Dynamical models of the middle atmosphere for tracer studies

Abstract

The distribution of ozone and other tracers depends on chemical factors and on the atmospheric circulations, which themselves depend, through heat sources and sinks, on the ozone amount. Examples of the sensitivity of ozone to variations in the dynamics are given from a two dimensional model. The total ozone depends greatly on the heating rate in the equatorial low er stratosphere; changes of tens of milli atmosphere centimetres f arise from alteration of only a few tens of kelvins per day in heating. The effects of variations in m om entum flux and mixing-surface slope are also discussed. T he current ability to model dynamics for tracer studies is reviewed. Only three dimensional models provide a consistent formulation; they simulate many observed atmospheric phenomena broadly b u t not in detail. However, the amount of computing needed precludes all but the simplest treatment of chem istry. Two dimensional models offer a valuable com promise. Although these are rather empirical and not adequately based on sound physical principles, good agreement can be found in practice between observed and modelled ozone, winds and temperatures. The calculation of model-dependent meridional circulations in two dimensional models is shown to be important, for which a parametrization of the momentum flux is an essential prerequisite. Some observations are given tha t suggest that a parametrization based on potential vorticity fluxes may be possible. This would be consistent with the usual formulation of tracer transport.