SESSION OVERVIEW

Topic 1: Which dynamical and meteorological processes govern the chemical compostition of the mid-latitude UTLS?

Session convenors: Heini Wernli & Horst Fischer
(wernli@uni-mainz.de & hofi@mpch-mainz.mpg.de)

On a large scale (temporally and spatially) the chemical composition of the mid-latitude UTLS is influenced by the downward transport of trace gases via the large-scale stratospheric circulation and the upward transport of trace constitutents from the troposphere by dynamical processes such as frontal uplift and deep convection. Coupling of air masses between the sub-tropical UT and the mid-latitude LS may also be important. Many important details of these transport processes still need to be understood. Analyses of various datasets are now providing insights into the causes of large-scale seasonal and possibly inter-annual variability in transport processes and chemical composition. New and upcoming satellite data are important assets for understanding the role of large-scale dynamical processes in controlling the chemical variability in this region. The extent to which small-scale processes (eg. gravity wave breaking near the tropopause, turbulence in the vicinity of jet-streams, radiative processes associated with upper level clouds and condensation) play a role in governing the composition and exchange within the extra-tropical UTLS is also not well known. In addition, there is increasing evidence that deep convection or convection embedded in frontal systems could be important.