A Comparison of Small‐ and Medium‐Scale Gravity Wave Interactions in the Linear and Nonlinear Limits

Abstract

A 2‐D numerical model is used to compare interactions between small‐scale (SS) (25 km horizontal wavelength, 10 min period) and medium‐scale (MS, 250 km horizontal wavelength, 90 min period) gravity waves (GWs) in the Mesosphere and Lower Thermosphere within three different limits. First, the MS wave is specified as a static, horizontally homogeneous ambient atmospheric feature; second, a linear interaction is investigated between excited, time‐dependent SS and MS waves, and third, a fully nonlinear interaction at finite amplitudes is considered. It is found that the finite‐amplitude wave interactions can cause SS wave breaking aligned with the phase fronts of the MS waves, which induces a permanent mean flow and shear that is periodic in altitude. This impedes SS wave propagation into in the upper thermosphere and dissipation by molecular diffusion, when compared to linear amplitude simulations. Linear cases also omit self‐acceleration‐related instabilities of the SS wave and secondary wave generation, which modulate the MS wavefield. Neither the linear or nonlinear cases resemble the static approximation, which, by reducing a dynamic wave interaction to a static representation that is vertically varying, produces variable momentum flux distributions that depend strongly upon the amplitude and phase of the larger‐scale wave. This is an approximation made by GW parameterization schemes, and results suggest that including time‐dependent effects and feedback mechanisms for interactions between resolved and parameterized waves will be an important area for future investigations especially as general circulation models begin to resolve MS GWs explicitly.

Stability Characteristics of the Mesopause Region Above the Andes

“…Superposition of waves is more likely to increase the occurrence of locally large wind shear. Heale and Snively (2018) suggested that a LF wave has a significant influence in preventing the HF wave momentum and energy from reaching the upper portions of the thermosphere. HF and MF GWs are more likely to break around mesopause with LF GWs present because the HF and MF waves can be strongly critical-level filtered by the strong winds associated with the LF wave.…”

3D Numerical Simulation of Secondary Wave Generation From Mountain Wave Breaking Over Europe

“…The purpose of these three simulations is to ascertain how the static tidal winds affect the shape of the secondary wave spectrum that can propagate into the thermosphere. We note that time‐dependence of the tidal field is omitted from these simulations, the inclusion of which has been shown to reduce effects such as critical level filtering and reflection (Broutman & Young, 1986; Eckermann, 1997; Heale & Snively, 2018; Liu et al., 2014; Ribstein et al., 2015; Senf & Achatz, 2011; Vanderhoff et al., 2008). However, instability and breaking occurs rapidly, and the majority of secondary waves produced are expected to have fast phase speeds and vertical group velocities relative to the dominant semi‐diurnal period of tides.…”

“…This alteration of the frequency can lead the wave to avoid critical level filtering (Heale & Snively, 2015; Huang et al., 2013), while changes in tidal amplitude and phase, for example, can lead to changes in the location of the critical level or cause it to disappear altogether. Numerous studies have shown that critical level filtering is reduced, and transmission increased, once time‐dependence of the background state is considered (Broutman & Young, 1986; Eckermann, 1997; Heale & Snively, 2018; Liu et al., 2014; Ribstein et al., 2015; Senf & Achatz, 2011; Vanderhoff et al., 2008). However, the degree to which this happens depends upon amount the background state changes over the period that a wave propagates through, or interacts with that state.…”