The foci of work in this part of StratoClim are (a) to quantify the distribution of natural sources of species that are relevant for the stratospheric composition (with special attention to sulfur containing compounds and VSLS) and better understand how these will be altered by climate change, (b) to improve our understanding of the processes controlling upward transport of trace gases and pollutants in the tropics and the entry of these substances into the stratosphere, (c) to develop numerically efficient modules for CCMs and ESMs for these processes, with focus on the sulfur cycle, aerosol formation and ozone chemistry parameterization, allowing them to be included in these models (WP 5).
Focus (a) will be addressed in cooperation with the project THREAT, which will provide surface emission modules for marine sulfur containing and halogenated species and assess how these emissions will depend on climate change. Results will be used by the StratoClim models to assess the impact of climate change-induced effects on UTS composition. Measurements of sulfur containing and halogenated species from biogenic emissions in the tropical Indian Ocean will be carried out during a planned ship campaign with RV Sonne in July-August 2014. An additional Sonne expedition is planned in the North Pacific during 2015 or 2016, to detect volcanic and marine emissions, and will be coordinated with the StratoClim aircraft campaign. Finally, the role of natural versus anthropogenic sources from sulfur emissions will be assessed.
Work addressing (b) will build on the information obtained from the StratoClim aircraft campaign and possibly other aircraft observations from US and British campaigns coordinated with this campaign, the StratoClim ground station observations (WP 2) and the StratoClim satellite data analysis (WP 3) along with information available from a range of further satellites, including MeghaTropiques, IASI, MIPAS, GOSAT, CALIOP, Cloudsat, ACE, MLS, OMI. The focus here will be on exploiting the new satellite products developed in StratoClim (WP 3), complemented with results from previous EU and US measurement campaigns, e.g., slow upwelling observed during APE-THESEO (Peter et al., 2003; Luo et al., 2003) or overshooting convection studied during SCOUT-O3 (Corti et al., 2008; DeReus et al., 2009).
Several complementary modelling approaches will be used: Lagrangian techniques ranging from multiannual cluster trajectories for identifying convective source regions, detailed microphysical and chemical modelling along trajectories including gas-to-particle conversion, and Lagrangian studies of the mixing properties of the UTS and their effect on pathways into the stratosphere. In this mixing regime, an important focus will be on ice nucleation during cirrus formation in upwelling air, which may be dominated either by nucleation on aerosol particles entering the UTS from below (e.g. via convection) or from above (e.g. the largest stratospheric aerosol particles sedimenting back into the troposphere (Cirisan et al., 2013), possibly delivering ice nuclei of meteoritic origin to the UT (Engel et al., 2013) instead of the normally considered upward transport of ice nuclei. UTS meteorological fields from operational analysis and reanalysis systems will be used and assessed throughout.
Eulerian non-global models, ranging from the cloud resolving scale to local and regional scale will be employed to zoom into regions of particular interest, examples being the campaign region or regions of particular vulnerability and large box experiments to study cloud sources for a distribution of large interacting meso-scale convective systems. A particular focus will be on the simulation of the impacts on the sulfur cycle and aerosol distribution by processes such as convection, clouds and gas to particle conversion. Finally, global CTMs will be used to address questions regarding the global budgets of sulfur species (in particular SO2, COS and DMS) and to investigate the evolution of large stratospheric injections of sulfur species.
Work addressing (c) will be based on the results of work from (a) and (b). Modules and parameterisations describing the processes investigated in (a) an (b) will be developed for CCMs (with a focus on a detailed representation of the processes) and for ESMs (with a focus on numerical efficiency while maintaining the basic physical properties of the processes and their potential links to climate change). This activity will ensure that the improved understanding of key processes results in a representation of them in global climate models and thus contributes to improved climate predictions (WP 5).