Use of on-line tracers as a diagnostic tool in general circulation model development 2. Transport between the troposphere and stratosphere

The effect of altering the vertical resolution of a general circulation model and processes associated with it are used to investigate mixing between the troposphere and stratosphere. Four on-line tracers are employed: chlorofluorocarbon-11 and SF6 for mixing from the troposphere into the stratosphere, Rn-222 for vertical mixing within the troposphere, and C-14 for mixing from the stratosphere into the troposphere. Four standard models are tested, with varying vertical resolution, gravity wave drag, and location of the model top, and additional subsidiary models are employed to examine specific features. The results show that proper vertical transport between the troposphere and stratosphere in the Goddard Institute for Space Studies models requires lifting the top of the model considerably out of the stratosphere and including gravity wave drag in the lower stratosphere. Increased vertical resolution without these aspects does not improve tropospheric-stratospheric exchange. The transport appears to be driven largely by the residual circulation within the stratosphere; associated Eliassen-Palm flux convergences require both realistic upward propagating energy from the troposphere and realistic pass-through possibilities. A 23-layer version with a top at the mesopause and incorporating gravity wave drag appears to have reasonable stratospheric-tropospheric exchange in terms of both the resulting tracer distributions and atmospheric mass fluxes.