Global-mean Vertical Tracer Mixing in 3D Planetary Atmospheres
Most current atmospheric chemistry models and haze and cloud formation models for solar system planets and extra-solar planets adopt a one-dimensional (1D) chemical-diffusion approach to approximate the global-mean vertical tracer transport. The physical underpinning of the key parameter in this framework, eddy diffusivity (Kzz), is usually obscure. In this presentation we will talk about our recent work on the vertical tracer transport in 3D atmospheres and analytically predict the global-mean effective eddy diffusivity for 1D models. We find that Kzz strongly depends on the large-scale circulation strength, horizontal mixing due to eddies and waves and local tracer sources and sinks due to chemistry and microphysics. We will also show that the global-mean vertical tracer mixing does not always behave diffusively. If the chemical processes are non-uniformly distributed across the globe (e.g., photochemistry on tidally locked planets), a significant non-diffusive component might lead to a negative Kzz under the diffusive assumption in some situation. Even in the diffusive regime, the traditional assumption in the current 1D models that all chemical species are transported via the same Kzz generally breaks down. Instead, Kzz increases with tracer chemical lifetime and circulation strength but decreases with horizontal eddy mixing efficiency. We also performed numerical simulations of 2D and 3D tracer transport on fast-rotating zonal-symmetric planets and tidally locked exoplanets to confirm our analytical Kzz theory in a wide parameter space. Using species-dependent Kzz, we provide a new analytical theory of the dynamical quenching points for disequilibrium tracers on tidally locked planets from first principles.