Multiple Gravity-Wave Breaking Levels

Abstract

It is noted that gravity waves for which |u¯−c| (u¯=mean flow speed, c=wave phase speed) has a sharp minimum in the upper troposphere or lower stratosphere will have decaying amplitudes above this level despite exponentially decreasing mean density. Eventually this decay ceases and growth resumes. Thus, if a gravity breaks below the level of |u¯−c|min, it will cease breaking above this level. Breaking will, however, resume at some higher level. This second breaking level is a lower bound for the level of breaking in the mesosphere since waves too weak to break where |u¯−c|=|u¯−c|min will break at still higher levels in the mesosphere. Explicit calculations show the “second” breaking levels to be close to observed levels of mesospheric gravity wave breaking. Evidence is also cited for wave breaking in the lower atmosphere, and for the importance of this breaking in the momentum budget of the lower atmosphere.