Analysis On Mechanism Of A Stratus-fog Event Over The Yellow Sea And East China Sea

Various observational data, satellite data and reanalyzed data are used to researcha stratus-sea fog event over the Yellow Sea and East China Sea on June3,2011,including the temporal and spatial evolution of the cloud and fog, the weathersituation, water vapor source, subsidence and so on. Furthermore, the stratus-sea fogcase is simulated with high resolution by using WRF model and investigated with twoexperimental simulations by smoothing and increasing sea surface temperature (SST)to find out the role of subsidence, underlying surface thermal forcing and turbulencein the process.Before the formation of sea fog, there are clouds or rain in China southeast coastand East China Sea to the south of the estuary of the Changjiang River. There is astable high pressure over the Yellow Sea and East China Sea, south wind in the westflank of high pressure continuously transports sufficient water vapor from East ChinaSea to Yellow Sea in the low-level of marine-atmosphere boundary layer (MABL),which is the precondition for forming sea fog. Under the influence of high pressureover water and SST front, the air in low-level MABL moves northward companyingwith obviously subsidence, which force the stratus to decline. Meanwhile, there is alsoa high pressure over the China southeast coast which makes the air in high-levelMABL mainly from land. Stratus declines forcing by subsidence while air movesfrom land to sea along the north flank of the high pressure over land. The subsidenceis also benefit for forming and intensifying inversion in MABL. Inversion exists inhigh-level and suitable water vapor condition exists in low-level, under such condition,stratus declines to surface and turns into sea fog, and then sea fog maintains anddevelops.Comparing the result between experimental simulations and control run, it isindicated that SST can affect the vertical circulation within MABL. Warm side of seafront intensifies ascending motion, while cold side of sea front intensifies sinking motion. SST also can influence the turbulence in MABL, the turbulence becomesstronger (weaker) due to the less (more) stable stratification in the MABL on warm(cold) sea surface. When turbulence intensity is suitable, the stratus moves downwardto the surface and turns into sea fog under subsidence forcing, but if the turbulence istoo strong, stratus cannot reach the surface. So, the stratus-sea fog process is a resultof subsidence, underlying surface thermal forcing and turbulence. In addition, thechanges of turbulence in MABL have a direct impact on the height of MABL. Whenturbulence is stronger, the height is higher, while turbulence is weaker, the height islower.