Enhanced and Inhibited Gravity Wave Spectra

Abstract

Balloon measurements were used to investigate gravity waves in the upper troposphere and lower stratosphere above the Canadian high Arctic. The amount of gravity wave activity in the stratosphere was found to be related to particular meteorological conditions that influence the generation and propagation of mountain waves. Enhanced wave activity was observed to occur when there was a small change in wind direction in the troposphere and high wind speed at the ground. These conditions correspond to strong wave generation by flow over the ground of upward-propagating waves that do not encounter critical level filtering. Inhibited wave activity was observed when the wind direction changed by more than 180° in the troposphere or when the wind speed was relatively weak at the ground. These conditions correspond to critical level filtering and weak generation of upward-propagating waves. The vertical wavenumber spectrum of perturbation potential energy was enhanced at all resolved scales when the conditions were favorable for upward wave propagation. The average enhanced spectral magnitude increased in proportion to the increase in N2 (buoyancy frequency squared) between the troposphere and stratosphere. When the background atmospheric conditions inhibited upward wave propagation the spectral magnitude did not change between the troposphere and stratosphere, despite a factor of 4 increase in N2. The enhanced spectra are consistent with the current gravity wave paradigm but the inhibited spectra are not so readily explained.