Interpretation of gravity waves observed in the mesopause region at Starfire Optical Range, New Mexico: Strong evidence for nonseparable intrinsic (m, ω) spectra

Abstract

Sixty‐five hours of Na lidar observations of vertical and horizontal winds, temperature, and Na density were obtained during eight different nights in 1994 and 1995 at the Starfire Optical Range, New Mexico, using a 3.5 m diameter telescope. The high‐resolution data are used to study the spectra of gravity wave perturbations in the mesopause region. Wave activity was strong during the observations. The average variances of temperature, relative atmospheric density, horizontal wind, and vertical wind were 80 K2, 28 (%)2, 1100 m2/s2, and 4.3 m2/s2, respectively. The temperature, relative density, and horizontal wind spectra are generally consistent with the large body of published measurements and with the predictions of gravity wave theory. The observed temporal frequency (ω0) and vertical wave number (m) spectra of vertical winds are both very shallow. The indices of the ω0 spectra vary between −0.59±0.13 and −1.2±0.09, and the mean value is −0.76. The indices of the m spectra vary between −0.83±0.04 and −1.48±0.03, and the mean value is −1.1. In contrast, the indices of the horizontal wind m spectra vary between −2.8±0.10 and −3.2±0.13 with a mean of −3.0. These large differences imply that the underlying intrinsic spectra are not separable. However, the observed vertical wind m spectra are not consistent with the nonseparable theories which predict index values near +1. By using mathematical and numerical models, we show that the observed m spectra are distorted by Doppler and critical layer effects associated with the height‐varying mean wind field. This distortion is greatest at high values of m and leads to observed vertical wind m spectra which are steeper than the underlying intrinsic spectra. Although the intrinsic spectra are definitely shallower (i.e., indices more positive) than the observations, it is not possible to determine if the measurements are entirely consistent with any of the nonseparable wave dissipation theories.