Mixing Processes in the Extra Tropical Stratosphere

The general problem of trace gas mixing in the stratosphere is considered. Mixing is divided into two parts: mixing which occurs as part of the tracer realignment with the flow streamlines, and mixing which occurs as part of the evolution of the fluid flow. The mixing process is fundamentally driven by fluid strain or wind shear. The straining produces trace gas filaments which thin until the molecular viscosity scale is reached. At this point the trace gas comes into a dynamical (and chemical) balance with the ambient flow. The further evolution of the trace gas is highly correlated with dynamical tracers, such as potential vorticity, or with other long lived trace gases. Subsequent mixing of the trace gas follows the mixing of the dynamical tracers. Estimates the time required for the trace gas to reach dynamical equilibrium show that linear shear or even shear variation associated with the observed stratospheric energy spectrum cannot bring the trace gas to the dynamical balance as rapidly as is observed. Nonlinear phenomena producing random strain probably accelerate the collapse of trace gas filaments to viscous scales. This suggests that random strain models may be appropriate at meso and synoptic scales under appropriate conditions.