Mixing characteristics within the tropopause transition layer over the Asian summer monsoon region based on ozone and water vapor sounding data

The structure of the tropopause transition layer (TTL) over the Asian summer monsoon (ASM) anticyclone is analyzed on the basis of high-resolution balloon soundings of temperature, ozone, and water vapor at Kunming and Lhasa, China during 11 summer monsoon seasons between 2009 and 2019. Two definitions of the TTL, one based on thermal structure and one based on tracer correlations of O3 and H2O, are analyzed and compared. In tracer–tracer space, the air masses of mixed stratospheric and tropospheric characteristics are identified by using the O3–H2O correlations. The mixed air formed a transitional layer and shows strong spatial variation in the altitude space. Statistical result shows that the altitude range of mixed air parcels spans a layer of approximately 5.5 km depth between 12.5 km and 18 km. This vertical distribution of mixed air masses is basically consistent with thermally-based TTL, which takes the level of minimum stability (LMS) and the cold point tropopause (CPT) as its lower and upper boundaries, respectively. The thermal definition regards the TTL as a thermal transition zone, but does not consider the tracer mixing features between stratosphere and troposphere. Tracer correlations can identify the mixed air mass, which is a basic feature of the TTL, whether they are from the range of the thermally-based TTL, or below the LMS, or above the CPT. Based on the O3–H2O correlations, statistical analysis shows that about 30%–50% of air masses between the LMS and the CPT are identified as mixed. And up to 1–1.5 km above the CPT, a small fraction of air masses is also mixed, which means that air masses from the troposphere can be lifted and mixed into the free stratosphere within the ASM anticyclone region.