Gravity Waves Appearing in a High-Resolution GCM Simulation

Abstract

Global characteristics of gravity waves in the lower stratosphere are examined using a GCM with high resolution in both the horizontal (T106, corresponding to about 120 km) and the vertical (∼600 m). The bottom boundary condition of the model is that of an aquaplanet with perpetual February sea surface temperature. The simulated gravity waves are in good agreement with mesosphere–stratosphere–troposphere (MST) radar observations at a middle latitude on the gravity wave structure and on the frequency spectra as a function of height. The frequency spectra of simulated wind and temperature fluctuations are also examined as a function of latitude. Large values of spectral density are observed at frequencies higher than the inertial frequency (f) in a weak wind region around 20 km, which is consistent with the characteristics of internal gravity waves. An isolated peak is observed near f for horizontal wind spectra at latitudes higher than 10°, while the energy is distributed in a wide range of frequency at lower latitudes where f approaches zero. Further analysis is performed of those fluctuations having periods shorter than 24 h and those having vertical wavelengths smaller than 5 km. These are frequently analyzed as gravity waves using observation data. The distribution of energy and momentum fluxes in the latitude–height section is examined. The result indicates that short-period waves mostly propagate upward and poleward from the equatorial region. The wave energy reaches about 50° lat at the 27-km altitude. A negative (positive) maximum of vertical flux of meridional momentum ( ) is seen above the subtropical jet in the Northern (Southern) Hemisphere for small vertical-scale gravity waves. This is consistent with the preferred equatorward propagation of the wave indicated by a statistical analysis based on MST radar observations. The ratio of potential to kinetic energy maximizes over the equator and decreases poleward. The Eliassen–Palm flux divergence associated with gravity waves is decelerative above the subtropical jet, albeit small.