Secondary Generation of Gravity Waves Associated with the Breaking of Mountain Waves

Abstract

The generation of small-scale gravity waves associated with the breaking of mountain waves in the stratosphere has been simulated within a fully compressible, nonhydrostatic, two-dimensional numerical model. The model includes the mesosphere and offers sufficiently high spatial resolution to characterize the breaking and generation of gravity waves in the stratosphere. The mean flow is initialized by CIRA86 at 40°N in February. A bell-shaped mountain with 30-km half-width and 1.5-km height is located in the bottom of the modeled domain. The primary wave forced by the mountain propagates into the stratosphere with amplitude increasing with height and subsequently breaks in the lower stratosphere. After the primary wave breaking, significant wave activity is simulated in the stratosphere. These secondary waves are generated at both the upstream and the downstream edges of the breaking zone. Analysis is mostly focused on downstream small-scale waves. The horizontal and vertical wavelengths of the secondary gravity waves are 3?8 km and 3?20 km, respectively, and phase velocities are ?1.5 to +4 m s?1. The amplitudes of the vertical velocity component of the secondary waves are 0.1?0.2 m s?1 at altitudes of about 20 km. Theoretical consideration and model simulation suggest that the winter stratosphere can be a wave duct for small-scale gravity waves once generated there as in the present simulation, because zonal winds are minimized in the winter stratosphere between the height regions of the subtropical jet and the mesospheric jet. It is also suggested that both the convective instability and an instability related with normal-modes act separately in different areas to generate the secondary gravity waves.