Turbulence and stress owing to gravity wave and tidal breakdown

TLDR

Turbulence in the upper mesosphere is thought to arise from the unstable breakdown of tides and gravity waves, which should limit wave amplitude growth with height. The study extends previous models to include smaller‑amplitude waves, mean‑wind effects, and momentum deposition, using rocket observations to quantify gravity‑wave momentum and showing that tropospheric winds and mesosphere critical levels constrain wave phase speeds and turbulence generation. The results show eddy coefficients peaking at 50 km in winter and 70–80 km in summer or sudden warmings, with gravity‑wave accelerations of ~100 m s⁻¹ day⁻¹ sufficient to generate the observed warm winter and cold summer mesopauses.

Abstract

It has been suggested (Lindzen, 1967, 1968a, b ; Lindzen and Blake, 1971; Hodges, 1969) that turbulence in the upper mesosphere arises from the unstable breakdown of tides and gravity waves. Crudely speaking, it was expected that sufficient turbulence would be generated to prevent the growth of wave amplitude with height (roughly as (basic pressure) −1/2 ). This work has been extended to allow for the generation of turbulence by smaller amplitude waves, the effects of mean winds on the waves, and the effects of the waves on the mean momentum budget. The effects of mean winds, while of relatively small importance for tides, are crucial for internal gravity waves originating in the troposphere. Winds in the troposphere and stratosphere sharply limit the phase speeds of waves capable of reaching the upper mesosphere. In addition, the existence of critical levels in the mesosphere significantly limits the ability of gravity waves to generate turbulence, while the breakdown of gravity waves contributes to the development of critical levels. The results of the present study suggest that at middle latitudes in winter, eddy coefficients may peak at relatively low altitudes (50 km) and at higher altitudes in summer and during sudden warmings (70–80 km), and decrease with height rather sharply above these levels. Rocket observations are used to estimate momentum deposition by gravity waves. Accelerations of about 100 m/s/day are suggested. Such accelerations are entirely capable of producing the warm winter and cold summer mesopauses.