Results of two-dimensional and narrow three-dimensional (2-D and 2.5-D) simulations of a gravity wave (GW) packet localized in altitude and along its propagation direction employing a new, versatile compressible model are described. The simulations explore self-acceleration and instability dynamics in an idealized atmosphere at rest under mean solar conditions in a domain extending to an altitude of 260 km and 1,800 km horizontally without artificial dissipation. High resolution in the central 2.5-D domain enables the description of 3-D instability dynamics accounting for breaking, dissipation, and momentum deposition within the GW packet. 2-D results describe responses to localized self-acceleration effects, including generation of secondary GWs (SGWs) at larger scales able to propagate to much higher altitudes. 2.5-D results exhibit instability forms consistent with previous 3-D simulations of instability dynamics and cause SGW generation and propagation at smaller spatial scales to weaken significantly compared to the 2-D results. SGW responses at larger scales are driven primarily by GW/mean flow interactions arising at early stages of the self-acceleration dynamics prior to strong GW instabilities and dissipation. As a result, they exhibit similar responses in both the 2-D and 2.5-D simulations and readily propagate to high altitudes at large distances from the initial GW packet. A companion paper examines these dynamics for an initial GW packet localized in three dimensions and evolving in a representative 3-D tidal wind field.
Dong et al. (2020, https://doi.org/10.1029/2019JD030691) employed a new compressible model to examine gravity wave (GW) self‐acceleration dynamics, instabilities, secondary gravity wave (SGW) generation, and mean forcing for GW packets localized in two dimensions (2D). This paper extends the exploration of self‐acceleration dynamics to a GW packet localized in three dimensions (3D) propagating into tidal winds in the mesosphere and thermosphere. As in the 2D packet responses, 3D GW self‐acceleration dynamics are found to be significant and include 3D GW phase distortions, stalled GW vertical propagation, local instabilities, and SGW and acoustic wave generation. Additional 3D responses described here include refraction by tidal winds, localized 3D instabilities, asymmetric SGW propagation, reduced SGW and acoustic wave responses at higher altitudes relative to 2D responses, and forcing of transient, large‐scale, 3D mean responses that may have implications for chemical and microphysical processes operating on longer time scales.
describes high-resolution observations in the hydroxyl (OH) airglow layer of interactions among adjacent Kelvin-Helmholtz instabilities (KHI). The interactions in this case were apparently induced by gravity waves propagating nearly orthogonally to the KHI orientations, became strong as Kelvin-Helmholtz (KH) billows achieved large amplitudes, and included features named "tubes" and "knots" in early laboratory KHI studies. A numerical modeling study approximating the KHI environment and revealing the dynamics of knots and tubes is described here. These features arise where KH billows are misaligned along their axes or where two billows must merge with one. They bear a close resemblance to the observed instability dynamics and suggest that they are likely to occur wherever KHI formation is modulated by variable wind shears, stability, or larger-scale motions. Small-scale features typical of those in turbulence develop in association with the formation of the knots and tubes earlier and more rapidly than those accompanying individual billows, supporting an earlier conjecture that tubes and knots are commonly major sources of intense turbulent dissipation accompanying KHI events in the atmosphere.
The effects of isothermal stratification strength on vorticity dynamics for single-mode Rayleigh-Taylor instability (RTI) are examined using two dimensional fully compressible wavelet-based direct numerical simulations. The simulations model low Atwood number (A = 0.04) RTI development for four different stratification strengths, corresponding to Mach numbers from 0.3 (weakly stratified) to 1.2 (strongly stratified), and for three different perturbation Reynolds numbers, from 5,000 to 20,000. All simulations use adaptive wavelet-based mesh refinement to achieve very fine spatial resolutions at relatively low computational cost. For all stratifications, the bubble and spike go through the exponential growth regime, followed by a slowing of the RTI evolution. For the weakest stratification, this slow-down is then followed by a re-acceleration, while for stronger stratifications the suppression of RTI growth continues. Bubble and spike asymmetries are observed for weak stratifications, with bubble and spike growth rates becoming increasingly similar as the stratification strength increases. For the range of cases studied, there is relatively little effect of Reynolds number on bubble and spike heights, although the formation of secondary vortices becomes more pronounced as Reynolds number increases. The underlying dynamics are analyzed in detail through an examination of the vorticity transport equation, revealing that incompressible baroclinicity drives RTI growth for small and moderate stratifications, but increasingly leads to the suppression of vorticity production and RTI growth for stronger stratifications. These variations in baroclinicity are used to explain the suppression of RTI growth for strong stratifications, as well as the anomalous asymmetry in bubble and spike growth rates for weak stratifications.
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