Study On The Formation Mechanism Of Sea Fog Over The Yellow Sea Related To High Pressure

The sea fog related to high pressure is a main type of sea fogs over the Yellow Sea. However, research works on modeling and formation mechanisms of this type of sea fog is rare.In this thesis,two sea fog cases of this type are selected and investigated.First, the evolution of sea fog patch, the high pressure and the inversion are carefully analyzed by using satellite data, surface observations and soundings at islands and along coasts,and the final analysis data (FNL) from National Center of Environmental Prediction (NCEP).Second, the process and three-dimensional structures of the sea fog cases are simulated with high resolution by using Weather Research & Forecasting model (WRF), and the formation mechanisms are studied in detail.The effects of advection, turbulence, radiation of long and short wave are investigated. Last, commons and differences between these two sea fog cases and a typical advection sea fog case relative with a warm front are further discussed.Observations show that the formation and development of the sea fog with high pressure are obviously controlled by the high pressure and its change.The differences of the temperature advection at the different heights result in an inversion which develops over the sea prior to the sea fog formation.When the sea fog occurs,the Yellow Sea is controlled by this shallow high pressure, the surface wind speed is almost below 4m/s,and the sea fog area changes with the movement and the strength variation of this high pressure, and most of the fog area locates in the west and north of this high pressure, and the fog area expands when the high gets weaker. The analysis of the FNL data shows that most water vapors contributing to the sea fog are supplied from the local sea.The results of comparison with the satellite picture and station observations show that WRF with good initial filed provided by three dimensional variational (3DVAR) successfully reproduces the processes of the sea fog.Based on the modeling results,a clear fog formation mechanism can be drawn as follows.With the development of mechanic turbulence caused by the vertical shear of the sea surface wind, the sea fog is initially formed under the inversion, and turbulence cooling plays the most important role. Once the sea fog get thicker, the cooling effect of the long wave radiation at the top of fog maintains the sea fog developing, and a suitable cold-air advection also has a cooling effect for the formation of the sea fog against the wind, especially at higher height. After the air temperature of the fog body decreases below sea surface temperature (SST),the ocean surface does not cool the air any longer, and its main role is to provide moisture through evaporation process.When the fog patch is controlled by the cold advection of the high pressure, there is an updraft against the wind due to convergence shift, the cold advection also uplift the bottom of the inversion, so the sea fog top rises too.More liquid water is produced due to the long wave radiation at the top of the sea fog, then they fall down and forward due to gravitation and advection,the sea fog develops down along the wind. When the fog patch is controlled by the warm advection because of the moving of the high pressure, the cooling effect of turbulence near sea surface and the long wave radiation at the top of fog cause the sea fog developing down the wind, the bottom of the inversion fall down due to the downdraft near the center of the high pressure and the warm advection, the top of the sea fog falls down too.In addition, a series of sensitive experiments show the follwing conclusions:the suitable turbulence strength at the sea surface is favorable for the sea fog formation, while too stronger turbulence may dissipate it; the short wave radiation has little influence for the formation, while long wave radiation is a key factor; the SST will influence the formation to some extent.Compared to a rapid changing sea fog with a warm front, because of weaker temperature advection and wind, the expanding and dissipating of these two cases is relatively slow. However, the main physical mechanism of them is still advection cooling effect. Moreover, their formation and development are strongly dependent on a weather system and its changing. Thus it can be said that the sea fog is a product of a changing weather system.