Stokes Diffusion by Atmospheric Internal Gravity Waves

Abstract

Owing to the stochastic action of Stokes drift induced by a spectrum of noninteracting gravity waves, a parcel of air subject to a spectrum of randomly superposed waves will be displaced, on average, an increasing distance from an initial position as time goes on. The effect of this drift on an ensemble of parcels is to induce a diffusive-like growth in time of the mean-square separation between parcels. We refer to this process as Stokes diffusion. We have calculated trajectories for an ensemble of parcels moving under the influence of a spectrum of gravity waves and have found vertical diffusion coefficients inferred from the dispersion of parcels on the order of 10² m s⁻², which is comparable to the values usually attributed to turbulence. If a constituent is distributed as a function of potential temperature, the dispersion due to conservative waves is not apt to accomplish a significant vertical transport. However, if the constituent has a significant gradient on wave-perturbed potential temperature surfaces, this dispersion might be an important cause of vertical transport in the mesosphere.