Gravity Waves and Their Impact on the Atmospheric Flow

Abstract

To leading order the synoptic- and planetary-scale flow of the troposphere can be understood in terms of a zonal-mean circulation interacting with Rossby waves. In the stratosphere above the tropopause and especially in the mesosphere above the stratopause at about 50 km altitude, however, mesoscale gravity waves are a factor not to be ignored. Gravity waves are emitted by various processes in the troposphere, most prominently flow over mountains, convection, and so-called spontaneous imbalance near jets and fronts. From all of these processes gravity waves radiate upward into the middle atmosphere. Measurements and observations typically show strong gravity-wave activity in these altitudes. Their dynamics is a wide field, easily filling books on their own, that this chapter cannot cover by far. Instead, it gives a short overview on the empirical findings why it is relevant, then discusses the fundamental wave motions of an atmosphere at rest, moves on to the description of the interaction between mesoscale gravity waves and a synoptic-scale flow, and finally demonstrates how that theory can explain the observed gravity-wave effects in the middle atmosphere.