The dependence of tracer transport on grid refinement

The lifetime of sulphur in the strartosphere depends on volcanic emission parameters and aerosol microphysical processes, as well as on particle transport. These processes interact and determine the particle size and optical depth of the volcanic cloud.
While tracer transport, via advection and turbulence, affects mixing and dilution, it feeds back onto the aerosol concentration and the microphysical processes.  At the same time, the aerosols absorb terrestrial radiation, which heats the stratospheric aerosol layer and affects transport. Thus, all processes interact, feedback to each other and determine the concentration and particle size of the aerosol.
 
To disentangle the role of aerosol microphysical processes and heating from the role of transport in aerosol evolution, we introduced a passive tracer. This allows us to determine the impact of the grid size on stratospheric dynamics and tracer transport.  Therefore, we added emissions of the gas SF6 (sulphur hexafluoride) to ICON-XPP. SF6 is inert in the troposphere and lower stratosphere and is emitted in industrial production. This allows us to compare the simulated distribution of SF6 with observations.

Comparison with observations allows us to better understand the transport processes in the model. Important for the transport of a volcanic cloud is the velocity of the vertical transport in the tropical pipe or the meridional transport to higher latitudes. Since previous studies have shown a dependence of the transport in the tropical pipe on the model grid, we have performed simulations with different horizontal and vertical resolutions to determine the role of the grid. We had to tune the model for the different resolutions. In the process, we obtained different dynamical states in the lower tropical stratosphere, the region of the quasi-biennial oscillation (QBO). For example, we got only easterly or only westerly jets in the lower tropical stratosphere. These different jets allow us to see the effect of the QBO on the transport of SF6.