Tropopause and interchange of minor constituents in the upper troposphere and lower stratosphere (UTLS) region

Abstract

In this study, an attempt is made to understand the ‘freeze drying’ process in the tropopause region over India and in associated regions. GPS sonde/radiosonde data collected onboard research vessel Sagar Kanya (SK) and other land stations during two intensive campaigns, namely Bay of Bengal Monsoon Experiment (BOBMEX) and Arabian Monsoon Experiment (ARMEX) have been utilized. In addition, COSMIC/CHAMP satellite data have also been used. It is revealed that on several occasions, low tropopause temperature (<191 K) is observed over the ocean as well as on the land stations covering a wide area. During these campaigns, low tropopause temperature and low outgoing long wave radiation (OLR), which is an indicator of convective activity, do not overlap except in few cases of partial overlapping. Observation of low tropopause temperature close to region of convection suggests that the troposphere to stratosphere exchange of water vapour may be affected over these regions. The low tropopause temperature (<191 K) are observed to be associated with low water vapour mixing ratio indicating that ‘freeze drying’ mechanism may be operating over the area of low temperature indicating the significance of convection. In this context, analysis of AURA/MLS data have shown that during the deep convection, water vapour may reach very fast upward up to the tropopause. Another process commonly known as ‘subtropical break’, which eventually occurs over Indian subcontinent region facilitates the downward transport of minor constituents, such as ozone from lower stratosphere to upper troposphere during winter. SHADOZ network data as well as Indian ozonesonde data provide evidence of downward transport of ozone.