In strongly stratified geophysical fluids such as the stratosphere and the ocean, the vertical mixing of tracers is largely due to patches of turbulence that are intermittent in time and space. Heuristic models for this type of mixing are studied which extend that of Dewan (1981), in particular by allowing for imperfect mixing inside the turbulent patches. The theory of continuous-time random walks is used to derive closed-form results for the particle-position statistics. The particle dispersion is shown generally to be diffusive in the long-time limit, with a diffusivity that can be related to the statistics of the turbulent patches (size, lifetime, etc.) and so can be derived from microstructure measurements. The results are used to estimate the vertical diffusivity in the lower stratosphere from high-resolution balloon measurements. For short time, the particle dispersion is non-diffusive; this regime is also analysed, since a time-scale estimate indicates its importance, in particular for stratospheric mixing. A parameterisation of tracer mixing that accurately models the non-diffusive regime is presented; its coupling with advection is investigated.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics