Role of Wave-Induced Diffusion and Energy Flux in the Vertical Transport of Atmospheric Constituents in the Mesopause Region

Vertical constituent transport is characterized by solving the continuity equation for a minor species in the presence of waves, turbulence, molecular diffusion, and chemical sources and sinks. Random vertical displacement fluctuations induced by waves contribute to vertical mixing of the atmosphere similar to turbulence. The effective wave diffusivity is proportional to the sensible heat flux. Because waves induce pressure fluctuations, diffusivity can also be expressed in terms of the wave energy and enthalpy fluxes. Waves induce strong down gradient mixing in regions where dissipation is significant and weak up gradient mixing where dissipation is negligible. At midlatitudes wave diffusivity is maximum in the upper mesosphere near 88 km where the annual mean is ~130 m²/s. Waves also enhance molecular diffusion of reactive species in regions of slow chemical production and reduce eddy mixing in regions of fast chemical production. The enhancement/reduction factor is proportional to the perturbed lapse rate variance and varies between 15 and 45% at mesopause heights. Wave-induced chemical transport is related to the chemical production/loss rates and temperature fluctuation variance. The vertical transport of atomic oxygen, meteoric Fe, and meteoric Na is assessed. Analytical predictions compare favorably with models of the chemical loss of O and the meteoric influxes of Fe and Na and with lidar measurements of the vertical fluxes of Fe and Na. Predicted seasonal variations of wave diffusivity at 97.5 km compare favorably with the total diffusivity inferred from satellite observations of neutral density and the O/N₂ ratio in the thermosphere.