ON THE ESTIMATION OF TRANSPORT CHARACTERISTICS OF ATMOSPHERIC DATA SETS

Transport characteristics of an atmospheric general circulation model have been estimated by analysing 30 000 trajectories over several mont hs. These trajectories have been used to construct an advection-diffus ion operator for the zonally-averaged transport of passive tracers. Se veral different methods have been tried for this purpose. The most pro mising seems to be a method where eddy-flux statistics for a large num ber of tracers are generated and used to End an optimized flux-gradien t relationship. An alternative approach based on Lagrangian averaging is shown to be much less robust with respect to changes in the details of the procedure. The meridional circulation of the advection-diffusi on operator is direct almost everywhere, but is very weak outside the tropics. Diffusion coefficients show a quite complicated structure wit h minima near the jet maxima. Certain limitations of the advection-dif fusion model have been identified. In particular, there seems to be pr oblems related to Walker cells. The use of a large number of different tracers has also allowed us to improve on the flux-gradient relations hip by including higher order terms. The relevance of the advection-di ffusion approach for the transport of potential vorticity has been tes ted. The Stokes drift is shown to be very important for potential vort icity fluxes near the tropopause.