Abstract:Using the ground based weather phenomena (fog) observations data of 76 stations over Shaanxi Province, and the monthly re analyzed NCEP/NCAR reanalysis dataset from 1960 to 2010, by menas of the EOF, wavelet, and regression analysis methods, the spatial temporal change characteristics of Shaanxi fogs and associated atmospheric circulation abnormity in the fog prone season are studied. The results show that: (1) Shaanxi fogs have obviously regional distributional feature, with three high and low frequency centers of fog days; rivers have significant influence on fog distribution, but it is not the main cause. (2) The average number of fog days reaches the peak from the middle and late 1980s to 1990s; the number of autumn fog days accounts more than 665% of fog days for the whole year. (3) The SEOF analysis shows that the number of fog days differs from one place to another. From August to October, fogs mainly appear in the north of Guanzhong, reaching the peak in September, while from October to December in the south of Shaanxi and in Guanzhong, reaching the peak in November. (4) The inter annual variability of Shaanxi fog mainly includes two modals: EOF1 and EOF2 (5) The circulation anomalies are associated to EOF1, which shows that SLP and 500 hPa geopotential heights over the land at middle latitudes in East Asia are abnormally higher. Shaanxi lies near the center of anticyclonic circulation over the 850 hPa average wind field, so when the anticyclonic anomalous circulation is in the west north and the northerly abnormal component is stronger, and the water vapor transport from the sea is in the south, fogs in Shaanxi are relatively rare in the north, and abundant in the central and southern Shaanxi.
