Research On The Physical Mechanism Of The Interannual And Interdecadal Variation Of Foggy Days In Autumn And Winter In Southwest China

Fog is a severe weather hazard.This paper uses statistical analysis methods such as correlation analysis and synthetic analysis to analyze the fog days data of 503 Chinese ground observation stations from 1958 to 2007..By analyzing the temporal and spatial characteristics of fog days in autumn and winter in China,it is found that the frequency of fog days in Southwest China is higher than that in other regions of China,and there is a significant climate variability.The variability is mainly reflected in the interannual and interdecadal scales,and there are significant differences in the climatic conditions of fog days in different scales.At the same time,there are differences in the climatic conditions of interannual fog days in Southwest China under different interdecadal backgrounds.The conclusions are as follows:(1)There is a significant correlation between the interdecadal fog days and AMO and PDO in Southwest China.AMO has a negative correlation with interdecadal fog days,while PDO has a positive correlation with it.When AMO is in the negative phase,it can cause north wind anomaly,which is conducive to the decrease of temperature and specific humidity in the lower layers.Moreover,the decrease of humidity is faster than the decrease of temperature,which is easy to cause supersaturation in the lower layers.The cooling rate of temperature in the lower layer is faster than that in the middle and upper layers,which is conducive to atmospheric stability.Therefore,fog days increase in Southwest China.When PDO is in the positive phase,there is anomalous northerly wind from the lower to the upper layers,and its effect on the decrease of temperature and specific humidity in the lower layers is weaker than that of negative AMO,but it is conducive to the stability of the whole layers,so it is also conducive to the increase of fog days in Southwest China.When the negative AMO phase and the positive PDO phase are superimposed,it is very conducive to the increase of fog days in Southwest China,and the corresponding period is 1972-1995.(2)On the interannual scale,when the precipitation in South China increases abnormally,there will be an increase in foggy days on the north side of the precipitation anomaly center.When the warm and humid air flows northward,it is conducive to the phenomenon of warming and humidification in the lower layers of Southwest China,and the humidification rate is faster than the warming rate.The southwest China is prone to supersaturation,and the abnormal downdraft appears on the north side of the abnormal precipitation center,so the fog days in Southwest China increase.When the precipitation in South China decreases abnormally,the fog days in Southwest China decrease due to the above mechanism.Especially in the negative phase of AMO and positive phase of PDO,El Nino intensity is stronger,EP ENSO occurs frequently,and southwest warm and humid air flows northward,which makes southwest fog days show a significant increase in interannual scale.(3)Comparing the physical mechanism of the interdecadal and interannual variation of fog days in Southwest China,it is found that on the interdecadal scale it is easy to form fog days when the temperature and humidity in Southwest China decrease which is caused by the southward cold air flows.The temperature drops faster than the specific humidity so that the air is easily saturated.On the interannual scale,it is easy to form fog days when the temperature and humidity in Southwest China increase which is caused by the northward warm and humid air flows.The humidity increases faster than the specific humidity so that the air is easily saturated.In these two different time scales of climate variability,the physical mechanism of fog days in Southwest China is significantly different,and the annual fog day change is a phenomenon superimposed by multiple scales.This paper provides a basis for understanding the climate change of fog days in Southwest China at different scales,and provides a reference for the future climate prediction of fog days in Southwest China.