Stratosphere (ICE-4)

Simulating the Earth system and climate requires numerical solution of the complex, coupled differential equations describing the system. Continuous improvement of the detailed representation of the various dynamical and chemical processes in the climate system in Earth system models is crucial for enhancing the reliability of model projections.

Model development activities at ICE-4 aim at improving the representation of climate-relevant atmospheric processes in models of different complexity. The concentrations and distributions of trace gases in the atmosphere are controlled by transport processes and chemical reactions on various time scales which, in turn, impact radiation and dynamics.

A particular challenge when simulating trace gas transport in the upper troposphere and lower stratosphere is the frequent occurrence of small-scale filaments and steep gradients, for example around the tropopause and at the edge of the polar vortex. These structures are usually not well resolved in global models and related errors can cause biases in simulated climate due to chemistry-climate coupling. To improve the representation of trace gas transport in models, activities at ICE-4 have been focused on developing the Chemical Lagrangian Model of the Stratosphere (CLaMS). In CLaMS, the trace gas transport is formulated in a Lagrangian framework, i.e. on an irregular, adaptive grid which moves with the flow and requires the calculation of a large number of air parcel trajectories at each time step. In addition, CLaMS includes a sophisticated characterization of small-scale atmospheric mixing, related to deformations and shear in the large-scale flow, and modules for chemistry and microphysical processes.

CLaMS can be used as a conceptual trajectory model, in an offline, reanalysis-driven chemistry transport model version or online in a climate model version. This hierarchy of models is applied to analyse global satellite data and high-resolution in-situ aircraft and balloon measurements, as well as to investigate chemistry-climate coupling processes in the coupled Earth system.

CLaMS is also frequently applied for planning the flights of atmospheric research aircraft and balloons. For improving atmospheric trace gas transport in Earth system models, CLaMS has been coupled into the ECHAM MESSy Atmospheric Chemistry (EMAC) model, a state-of-the-art community Earth system model. EMAC allows flexible coupling of different process modules (e.g. for chemistry or transport processes) via the Modular Earth Submodel System (MESSy) infrastructure.

Currently, work is ongoing to carry these developments over to the ICOsahedral Non-hydrostatic (ICON) model. This work is carried out in close collaboration with national and international partners. The application of a Lagrangian transport representation in EMAC has been shown to lead to significant improvements in simulating stratospheric transport, in particular for water vapour in the tropopause region.