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In this study,a stratus-sea fog event which occurred on 3 June 2011 over the Yellow and East China Seas is investigated observationally and numerically,especially focus on the effects of background circulations and the SSTF (sea surface temperature front) on the transition of stratus into sea fog.The background circulation is helpful for water vapor transports to the Yellow Sea with southerly wind,which makes a physical base of sea fogs/stratus formation.The subsidence occurred above the ABL (atmospheric boundary layer) induced by the synoptic background circulation contributes to the temperature inversion on the top of the stratus,and the persistent downward motion makes the inversion getting lower and stronger,thus lowering of the stratus.Forced by the SSTF,there is a secondary circulation within the marine ABL,the sinking branch of which on the cold flank of SSTF helps the stratus further lowering.The cooling effect of cold sea surface counteracts the temperature increasing induced by subsidence and thus helps the transition from stratus to sea fog.After smoothing SSTF,the secondary circulation within the marine atmospheric boundary layer and the colder water disappear while the subsidence above the ABL still exists and the stratus cannot transform into sea fog.The transition of stratus into sea fog in the Yellow and East China Seas is the combined result of subsidence and low-level turbulence,if the turbulence is too stronger,stratus can’t convert into sea fog with subsidence.In addition,with global warming,sea fog frequency will become lower,probably because the strength of SSTF will be weaken and thus the subsidence of secondary circulation will be weaken as well.