Intermittent mixing, large-scale advection and stratospheric tracer distributions (formerly paper P6.12)

Small-scale mixing in the stratosphere mainly takes place within intermittent, localised turbulent patches which are caused by gravity-wave breaking or dynamical instabilities. A simple heuristic model of this mixing is developed and employed to examine the effect of small-scale mixing on large-scale distributions of stratospheric tracers. The model is based on random-walk ideas, and it leads to an analogue of the usual advection-diffusion equation in which the diffusion operator is replaced by another operator which takes into account the intermittency of the mixing. By examining the behaviour of tracer distributions in some idealised flows we show how intermittency makes mixing less effective in damping the small-scale tracer fluctuations which arise through advection. This has an impact for stratospheric tracer distributions which we demonstrate using numerical simulations based on observed stratospheric winds for the advection. Specifically, the new model of mixing leads to a tracer spectrum which is shallower, and closer to the observed k^-2 spectrum, than the spectrum obtained with a diffusive parameterisation of the mixing. This indicates, in particular, that tracer filaments significantly thinner than the diffusive scale (10 km or so) can survive dissipation.