Regional Dense Fog Time Sequence with Its Annual and Decadal Variation in the Sichuan Basin

According to the definition in the Surface Meteorological Observation Manual, dense fog is a kind of disastrous weather phenomenon that the visibility is less than 500 meters caused by abundant tiny water drops floating in the low atmosphere layer. Since the 1980s, the traffic has increased sharply with the developing of the national economy, and the damage caused by dense fog becomes more and more severe in China. As a result, much attention has been paid to studies on dense fog.Based on the available observation data set from 20 meteorology stations located in the Sichuan Basin where dense fog occurs frequently during 1954-2005, the time series of regional dense fog has been constructed under the criterion, during the same observation day there are 5 or more stations where dense fog emerge together, in the Sichuan Basin. Furthermore, the annual and decadal variation of this time series and possible change mechanism are discussed.Results show that most regional dense fogs occur mainly during autumn and winter (September to next February), and account for 92.8% of the annual amount. In summer, dense fog events are not prone to happening, only accout for 1.4%. So it means that the time sequence of dense fog in the Sichuan Basin spans two years, from July to June of next year. Taking the 51 years as a whole, regional dense fog has an increasing tendency with 2.96 days per decade in the Sichuan Basin. During 1954-1976, regional dense fog events are less than the average of 51 years, keeping a negative phase and 18.2 days per year, especially the 1960s is the least ten years, only 15 days per year. On the contrary, dense fogs are frequent and severe through 1980s and 1990s, keeping a positive phase. In recent years, dense fogs in the Sichuan Basin are decreasing, but the number is still more than that in the 1960s. There are significant negative correlation coefficients, not only at inter-decadal scales but also inter-annual scales, between dense fogs and visibilities of background atmosphere with no fog weather in autumn and winter. It explains that in the past 51 years the number of condensation nucleus is the decisive factor on the tendency of dense fogs in the Sichuan Basin. At the same time, the relative humidity of atmosphere is another main factor for dense fog change at inter-decadal scales despite the limit influence weaker at inter-annual scales, and the local temperature warming tendency doesn't exert obvious influence on dense fogs. In recent years, the change of condensation nucleus, relative humidity and temperature lead to the decline of regional dense fogs in the Sichuan Basin.