An Overview of OCTAV-UTLS (Observed Composition Trends and Variability in the UTLS), a SPARC Emerging Activity

The distribution of tracers in the upper troposphere and lower stratosphere (UTLS) shows large spatial and temporal variability because of interactions of transport, chemical, and mixing processes near the tropopause, as well as variations in the location of the tropopause itself. This strongly affects quantitative estimates of the impact of radiatively active substances, including ozone and water vapour, on surface temperatures, and complicates diagnosis of dynamical processes such as stratosphere troposphere exchange (STE). The community thus faces the challenge of optimally exploiting the existing portfolio of observations to better understand the physical composition of the UTLS, including past long-term changes in trace gas distributions and the processes that control them. The Stratosphere-troposphere Processes And their Role in Climate (SPARC) emerging activity OCTAV-UTLS (Observed Composition Trends and Variability in the UTLS) aims to improve our quantitative understanding of the UTLS’s importance to global climate and the radiation budget. This particularly includes both the impacts of STE processes on variability in UTLS composition and the effects of STE and long-range transport on upper tropospheric ozone and pollution distributions. Achieving these goals requires detailed characterization of existing UTLS measurements from aircraft, ground-based, balloon, and satellite platforms, focusing on understanding how their quality and sampling characteristics (spatial and temporal coverage, resolution) affect their representation of UTLS processes. As a central task for OCTAV-UTLS, we are developing and applying common metrics to compare UTLS data using geophysically-based coordinate systems including tropopause and upper tropospheric jet relative coordinates. This approach leverages the meteorological context to provide a framework for comparing measurements with widely varying instrument specific sampling patterns and to derive maximum information on the relationships of UTLS composition to dynamical variability. In addition to assessing present day measurement capabilities, OCTAV-UTLS will determine gaps in current geographical/temporal sampling of the UTLS region that limit our ability to determine atmospheric composition variability and trends. This talk will provide an overview of the OCTAV-UTLS activity and some examples of initial calculations of geophysically-based coordinates and comparisons of remapped data.