| Sea fog is a weather phenomenon wherein tiny water droplets suspend in the vicinity of the ocean surface leading to the horizontal atmospheric visibility to below 1 km. The Yellow Sea is one of the most frequent sea fog occurring regions among the coastal seas of China. However, more and more economic losses associated with the impact of sea fog on land, aviation, and marine transportation has been conducted as more and more human activities reach the coastal and marine areas. In the present study, six sea fog cases from 2003 to 2008 have been investigated observationally. The WRFV3 model has been employed to simulate two of these six cases occurring during the period of 8 to 11 March 2005, 6 to 12 July 2008 respectively. The comparisons between the cases have been conducted.Several influencing factors, i.e., horizontal visibility, sea level pressure, temperature, as well as relative humidity, have been analyzed for six sea fog cases over the Yellow Sea. It suggested that according to the variability of the sea level pressure at CT station, six fog cases can be divided into two types: pressure-weakening type and pressure-strengthening type. The former type happened in spring and the latter type in summer. Prevailing southerly winds, accompanied with the well-positioned NPH, may supply a large amount of warm water vapor for the fog formation and maintenance. Two major reasons for the fog dissipation in the present study are wind direction change (i.e., from southerly to northerly) and the weakening atmospheric stability (i.e., the vanish of the inversion layer). The favorable conditions for fog formation and maintenance over the Yellow Sea is as following: (1) 2-10 m/s southerlies, (2) the well positioned NPH (25-35°N, 120-140°E), (3) the inversion layer (7-20℃/km,2-6℃/300m), (4) the south-north SST gradient (4-16℃). These conditions are vital to the formation of advection cooling sea fog over the Yellow Sea.One of the spring fog cases happened during the period of 08 to 11 March 2005 has been simulated and analyzed. It suggested that the low level jet (LLJ) happened during this fog episode played an important role in the formation and dissipation of this case. During the onset and mature stages, the pressure gradient under 925 hPa over the Yellow Sea is extremely large. The southerly LLJ occurred over the Yellow Sea and intensified as the pressure gradient increased. During the dissipation stage, the northerly LLJ under 850 hPa occurred over the Yellow Sea and fastened the dissipation of this case. During the onset period, sea fog extended vertically with the aid of turbulence. The mechanical production from wind shear caused by LLJ generated the turbulence kinetic energy (TKE). Under the suppression of the inversion layer, the fog layer could only reach the bottom of the inversion layer without enough turbulence. In the topography run, the LLJ has been weakened. Consequently, the turbulence has been weakened. Hence, the height of the fog is lower than that in the control run. During the dissipation period, the inversion layer vanished gradually. The mechanical production and the buoyancy term together generated the turbulence. Sea fog became low stratus with the aid of turbulence. The air mass of the inversion layer is from the southern land of China according to the back trajectory analysis. In the topography run, the inversion layer was weakened compared with that in the control run. The intensity of sea fog was weakened. The inversion layer is weakened due to the vanishing of the adiabatic warming effect caused by the removal of the mountain topography in Fujian area. The reason for the weakening of sea fog is that the moisture support has been partly brought into the southern coastal area of China by the southeast wind due to the removal of the mountain in that area.The summer case happened during the period of 6 to 12 July 2008 has been simulated and analyzed. It is found that turbulence plays an important role in the vertical extension of sea fog. During the onset period, turbulence is generated by the mechanical production. During the dissipation stage, the inversion layer vanished and sea fog became low stratus with the aid of turbulence. There exists conditional instability in the lower level atmosphere. The buoyancy term and mechanical production together generated the turbulence. However, TKE is relatively small, ranging from 0.01 to 0.25 m2s-2. The air mass of the inversion layer is from the ocean east off of Taiwan based on the back trajectory analysis. In the topography run, the inversion layer was strengthened, the intensity of sea fog was weakened. The inversion layer is strengthened probably due to the modification of the atmospheric circulation after the removal of the mountain area. The reason for the weakening of sea fog is the same as that in the spring case.Comparisons on synoptic processes, sea fog characteristics, the atmospheric stability, the formation and dissipation mechanisms as well as the topography test results between the spring and summer cases has been conducted. It is found that high pressure dominated over the Yellow Sea in 850 hPa and sea level for both cases and southerly prevails over the Yellow Sea during the onset period. The high pressure of spring case centered south of 30°N, whereas the high pressure of summer case centered over north of 30°N, i.e., the east Yellow sea and the Korean peninsula. The high pressure is stronger in spring case than that in summer case. The pressure gradient over the Yellow Sea is stronger in spring case than that in summer case. Comparing the characteristics of sea fog between spring case and summer case, the fog layer is deeper in summer than that in spring. The general height of sea fog in summer case is about 200 m and it could reach 550 m at some period during the whole fog life. In spring case, the general height is about 100 m and 450 m is the highest level. The cloud water mixing ratio is larger in summer case than that in spring case. Sea fog last longer in summer case than in spring case. The air-sea temperature difference is larger in spring case than that in summer case. The moisture support is from south East Sea for both cases. Comparing the atmospheric stability, it is found that in spring case the stability in the lower level is more stable than that in summer case. The air mass of the inversion layer for spring case is from southern China, whereas it is from the ocean east off of Taiwan island for summer case. There exists LLJ in spring case, but show no evidence of LLJ in summer case. Comparisons on the mechanism between these two cases showed that the turbulence plays the same role in both cases. However, the turbulence is stronger in spring case than that in summer case. The topography test showed that the inversion layer in spring case is weakened, whereas it is strengthened in summer case. The intensity of sea fog is weakened for both cases.
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