The Observational And Numerical Study Of Sea Fog Events Over The Yellow Sea During Spring And Summer

In this thesis, the formation, development and physical process of two sea fog cases over the Yellow Sea which occur on 1 May 2008 and 7 July 2008, are investigated. The analyses of sea fog process are clearer by using new observational data. Modeling results shows that the WRF model has enough ability to document the sea fog process. Then the two cases of sea fog are taken as representations of sea fog occur on spring and summer. The different characters and the influence of thermal effects of SST as underlaying surface especially between spring and summer cases are investigated by using WRF model. The following results are obtained:(1) The weather situations are similar between spring and summer sea fog cases in this thesis when sea fog formation. There is a closed high pressure system over the Yellow Sea both at 1000hPa and 850hPa level in the geopotential topography. The high pressure of spring is caused by local effect, but the high pressure of summer is caused by subtropical high. The inversion layer is formed easily by the downward airflow caused by high pressure system. At the same time the anticyclonic circumfluence brings water vapor from south to the Yellow Sea which provides humidity condition for sea fog formation. There is also a cold area over the Yellow Sea. The water vapor condensate easily and translate into sea fog when temperature goes down.(2) After analyzing buoy observations and modeling results, it is indicated that air temperature is higher than SST obviously before and after sea fog formation whether on spring or summer. The temperature goes down when sea fog occur. Long wave radiation is the main reason for temperature reducing. The difference between SST and air temperature is smaller in spring case. Air temperature is more than SST 0.5℃～1℃. And SST is even higher than air temperature sometimes. But air temperature is always higher than SST 1℃～2℃in summer case, and the difference between SST and air temperature is larger.(3) After analyzing sounding observations and modeling results, it is indicated that there is inversion layer in boundary layer when sea fog formed whether on spring or summer. The inversion layer is higher and stronger in spring case than summer. The trajectory experiment show that air mass at low level comes from south of Yellow Sea which is cooler and moister, and air mass at high level comes from continent west to Yellow Sea which is warmer and drier. As a result the inversion layer formed on the Yellow Sea(5℃～8℃). For this consideration the inversion layer is weaker in summer because air mass both of low and high level come from south of Yellow Sea(1℃～2℃).(4) After analyzing sounding observations and modeling results, it is indicated that the height of sea fog in summer is higher than spring. The height is 200m in spring, and could reach 400m in summer. The content of water vapor in fog area is also higher in summer than spring. It is imply that sea fog in summer develops better than spring. So the temperature in fog area in summer is higher than spring because water vapor releases more latent heat during condensation.(5) After analyzing sounding observations and modeling results, it is indicated that the stability of boundary layer in fog area in summer is weaker than spring. The value of ?θv /?z in fog area could reach more than 0.05K/m in spring but only 0.01K/m in summer. The Richardson number shows that the turbulence layer in fog area is lower which sticking the sea surface in spring while turbulence layer could reach 100m-300m where is the top of the fog area. For the height of fog in spring is quite low, cooling of long wave radiation could reach bottom of fog area. While this effect couldn't happen in summer because of height of fog is higher and turbulence layer is at top of fog. That's the reason why the difference between SST and air temperature in summer is higher than spring.(6) After sensitive experiments of SST, it is indicated that the area of sea fog is larger when SST increases and smaller when SST reduces. The effect of SST is weaker to the height of sea fog. And the effect of SST has relationship with the content of water vapor. In spring case, area of fog reduces and stability weakens when increasing SST, in the opposite area of fog enlarges and stability enhances when reducing SST in the low water vapor condition ( q < 0.5g/kg). The effect of SST to the stability is weak in the high water vapor condition. For this consideration, changes of SST has less effect to sea fog in summer than spring because of content of water vapor is higher than spring.(7) In addition, after sensitive experiments of WRF Parameterizations, a suitable Specifications of WRF Parameterizations for sea fog simulation has been founded.