Propagation factors influencing observations of resonances of atmospheric gravity waves

Abstract

<p>Observations of the ionosphere with the airglow, GPS-TEC, and HF radar techniques reveal a resonance-kind response of the middle and upper atmosphere to broad-band excitation by earthquakes, volcano eruptions, and convective storms. The resonances occur at such frequencies that an atmospheric wave, which is radiated at the ground level and is reflected from a turning point in the middle or upper atmosphere, upon return to the ground level satisfies boundary conditions on the ground. The "buoyancy" resonances (resonances of atmospheric gravity waves) with periods from several minutes and up to several hours arise in addition to well-known "acoustic" resonances with periods of about 3–4 minutes. The buoyancy resonances occur on the gravity branch of the dispersion relation for the acoustic-gravity waves. Infragravity waves in the ocean covering the same frequency band may serve as an efficient source of excitation of the buoyancy resonances. We have obtained dispersion relations for buoyancy resonances earlier. In this paper we investigate the influence of specific propagation characteristics of the gravity waves (their oblique propagation and dissipative attenuation) on conditions of their observation. We use  asymptotic (WKB and ray tracing) methods to investigate relationship between the gravity wave skip distance and the dimensions of typical infragravity wave packets in the oceans and find that conditions can be met for interaction of the same atmospheric wave packet with the same ocean wave packet. The dissipative attenuation eliminates some of the resonance modes, but still many of them remain intact. We use numerical solutions of the full wave equation to confirm results obtained by asymptotic methods. Calculations of this kind demonstrate a possibility of resonance-like behavior of the gravity waves in situations when partial reflections (caused by extrema of the refractive index) appear in addition to the total reflection. Unlike acoustic resonances, buoyancy resonances exhibit high sensitivity to the wind velocity profile and its variations. Non-stationarity of the atmosphere is an important factor limiting possibilities to observe the buoyancy resonances. Nevertheless, relatively low threshold for meeting all other conditions for their appearance and temporal/geographical diversity of the atmosphere makes it still quite probable to see their manifestations. The resonances correspond to most efficient coupling between the atmosphere and its lower boundary and are promising for detection of such coupling.</p>