Tsunami‐driven gravity waves in the presence of vertically varying background and tidal wind structures

Abstract

Many characteristics of tsunami‐driven gravity waves (TDGWs) enable them to easily propagate into the thermosphere and ionosphere with appreciable amplitudes capable of producing detectable perturbations in electron densities and total electron content. The impact of vertically varying background and tidal wind structures on TDGW propagation is investigated with a series of idealized background wind profiles to assess the relative importance of wave reflection, critical‐level approach, and dissipation. These numerical simulations employ a 2‐D nonlinear anelastic finite‐volume neutral atmosphere model which accounts for effects accompanying vertical gravity wave (GW) propagation such as amplitude growth with altitude. The GWs are excited by an idealized tsunami forcing with a 50 cm sea surface displacement, a 400 km horizontal wavelength, and a phase speed of 200 ms−1 consistent with previous studies of the tsunami generated by the 26 December 2004 Sumatra earthquake. Results indicate that rather than partial reflection and trapping, the dominant process governing TDGW propagation to thermospheric altitudes is refraction to larger and smaller vertical scales, resulting in respectively larger and smaller vertical group velocities and respectively reduced and increased viscous dissipation. Under all considered background wind profiles, TDGWs were able to attain ionospheric altitudes with appreciable amplitudes. Finally, evidence of nonlinear effects is observed and the conditions leading to their formation is discussed.

The Dynamics of Nonlinear Atmospheric Acoustic‐Gravity Waves Generated by Tsunamis Over Realistic Bathymetry

“…At thermospheric heights, there is a notable variability of TAGW phase speeds (Figures 2g–2j) and amplitudes (Figures 4d–4f), which result from bathymetry variations and nonlinear effects (nothing that fluid velocities are comparable to the phase velocity). Although the dispersive nature of the TAGW packet also leads to its spreading (Laughman et al, 2017), this effect takes relatively longer distances prior to becoming clearly discernible.…”

Middle-Scale Ionospheric Disturbances Observed by the Oblique-Incidence Ionosonde Detection Network in North China after the 2011 Tohoku Tsunamigenic Earthquake

“…Firstly, detection of gravity waves in the ionosphere largely depends on the anisotropic coupling process between the neutral atmosphere and the charged ions controlled by the magnetic field orientation [ 37 ]. Secondly, the modulation effect of neutral winds, heat conduction, chemical reactions, and bathymetric effects can result in anisotropic and nonlinear ionospheric behavior of gravity waves [ 10 , 38 , 39 , 40 , 41 ]. Moreover, the relatively sparse observation points in the map would also result in deformation of the observed disturbances.…”

Gravity Wave‐Induced Ionospheric Irregularities in the Postsunset Equatorial Valley Region

“…The assumed isothermal T ( z ) = 240 K at lower altitudes yields N 2 =( g / T )(d T /d z + g / c p ) = 4×10 −4 s −2 , where N is the buoyancy frequency, T b =2 π / N = 314 s is the buoyancy period, g is the gravitational constant, c p is the specific heat at constant pressure, and H = R T / g = 7 km is density‐scale height. The tidal fields are those employed in the study by Laughman et al ().…”