Numerical Study On The Formation Mechanism Of Marine Atmospheric Duct During The Yellow Sea Fog Processes

The structure of marine atmospheric boundary layer represented inversion and thehumidity declined sharply over the sea fog when the sea fog happened, and on the top of seafog existed intense temperature and humidity gradient, this vertical structure of temperatureand humidity promoted the duct’s occurrence; Observations show that, when the sea foghappened it was often followed by duct, however, researchers always neglected the sea fogin the study of the duct. In this thesis, two duct cases in the sea fog process that occurredover Yellow Sea from1-3Jun2005and9-11Apr2009are selected and investigated.First, the evolution of the duct patch in the sea fog process and the weather situationwere carefully analyzed by using marine navigational radar, sounding data, satellite imagery,surface observation along the coast and the final analysis data(FNL) from National Center ofEnvironmental Prediction(NCEP). Second, the process and three-dimensional structures ofthe sea fog cases were simulated with high resolution by using Weather Research andForecasting model(WRF). Last, commons and differences between these two duct caseswere further discussed.Observations show that a typical atmospheric duct process occurred over the Yellow Seafrom1-3Jun2005. This duct process was associated with an advection sea fog event and thehorizontal distribution evolutions of the duct and the sea fog were almost unanimously; theanalysis of the FNL data shows that sea fog related to low pressure. While it was a typicaladvection cooling sea fog occurred over the Yellow Sea in9-11Apr2009, it was related tohigh pressure; further research revealed that sea fog was followed by the duct.The results of comparison by the satellite pictures and station observations along thecoast show that WRF with good initial filed provided by three dimensional variational(3DVAR) successfully reproduce the processes of the duct. Based on the modeling results, aclear duct formation mechanism can be drawn as follows: The sea fog related to low pressure in2005originated in the middle part of the Yellow Sea, due to weak inversion and minortemperature gradient, duct did not happen on most of the fog top; following with northern lowpressure moving easterly, the fog area expanded east forward with a structure of thin in westwhile thick in east. Because of the inversion and intense humidity gradient, duct happened onmost of the fog top with a well mixed sea fog as base layer. Over thin and thick sea fog,non-touching-sea-level surface duct and elevated duct exist respectively; influenced by thecold air in the western low pressure, sea fog gradually dissipated towards south with the baselayer getting thinner even vanished, which resulted in the inversion and intense humiditygradient declining. Thus the non-touching-sea-level surface duct was replaced by the weaktouching-sea-level surface duct. Because of the inversion and intense humidity gradientexisted over the Yellow Sea before the sea fog related to high pressure occurred in2009developed, touching-sea-level surface duct and non-touching-sea-level surface duct happenedover the Yellow Sea. The sea fog originated in the western of high pressure and followed byeastern low pressure moving easterly and becoming weak, the fog area expanded northforward. Though the long wave radiation at the top of fog strengthened the inversion, theconvergence shift as the fog area expanded north forward, and the water vapors well mixedresult in the weak humidity gradient on top of the sea fog, weak elevated duct existed on seafog top and even vanished. While the no fog area existed intense non-touching-sea-levelsurface duct; sea fog gradually dissipated for the dry air mass transported in sea fog area, thusthe non-touching-sea-level surface duct is replaced by the touching-sea-level surface duct.Comparing the two duct cases we can find: though two duct cases that happened in2005and2009related to low pressure and high pressure, respectively. The former was followedwith the Yellow sea fog and the latter was followed by Yellow sea fog. Both cases wereadvection sea fog, originated in the eastern low pressure and southwestern high pressure, withthe south wind brought warm air mass to the cold sea surface, the inversion formed, then theadvection sea fog happened. The subsidence effect was a key factor to promote the ductformation, behind the trough and the high-pressure weather system existed subsidencemovement, which increased the gradient of humidity and inversion; sea fog itself had thesame effect on the duct, the turbulence in sea fog and long wave radiation at the top of fogcaused the duct type and height changed directly. In two duct cases, the long and short wave radiation had influence on the distribution and the strength of the inversion, then thedistribution and development of sea fog changed, it indirectly caused changes of the duct’sdistribution, type and strength.