaining. The SAT is even lower than the SST in dense fog with the lowest visibility. The digital soundings indicate that stronger turbulence occurs below 150 m in the lower levels of the marine atmospheric boundary layer (MABL) during fog, reflecting that weaker stability is possibly in favor of sea fog generation. The differences between the SAT and the SST become greater distinctly and the turbulence is weakened in the intensity with its developing levels rising in the dissipation phase of fog. The local airsea interaction mechanism of momentum transport downward could play a role in the fog process, with higher wind speed as the result responding to warmer SST that is one of the conditions favoring better visibility.The numerical simulations with the WRF reproduce the variations of fog area and the horizontal visibility with time, which are consistent with the observations by satellite and from buoy. The difference between the SAT and the SST is less in fog areas than that out of fog areas. The effect of sea surface thermal condition on the sea fog is investigated quantitatively through sensitive experiments. The results show that the stability and turbulence under 100 m altitude are sensitive to the variations in the SST. The degree of the sensitivity depends on the water vapor content in the lower levels of the MABL. Under the conditions of lower humidity ( ) in the thin fog area, the stability can decrease (increase) obviously ( ( )) and the thin sea fog patch shrinks (enlarges) evidently with the SST rising (dropping) 1℃; while in the dense fog patch with a higher humidity ( ), the effect of changing in SST on the stability is not so obviouse, and the dense sea fog can be maintained. Therefore, if the difference of SST and SAT is small, even SST is higher than SAT, and if the sea fog still exists, the fog should be dense fog and the water vapor content is higher. This result is helpful to understanding of sea fog formation mechanism and forecasting.