TRANSPORT OF PASSIVE TRACERS IN BAROCLINIC WAVE LIFE CYCLES


E. M. Stone and W. J. Randel, National Center for Atmospheric Research, Boulder, Colorado

J. L. Stanford, Department of Physics and Astronomy, Iowa State University, Ames, Iowa


J. Atmos. Sci., 56, 1364-1381 (1999)


Abstract

The transport of passive tracers in idealized baroclinic wave life cycles is studied using output from the NCAR community climate model (CCM2). We simulate two life cycles, LCn and LCs, starting with baroclinically unstable initial conditions similar to those used by Thorncroft et al. (1993) in their study of two life cycle paradigms. LCn and LCs have different initial horizontal wind shear structures which result in distinctive non linear development. In terms of potential vorticity-potential temperature (PV-theta) diagnostics, the LCn case is characterized by thinning troughs which are advected anticyclonically and equatorward, while the LCs case has broadening troughs which wrap up cyclonically and poleward.

Four idealized passive tracers are included in the model to be advected by the semi-Lagrangian transport scheme of the CCM2, and their evolutions are investigated throughout the life cycles. Tracer budgets are analyzed in terms of the transformed Eulerian mean (TEM) constituent transport formalism. Results for both LCn and LCs show transport that is downgradient with respect to the background structure of the tracer field, but with a characteristic spatial structure which maximizes in the middle to high latitudes. For the idealized tropospheric tracers in this study, this represents a net upward and poleward transport that enhances concentrations at high latitudes. These results vary little with the initial distribution of the constituent field. The time tendency of the tracer is influenced most strongly by the eddy flux term, with the largest transport occurring during the non linear growth stage of the life cycle. We also study the transport of a lower-stratospheric tracer, to quantify stratosphere-troposphere exchange for baroclinic waves.