Effect of superposition on measurements of atmospheric gravity waves: A cautionary note and some reinterpretations

Recent studies of gravity waves in the upper stratosphere have produced some interesting, but as yet unexplained, results. The observation and analysis of apparently “monochromatic” waves is not at first sight consistent with the current conception that the wave motions at these heights are due to a spectrum of superposed waves with a wide range of frequencies. Recent studies have suggested that regardless of season or location, inertia gravity waves with intrinsic frequencies around 3 times the inertial frequency appear to dominate the wave field. We address these issues using both digital simulation and analytical analysis, first showing how seemingly “monochromatic” waves can arise naturally from a broad spectrum of waves. Critical reappraisal of the analysis technique used to infer the peak in wave activity near 3 times the inertial frequency shows that this result is not definitive. This peak is demonstrated to be a statistical manifestation of wave superposition. These results in fact indicate azimuthal directionality in wave propagation, an important measurement of a little‐known quantity. Strong directionality in winter and at the equator is revealed. The equatorial results suggest a deficiency in inertia gravity waves and a preponderance of Kelvin wave and/or mixed Rossby‐gravity wave motions.