Identification Of The Beginning Of The Sea-fog Season Over The Yellow Sea And Its Inter-annual Variation

Sea fog is a great harmful weather phenomena on the sea. The north Yellow Sea is a sea where the sea fog occurs frequently near China, so to enhance and control formation and dissipation mechanisms of the sea fog is important to protect the maritime transport and port security there. In such case, if mastering the beginning time of the sea fog season, one can provide reference and guidance to the offshore production operations. Therefore, it is meaningful of making some effort to identify the beginning of the sea-fog season in terms of its important academic significance and economic and social value.By using the atmospheric visibility data of 14 weather stations in Shandong Province from 1979 to 2007, NCEP/NCAR and JRA reanalysis data, the criterion of the beginning season of the Yellow Sea fog at pentad time-scale is put forward in this paper. Also some possible factors which could impact on the criterion are analyzed.The results showed that:(1) The standards of the beginning of the Yellow Sea fog season is established, and a index is defined based on the accumulation of pentad average fog days anomalies, in order to indicate the trend of the frequency of fog days in one year. On the condition that the temperature difference between the sea and air is less than 0℃, when the index of accumulation of pentad average fog days anomalies increased more than two pentads, then the pentad when the index begins to increase and the fog occurs is the beginning of the fog season. According to this standard, the climatic average beginning of the Yellow Sea fog season can be determined at pentad 20.(2) In pentad 21-22, the direction of the prevailing wind over the north-west Yellow Sea is southeast, the humidity advection over the Yellow Sea is positive, and the temperature advection at 925hPa is conducive to the establishment of the inversion layer, all of which are conducive to the formation of advection fog. However, in pentad 18-19, neither obvious southerlies nor positive moisture advection at 925hPa level can be detected over the Yellow Sea, these features are distinctly different from those of the pentad 21-22.(3) The beginning of the fog season varied annually. The years 2007 and 2005 are selected as the early fog-season year and late fog-season year. When compared with each other, the following features can be found. On the condition of preferred sea-air temperature difference and static stability, the stronger southerlies and positive humidity advection before the fog-season begins can transport adequate moisture to the north Yellow Sea, which plays a crucial role in the beginning of the fog-season.(4) The composite analyses of the early and late fog-season years indicate that the constant southerlies and moisture transportation over the Yellow Sea may influence the beginning of the fog-season, which are consistent with those of the above-mentioned typical years. Furthermore, the distribution of the sea surface temperature can not be ignored. When the positive (negative)sea surface temperature (SST) anomalies over the East China Sea are greater than the SSTA over the Yellow Sea, and the positive (negative) the fog season of the northern Yellow Sea starts earlier (later). The negative sea-air temperature difference is the prerequisite for the beginning of the fog season, but it does not affect it obviously. Also, the inversion plays an important role, the inversion is more close to the ground in the relatively early fog season years. And the higher the intensity of the inversion is, the earlier the fog-season begins.