Study On The Formation Mechanism Of Typical Advection Fog Occurred Over The Yellow Sea.

Advection fog is a main type of sea fog which occurs over the Yellow Sea. Its formation and development closely links to the evolution of weather systems, and the existence of inversion often exists prior to its occurrence. Since the inversion layer is a necessary condition for advection fog formation, it is very important to analyze its formation and evolution influenced by weather systems. There is only a few works on the relationship between a specific sea fog and the weather systems including inversion currently. Advection fog occurs over the Yellow Sea is very sensitive to initial conditions, while its formation and evolution is a complex micro-physical process which includes dynamic and thermal effects. Understanding of its formation mechanisms is not deep enough, especially the co action of turbulence and radiation is still not clear.In this thesis, from a lot of cases of typical advection fog over the Yellow Sea, three steady sea fog (occurring during 2-3 Apr 2002,11-13 Apr 2004 and 27-28 Mar 2005, respectively) are closed for study. According to the facts which based on conventional and satellite remote sensing observations, combining the weather situation given by FNL (final analysis data) reanalysis field data, as well as in virtue of the WRF (Weather Research and Forecasting) mesoscale numerical simulation with high resolution, the cause of inversion formation and change which happened prior to the occurrence of sea fog is analyzed, and the relationship between the occurrence of sea fog and the evolution of weather systems is revealed, and the effect of turbulence and radiation process during the sea fog formation and development is explored.According to the detailed analysis of the observed facts, a primary similarity of the fog area evolution, the corresponding changes in weather systems and the formation of inversion layer of the three fog cases is found. They have typical characteristics of stratospheric cooling sea fog. Such as their formation were impacted by the southeast high-pressure system. So the southerly winds over the Yellow Sea resulted in warm advection, leading to warm air mass over cold sea surface. The northern system evolved eastward and southward, which leaded to strong northerly winds to dissipate sea fog. During the northern low pressure and southwest high pressure moved eastward, inversion layer began to form as earlier as 2-3 days before the sea fog occurrence. It first appeared in the area southwest of Shandong Peninsula. The south high pressure system moved over the sea, which caused southwesterly flow of cold air about 300m close to the ground toward the central-north of Yellow Sea, while there also existed warm advection higher than 300m, the inversion layer gradually developed and maintained steadily.The WRF model is employed to perform three control numerical experiments and a series of sensitivity experiments, and a high-quality initial conditions is created by cycling 3DVAR (Three Dimensional Variation) assimilation approach. Contrasting to the visible light cloud and some sounding observations, the simulation results have high reliability. Based on the results of control experiments and some sensitivity ones, it is found that the physical mechanism of sea fog formation and maintenance is very similar after analyzing the details of the mechanism of sea fog formation and development. The sea fog formed at the bottom of the inversion layer over the central-south part of Yellow Sea first, the mechanical turbulence caused by vertical wind shear is the primary mechanism for its formation. Fog field developed to the north as well as to the south, and mainly due to the turbulent mixing made the air cool off in stability layer. Once it formed to a certain thickness, long-wave radiation cooling effect at fog top was becoming main force for their maintain and upward developing. The long-wave radiation can cause the air near the fog top to cool down, and then water vapor is condensed to make the fog develop upward; at the same time, the area of high cloud water appears in the upper part of fog body, then cold air sinked, leading to turbulence movement, so that the fog temperature gradually decreased below than SST. Sea surface becomes relatively warm, it began to heat the bottom of fog, where the cloud water reduced. Simultaneously the sea surface evaporation provides water vapor to maintain sea fog. The results of sensitivity experiments also reveal some effects due to other factors. For example, strong turbulence is not conducive to sea fog formation, solar short radiation contributes significantly to the maintenance of strong stability, so it is more favorable for sea fog formation and large-scale expansion after its closure. When the gradient between air temperature (AT) and SST decreases, the weaker stability is conducive to the formation of sea fog, meanwhile the increased one makes it not. The role of latent heat release can reduce the density of sea fog.Much more stable weather patterns led to these advection fogs maintaining 2-3 days, while other rapid changing weather systems often led to rapid movements of sea fog area. For example, there was a sea fog episode occurred on Mar 9,2005 over the Yellow Sea, which was influenced by a warm front. After comparison, some obvious differences are found as follows:For the Mar 9,2005 episode, the northward strong warm advection caused the inversion layer gradually processed to the north; the sea fog zone occurred close to the bottom of inversion layer and developed to the north complying with the warm air. For these cases, the inversion almost covered the whole range of the Yellow Sea and the intensity was weak. The fog first occurred under the inversion layer over the south central Yellow Sea. There was some distance between fog and the weak warm air from the south. The turbulent and long wave radiation cooling made the sea fog extend both northward and southward.