Sources of gravity waves simulated by a high-resolution GCM

We performed an integration of a GCM with high horizontal (T106) and vertical (600m in the upper troposphere and lower stratosphere) resolution to simulate gravity waves. The boundary condition of the GCM is that of an aqua planet with a perpetual February SST condition without longitudinal dependence. The reality of simulated waves was confirmed by comparison with MST radar observations. Statistical characteristics of gravity waves such as momentum and energy fluxes in the meridional cross-section and power spectra as a function of latitude were shown (Sato, Kumakura and Takahashi, 1999, JAS).

The purpose of this study is to identify the sources of the gravity waves appearing in the GCM, especially at middle and high latitudes in the winter lower stratosphere by examining three dimensional phase movement. In the middle latitude region where baroclinic waves appear around the midlatitude jet stream, the zonal propagation of short-period (10-40h) meridional wind components which are considered due mostly to gravity waves, depends on altitudes. The gravity waves propagate eastward between the jet level (about 10 km) and an altitude of 19 km and westward above. The transition level of 19 km corresponds to the edge of theoretically unreachable region of gravity waves generated associated with vigorous tropical convection. A hovmoller diagram of the short-period components above 19 km sometimes show a feature of interference of multiple waves. A movie of meridional cross-section of horizontal divergence components shows that one of the waves above 19km is propagating upward and poleward from the equatorial region and another is propagating downward and equatorward from the stratospheric polar night jet. A movie of the polar stereo map of the horizontal divergence component suggests that gravity waves are generated around the polar vortex in particular the curvature is large due to the modification by planetary-scale waves.