| The large scale synoptic background, near ground layer weather element, boundary layer structure and microphysical characteristics of a sustained sea fog event that occurred in the north part of the South China Sea on 24～25,march,2007,are analyzed by using the field observation and NCEP re-analysis data, and have a numerical study by the Weather Research Forecast Model (WRF) in the end. The following text is the main conclusions.(1) This sustained sea fog event is accompanied by southwest vortex developing southerly, and warm wet air flow continuously transport into the South China Sea both of them have advantage for the forming and developing of the sea fog. There is a good negative correlativity with relative humidity and visibility.(2) With control of southwest vortex, the aerosphere boundary layer has a feebleness static stabilization. The sea fog can develop and maturate in feebleness static stabilization, temperature inversion and strong stable stratification is not the sine qua non. There is notable difference between fog horizon forming by condensation cooling and typical fog horizon, and there is superadiabatic in condensation cooling layer.(3) In the developing phase, there is a obvious night lower tropospheric jet in the lower boundary layer. In one part, the lower tropospheric jet can choke back lower fog developing vertically, then fog horizon become illegible , and in another part, the higher aerosphere may cooling through dynamic forcing mixing, that is advantage for fog developing vertically.(4) The fully mixing fog occur in the condition of when boundary layer vapor is saturated, the fog horizon vapor condensation and long-wave radiation cooling enhanced. The fully mixing fog layer can reach the height of 600 m.(5) As there is not a strong temperature inversion layer, the fog horizon structure is different between forming form condensation and the typical one. The dry and wet air may enter into fog layer, when wind shear above fog horizon become stronger and air temperature rising, but radiation cooling also can renew fog layer. (6) The profound fog layer can reduce the dissipation effect from solar short-wave radiation. For one part, solar radiation can enhance higher aerosphere temperature and fog-drop vaporize, but air temperature increasing is also in favor of the lower air stabilization, and ground fog layer continue maintaining, until sea fog layer being gnawed from high layer to low layer by solar radiation.(7) In the developing and fully mixing phase, the number density and water content of fog-drop have an inconspicuous change, but the number of big fog-drop and water content reduce obviously, The sea fog-drops represent local features, the microphysical structure of the South China Sea is lower than other areas.(8) The sea surface temperature is under 24℃along the sea fog event, and air water temperature range is between 2℃in the developing and maturating phase.(9) Thought wiping off radiation calculate, we find that day short wave radiation have strong effect on fog dissipation and night long wave radiation have effect on fog layer developing vertically. |
