The spring Yellow Sea fog：synoptic and air–sea characteristics associated with different airflow paths

The fog occurs frequently over the Yellow Sea in spring（April–May）, a climatical period of Asian monsoon transition. A comprehensive survey of the characteristic weather pattern and the air-sea condition is provided associated with the fog for the period of 1960–2006. The sea fog is categorized by airflow pathways of backward trajectory cluster analysis with the surface observations derived from international comprehensive oceanatmosphere dataset（I_COADS） I_COADS datasets and contemporaneous wind fields from the National Centers for Environmental Prediction（NCEP）/National Center for Atmospheric Research（NCAR） reanalysis. On the basis of the airflow paths, the large-scale lower-tropospheric circulation patterns and the associated surface divergence,the distribution of a vertical humidity, the horizontal water vapor transportation and the air-sea temperature difference are investigated and the major findings are summarized as follows.（1） Four primary clusters of the airflow paths that lead to spring sea fog formation are identified. They are originated from the northwest, east,southeast and southwest of the Yellow Sea, respectively.（2） Springtime Yellow Sea fog occurs under two typical weather patterns： the Yellow Sea high（YSH） and cyclone and anticyclone couplet（CAC）. Each pattern appears by about equal chance in April but the YSH occurrence drops to around one third and the CAC rises to around two third of chance in May.（3） The common feature in the two types of synoptic conditions is that surface divergence center is located over the Yellow Sea.（4） For the YSH type of fog, water vapor comes mainly from local evaporation with a well-defined dry layer present in the lower atmosphere; for the CAC type of fog, however, water vapor comes mainly from areas outside the Yellow Sea with a thick surface layer of high humidity.（5） With the differences in weather patterns and its associated vertical distribution of the humidity and the transportation of water vapor, there are two types of sea fogs. Most fogs of the CAC types are ＂warm＂ fog, while fogs of YSH type have nearly equal chance to be ＂warm＂ and ＂cold＂ fog.

Analysis of Climatic Characteristics of Fog and Haze Weather in Shaoyang in the Past 60 Years and Its Forecast

Based on conventional observation data and NCEP reanalysis data at 10 national basic stations and reference stations of Shaoyang City during 1951-2014,300 cases of typical regional dense fog process appeared in the history were selected. From meteorological factors and weather situation,temporal-spatial distribution characteristics and trend change characteristics of dense fog in Shaoyang were analyzed. The results showed that（ 1） temporal-spatial distribution of dense fog in Shaoyang region was uneven,and interannual variability of fog days had large volatility and bad periodicity; dense fog days in Shaoyang region was obviously more in winter half year and less in summer half year. Dense fog was the most in November and the least in July. Dense fog mostly concentrated during 03： 00-09： 00; appearance time mostly concentrated during 05： 00-07： 30,and dissipation time mostly appeared after 08： 30. Dense fog appeared early and dissipated late in winter half year,and vice verse in summer half year.（ 2） Seen from meteorological factors,ground and 850 h Pa of wind velocity was generally 0-3 m/s,which was all small. Moreover,there existed temperature inversion from ground to 850 h Pa. Relative humidity on dense fog day was larger,and precipitation or cloudy day mostly appeared in prior day.（ 3） There were four kinds of ground weather situation forming dense fog： uniform pressure field type,cold and high pressure bottom type,cold and high pressure rear type,frontal type. Based on grasping change characteristics,rule and formation reason of dense fog,some forecast focus was found.