Numerical simulation of gravity wave breaking in the lower thermosphere

Abstract

.[1] Numerical simulations are used to study gravity wave (GW) propagation, instability, and breaking in the lower thermosphere. Compressible effects are accounted for via an anelastic formulation of the equations of motion and we employ a realistic description of the background thermodynamic state. An initially low-amplitude, monochromatic GW with horizontal wavelength 60 km and intrinsic frequency N/3.7 is introduced at the lower boundary and allowed to propagate to higher altitudes. The GW steepens as it propagates upward and displays instability and breaking over the altitude range $120-160 km. The effects of momentum deposition due to viscous attenuation and wave breaking are studied by comparing simulations which either include or exclude induced changes to the mean wind. These two cases also bound the range of expected behavior for horizontally localized GW packets. When induced changes to the mean wind are excluded, instability and turbulence occur over a broad altitude range spanning several vertical wavelengths. In contrast, the region of instability and turbulence is confined to a much more limited altitude range when induced mean wind effects are included. Wave breaking and turbulence in this case are largely confined within a shear layer formed by GW momentum transport. In time, the shear layer evolves into a critical level which consumes nearly all of the incident GW energy.Citation: Lund, T. S., and D. C. Fritts (2012), Numerical simulation of gravity wave breaking in the lower thermosphere,

Simulations of large winds and wind shears induced by gravity wave breaking in the mesosphere and lower thermosphere (MLT) region

“…This value is also consistent with the in situ experimental result of α = 2.8 presented by Philbrick et al (1984) and reanalyzed by Weinstock (1988). The maximum degree of super-saturation reported here is also supported by a recent 3-D model in which α = 1.6 (Lund and Fritts, 2012). Super-saturation is possibly a nonlinear effect, but was also present in quasi-linear models (Fritts and Dunkerton, 1984).…”

“…We chose an observed value of 100 m 2 s −1 (Keneshea et al, 1979;Hocking, 1987;Fukao et al, 1994), which produces a GW overturning time of ∼ 2.9 times of τ gw . This is close to previously reported values of ∼ 2.5 times of τ gw from 3-D models Lund and Fritts, 2012). Our numerical experiments also show that the increasing eddy diffusion reduces the overturning time and the magnitudes of winds and wind shears in the MLT region.…”

“…For the eddy diffusion, its magnitude varies in the range of 10-1000 m 2 s −1 in the MLT region, depending on seasons and latitudes (Hocking, 1987;Xu et al, 2009). The proper magnitude of eddy diffusion in the 2-D model is adjusted to obtain a realistic overturning time Fritts et al, 1996;Lund and Fritts, 2012). We chose an observed value of 100 m 2 s −1 (Keneshea et al, 1979;Hocking, 1987;Fukao et al, 1994), which produces a GW overturning time of ∼ 2.9 times of τ gw .…”

The breakdown of the anelastic approximation in rotating compressible convection: implications for astrophysical systems

“…Coupled with the mean ideal gas law the above equations yield 12) where the (non-dimensional) polytropic index is defined by is first observed [25]; these values are related by…”

“…The AE are now a commonly employed tool for theoretical and numerical investigations of both stably stratified gravity wave dynamics [12,13] and unstably stratified convection dynamics [14]. Until recently, however, few investigations have been carried out that make one-to-one comparisons between the NSE and the AE for the case of compressible convection (e.g.…”