黄海春季海雾的年际变化研究

INTERANNUAL CHANGE OF SEA FOG OVER THE YELLOW SEA IN SPRING

利用黄海沿岸有代表性测站的常规观测资料和NCEP/NCAR资料,对黄海春季海雾年际变化进行了分析发现,雾多年份冬季环流减弱、低层流场向黄海为偏南向流入、中低层水汽充足、层结稳定;有雾时气温水温差在0.5—2.2℃范围内,地面风向以SESE为主。分析结果表明,在春季黄海雾形成过程中,高空环流提供了暖湿空气的输送条件,低层流场及地面风场的分布有利于来自西太平洋低纬地区的水汽向黄海海区输送;中低层水汽充沛,层结稳定,水气温差在一定范围内有利于海雾的形成和维持。

Sea fog is sometimes dangerous condition for aviation and ocean shipping. It occurs mostly between April and July every year, with evident interannual change. It can provide reliable basis for short-range climatic forecast, in our case, to understand the climatic condition of sea fog occurrence over the Yellow Sea. Because of the data insufficiency in the area, representative conventional observations along the Yellow Sea coast including Chaoliandao, Qingdao, Haiyangdao were used with NCEP/NCAR data to analyze the interannual change of sea fog over the Yellow Sea in spring. Good correlation was revealed in Chaoliandao, Qingdao and Haiyangdao by statistical analysis, which shows that the interannual change of the Yellow Sea fog is consistent in space, and the climatic background that sea fog occurs across the Yellow Sea is coherent. Therefore, Chaoliandao was taken as the representative station and the data there was used to analyze the interannual change. Because spring time is a transitional stage of atmospheric circulation from winter to summer, and data in May are coherent to the data of whole spring, so May was taken as the representative of spring to analyze the interannual change of sea fog. Abnormity can be evaluated in different scales by the relation between anomaly and standard deviation based on abnormal standard presented by WMO (Word Meteorological Organization). Based on the standardized value, if it is greater than or equal to 1.2, more foggy days year is defined, if less than or equal to-1.2, less foggy days year is considered. The authors found that the climatic conditions favoring the fog formation are weak winter circulation, northward trasportaion of ample lower level vapors and stable stratification. When sea fog occurs, wind direction is range from S to ESE and the temperature difference between air and sea is range from 0.5℃ to 2.2℃. Several conclusions are listed as follows: 1. The interannual change of spring sea fog over the Yellow Sea is related to the variability of atmospheric circulation. Abnomal circulation is accompanied by abnormal foggy year. Weak winter circulation is favorable for the northward transport of warm and wet air current from lower latitude producing more foggy days, and vise versa. 2. The vapor needed by the sea fog formation and maintenance comes mostly from lower latitude of the Pacific instead of locally provided. Favorable stream field in the lower level provides richer vapor to the Yellow Sea and generate more foggy days, and vise versa. 3. In the years of more foggy days, air stratification is stable, and the middle and low levels are moister. The pattern of the east high and west low SLP (surface level pressure) is favorable for moist air transportation from lower latitude and coagulation of the fog in the Yellow Sea. 4. Spring sea fog in the Yellow Sea occurs mostly when wind direction is between S and ESE and wind speed ranges from 2m/s to 10m/s, which is obviously different from the radiation fog. 5. Case study in the paper showed that sea fog is easy to occur when the temperature difference between air and sea is between 0.5℃ and 2.2℃.