Gravity Wave Dynamics in a Mesospheric Inversion Layer: 2. Instabilities, Turbulence, Fluxes, and Mixing

Abstract

A companion paper by Fritts, Laughman, et al. (2017) employed an anelastic numerical model to explore the dynamics of gravity waves (GWs) encountering a mesospheric inversion layer (MIL) having a moderate static stability enhancement and a layer of weaker static stability above. That study revealed that MIL responses, including GW transmission, reflection, and instabilities, are sensitive functions of GW parameters. This paper expands on two of the Fritts, Laughman, et al. (2017) simulations to examine GW instability dynamics and turbulence in the MIL; forcing of the mean wind and stability environments by GW, instability, and turbulence fluxes; and associated heat and momentum transports. These direct numerical simulations resolve turbulence inertial‐range scales and yield the following results: GW breaking and turbulence in the MIL occur below where they would otherwise, due to enhancements of GW amplitudes and shears in the MIL. 2‐D GW and instability heat and momentum fluxes are ~20–30 times larger than 3‐D instability and turbulence fluxes. Mean fields are driven largely by 2‐D GW and instability dynamics rather than 3‐D instabilities and turbulence. 2‐D and 3‐D heat fluxes in regions of strong turbulence yield small departures from initial T(z) and N2(z) profiles, hence do not yield nearly adiabatic “mixed” layers. Our MIL results are consistent with the relation between the turbulent vertical velocity variance and energy dissipation rate proposed by Weinstock (1981) for the limited intervals evaluated.

On the short-term variability of turbulence and temperature in the winter mesosphere

“…However, it is significant that most turbulent layers were found in this stable region, which also contained large temperature variations, and there was almost no turbulent activity in the weakly stable region (small N 2 ) above. This is in agreement with the modeling in Fritts et al (2018b), who also found most turbulence in the strongly stratified region. The turbulent energy dissipation rate was 1-10 mW kg −1 , in agreement with many previous in situ neutral turbulence measurements in the winter mesosphere (Lübken, 1997;Szewczyk, 2015).…”

“…Almost all of our turbulence layers were observed in the more stable region below 80 km. The perturbations in stability due to GW interactions resemble the individual N 2 (z) profiles shown by Fritts et al (2018b) ness is expected for turbulence over this spatial domain, and is also found in the multi-scale simulations. We also found significant differences in the turbulence strength and layer distribution between the first and second flights.…”

“…The stable region between 68 and 82 km did not have a persistent positive temperature gradient as in major MIL events and as modeled by Liu et al (2000) and Fritts et al (2018b). However, it is significant that most turbulent layers were found in this stable region, which also contained large temperature variations, and there was almost no turbulent activity in the weakly stable region (small N 2 ) above.…”

“…An overturning event in the sodium layer coincident with a near-adiabatic layer between 75 and 80 km suggested that it may have been accompanied by strong downward turbulent heat flux ; however, the new simulations by Fritts et al (2018b) did not produce significant heat fluxes above the temperature inversion. On the other hand, a turbulent layer was observed in this earlier experiment between 88 and 90 km, with energy dissipation rates up to 30 mW kg −1 , observed in neutral and electron fluctuations.…”

“…A strong temperature inversion between 86 and 89 km and quasi-adiabatic layer between 89 and 91 km was strongly turbulent, especially in the adiabatic region. It was concluded that gravity wave breaking and turbulent heating was creating or maintaining the inversion layer, also at odds with the recent modeling results by Fritts et al (2018b). Previously, Liu et al (2000) had carried out a 2-D modeling study of gravity wave-tidal interaction that produced extremely high GW heat fluxes and adiabatic gradients.…”