Research On The Characteristics Of Temperature Inversion In The Sichuan Basin

The characteristics of temperature inversion in the Sichuan Basin are unique due to its special topography and special location close to the eastern edge of the Tibetan Plateau.For the deep basin of Sichuan Basin,it is necessary to study not only the temperature inversion of the boundary layer,but also the temperature inversion of the troposphere layer to fully understand the atmospheric structure of the entire basin.This paper uses the sounding data of Chengdu Wenjiang Station,Chongqing Shapingba Station,Dazhou Dachuan Station,and Yibin Yibin Station from 2015 to 2018,as well as the air quality index and pollutant concentration data of the ambient air quality monitoring station during the same period.The characteristics of temperature inversion from the boundary layer to the middle-lower troposphere and the multi-layer temperature inversion in the Sichuan Basin were studied to evaluate the effects of different types of temperature inversion and vertical structure on the level of atmospheric pollution and the concentration of various pollutants.And explore the causes of the formation of inversion in the lower troposphere from the atmospheric circulation situation.The main conclusions are as follows:(1)Temperature inversion from the boundary layer to the middle and lower troposphere in the Sichuan Basin are relatively common.It mainly appears in two height ranges,namely the ground to the height of 600 m and the range of 2200-3500 m.The vast majority of inversions occur below 5500m(91.4%).The annual average inversion frequency,thickness and strength are 74.4%,252.2m and 1.3 ?/100 m,respectively.Temperature inversion is the most,strongest and thickest in winter,but the opposite in summer.The temperature inversion at 08:00 is obviously more than 20:00.There is also the characteristic that multiple inversion layers appear simultaneously.(2)The inversion temperature is divided into three categories according to its bottom height.The lower troposphere inversion(LTI)has the highest frequency,followed by surface inversion(SI)and elevated inversion(EI)the least.The high LTI frequency is the unique feature of the Sichuan Basin's temperature inversion characteristics,which is different from the other three heavily polluted areas in China(that is,Beijing-Tianjin-Hebei,Yangtze River Delta,and Pearl River Delta).On average,LTI is the thickest and SI is the thinnest;while SI is the strongest and EI is the weakest.There are seasonal differences in frequency,thickness and intensity of different types of inversion.The diurnal variation of boundary layer temperature inversion(SI and EI)is significant.(3)The multi-layer temperature inversion in the Sichuan Basin is not uncommon— the average frequency of occurrence in 2015-2018 reached 20.1%,and the frequency of multilayer inversion in winter in Chengdu even reached 51.6% — but no attention has been paid in previous studies.Both multi-layer inversion and non-multi-layer inversion occur frequently in winter,but the multi-layer inversion at 20:00 is more than08:00.The frequency,thickness and strength characteristics of different temperature inversion vertical structures have obvious differences.The frequency of four types of multilayer inversion(type I,II,III,and IV mul-tilayer inversion)and three types of general inversion(only SI,only EI,only LTI)are 0.3%,12.2%,7.1%,0.5%,5.7%,5.4%and 43.2%.The two most frequent types — only LTI and type II multi-layer inversion— have a high frequency in autumn and winter,and a low frequency in spring and summer.(4)Temperature inversion has an important impact on the air pollution in the Sichuan Basin.When there is temperature inversion,the incidence of air pollution in the Sichuan Basin is much higher than when there is no temperature inversion.The superposition effect of inversion temperature has a greater impact on atmospheric pollution.Multi-layer inversion(especially type II and IV multi-layer inversion)especially increases the possibility of moderate and heavy pollution.There are significant differences in the effects of different temperature inversion conditions on 6pollutants,and the concentration of pollutants is also related to the thickness and intensity of the temperature inversion.Type I and IV multi-layer inversions have the most obvious inhibitory effect on pollutant diffusion,followed by type II multi-layer inversion and only SI.In multilayer inversion,the type II multi-layer inversion(that is,the configuration of SI + LTI)has a high frequency,thickness and intensity in winter,and its superimposed inhibitory effect is obvious.It should be the focus of attention in the prediction and research of the impact of multilayer inversion on atmospheric pollution in the Sichuan Basin.(5)The analysis of the atmospheric circulation situation on the Tibetan Plateau and the Sichuan Basin shows that the westerly warm advection from the Tibetan Plateau at550 h Pa is an important reason for the formation of LTI in the Sichuan Basin.