Stratospheric Mixing

Tropospheric air enters the stratosphere mainly in the tropics and this air moves polewards in the stratosphere. By mass conservation this transport circulation must be close by stratospheric air returning to the troposphere in polar and middle latitudes. 

The meridional mass circulation is driven by planetary waves. The wintertime polar vortex provides a transport barrier for this process. Breaking planetary waves form a surf zone around the polar edge. At Rossby wave critical layers strong horizontal mixing occurs.  Stratospheric mixing leads to filament structures with pronounced small-scale structures. Since the structures are of fine horizontal resolution, adequate sufficient horizontal resolution is needed for modelling them. The existence of widespread filamentary structure in the stratosphere has been linked to large-scale nonlinear advection acting on the sharp gradients characterising the polar vortex edge. 

We are developing a barotropic transport model of the stratosphere with a variable high horizontal resolution on a spherical domain.  Horizontal transport and mixing processes of a passive tracer will be investigated.  Because of the nonlinearity of many chemical reactions a good understanding of small-scale structures is important for the modelled Arctic ozone loss. It has been argued that the discrepancy between observed and modelled Ozone loss is the result of insufficient fine horizontal resolution in the models.  A semi-Lagrangian adaptive finite-element algorithm for the passive advection equation has been developed in Behrens (1996) with a grid refinement which maintains high accuracy and minimizes computational costs and memory requirements.