The most important elements of the UTS for surface climate are stratospheric ozone and aerosol, the properties of clouds and distribution of water vapour, as well as the "downward propagation" circulation anomalies from the stratosphere to the troposphere. Work will focus on two areas:
a) improving the understanding of the chemical and dynamical processes that determine the composition of the UTS and the formation, loss and redistribution of ozone, aerosol, water vapour and clouds, and how these processes will alter under climate change;
b) developing model tools that allow to fully include the interactive feedbacks from UTS ozone and aerosol on surface climate and use of these models to produce improved climate predictions.
Work in (a) consists of two sub-areas: (a1) processes that determine the overall flux into the UTS of aerosol precursor gases (in particular total sulfur) and primary aerosols (e.g. mineral or meteoritic) as well as substances that are important for the stratospheric ozone balance (totals of halogens, hydrogen and nitrogen containing compounds); (a2) the dynamical and microphysical processes that result in the redistribution of aerosol, water vapour and ozone in the UTS and the formation of clouds in the upper troposphere (UT). These sub-areas are the basis for the development of new tools in (b).
Flux of species into the stratosphere occurs mostly through the Tropical Tropopause Layer (TTL). Over recent years research has focussed on the processes that deliver halogens (mostly CFCs, halons, methylchloride, methylbromide and still not well constrained contributions of very short lived halogen containing species termed VSLS) and hydrogen containing compounds (mostly water vapour and methane) into the UTS. Worldwide, previous concerted projects focussed on the role of ozone, water vapour and VSLS, such as the EC-funded SCOUT-O3 (Vaughan et al. 2008) and SHIVA projects. With regard to these species, our research can build on the strong basis provided by the outcome of these very successful projects.
But the flux of total sulfur into the stratosphere and the properties of clouds in the UT have not been adequately addressed since the fundamental understanding of the dynamical and transport processes in the TTL emerged in the late 1990s and early 2000s, partly due to the involved complex chemical, microphysical and dynamical interactions in the tropical troposphere and tropopause region. For some of these interactions even a basic understanding is lacking. Overall transport through the TTL is a critical process for the introduction of species into the stratosphere and the processes remain controversial for both gaseous and particulate matter (Randel et al., 2010; Bourassa et al., 2013; Fromm et al., 2013; Vernier et al., 2013).
The flux of air into the stratosphere is not zonally symmetric. A large fraction is related to specific non zonal processes. A convenient way to define the entry of an individual air mass into the stratosphere is by its individual absolute temperature minimum reached during transport through the tropopause. In the “Lagrangian Cold Points” (LCPs) the last contact of the air with the ice phase occurs, i.e., flux into the stratosphere of water vapour and of soluble species co-condensing on the ice is determined by the last gravitational removal of ice particles in the LCPs. The process is complicated by the gravitational removal of ice not necessarily being complete, or by deep convection with overshoots, as discussed below. The key pathways that determine the stratospheric composition are through the Asian Monsoon (AM) Circulation in NH summer (Kremser et al., 2009; Randel et al., 2010) and through convection followed by radiatively driven ascent above the West Pacific (WP) warm pool and the Maritime Continent (MC) in NH winter (see also Fueglistaler et al. 2009). The field and process modelling activities within StratoClim will focus on these regions.
To reach the above mentioned overall goals, StratoClim features 8 Work Packages (WPs):