Summer Tibetan Plateau Tropopause Folds Events And Their Impact On Tropospheric Ozone

Tropopause folds are one of the key processes for the transport of stratospheric materials into the troposphere in mid-latitude regions.They can transport high ozone concentration air masses from the stratosphere to the troposphere and affect the regional atmospheric environment.As a hotspot for the occurrence of tropopause folds,whether the concentration of tropospheric ozone in the Tibetan Plateau（TP）is affected by tropopause folds has been a scientific concern in recent years.Based on a tropopause fold event captured by ozone sounding over the TP,this study analyzed the impact of this event on the ozone concentration in the TP troposphere and surface by combining ERA5 reanalysis data,WACCM model data,and WRF simulation.In addition,according to the previous studies,the subtropical high-altitude jet stream is closely related to the occurrence and development of tropopause folds,this article further used ERA5 reanalysis data to analyze the impact of high-altitude westerly jet stream of various positions and intensities in summer on tropopause folds.The WRF-Chem model and AIRS satellite data were used to verify the impact of tropopause folds on tropospheric ozone under different high-altitude westerly jet stream conditions in summer.The main conclusions obtained in this study are as follows:（1）On July 11,2020,a high concentration of ozone was observed at 9.63 km during an ozone sounding released in Minle County（38.44°N,100.65°E）,which is located at the northeastern TP.ERA5 reanalysis data shows that the high-ozone air observed was caused by a tropopause fold event.Both the O₃S simulation from the WACCM model and the passive tracer simulated by the WRF model indicated that the tropopause fold event observed in July 2020 contributed to surface ozone in the western and northern regions of TP.In addition,we further identify all the tropopause folds that occurred over the TP in each July from 2015 to 2020 and quantified their impacts on surface ozone.The results showed that in July the average increase in surface ozone concentration is 15.9 ppbv during the impact period of the tropopause folds.The site most affected by tropopause folds is Shannan City,followed by Nyingchi City.（2）The position index（WJMPI）and intensity index（WJII）of the summer subtropical westerly jet stream both showed significant interannual variations.The statistical results showed that when the position of the high-altitude westerly jet stream was moved,the occurrence location of tropopause folds also changed accordingly;when the intensity was different,the ozone concentration transported downward by tropopause folds varied.When the westerly jet stream was located to the north,tropopause folds mainly affected the tropospheric ozone in the northern part of the TP（north of 40°N）;when the high-altitude westerly jet stream was in the middle,the tropospheric ozone in the northern and central parts of the TP（north of30°N）might be affected;when the high-altitude westerly jet stream was located to the south,tropopause folds could affect the tropospheric ozone in the entire TP,even including its southern edge（north of 25°N）.When the high-altitude westerly jet stream was stronger,the depth of tropopause folds was lower（approximately 290h Pa）,and more ozone concentration was transported downward by tropopause folds,and vice versa.In addition,when the high-altitude westerly jet stream was in the middle or located to the south,the horizontal coverage of tropopause folds was relatively large,that could affect a wider range of troposphere in the central and southern parts of TP.（3）The effect of the intensity of the subtropical westerly jet stream on tropopause fold was verified by changing its intensity on July 11,2020,using the WRF-Chem model.The simulation results showed that as the high-altitude westerly jet stream intensified,the air flows with more high-concentration ozone were transported into the troposphere by tropopause fold,resulting in the increased ozone concentration in the troposphere（overall increase in ozone concentration by approximately 20 ppbv）.When the high-altitude westerly jet stream weakened,relatively less high-concentration ozone air was transported into the troposphere by tropopause fold,resulting in a weaker increase in ozone concentration in the troposphere;and the weaker the high-altitude westerly jet stream,the less the increase in ozone concentration in the troposphere（overall decrease in ozone concentration by approximately 20 ppbv and 24 ppbv,respectively）.Subsequently,three typical tropopause folds events were analyzed using AIRS satellite data,and the results showed that when the high-altitude westerly jet stream was located to the north,tropopause fold mainly affected the tropospheric ozone concentration in the northern part of the TP（41°N）;when the high-altitude westerly jet stream was centered,tropopause fold mainly affected the tropospheric ozone concentration in the northern part of the TP（38°N）;and when the high-altitude westerly jet stream was located to the south,tropopause fold mainly affected the tropospheric ozone air in the central part of the TP（34°N）.