E6 Atmospheric Waves - Internal and Gravity

Topic

Atmospheric gravity waves are disturbances that exist because, in stratified air, buoyancy acts as a restoring force: any air parcel displaced vertically from its equilibrium position experiences a buoyant force pushing it back, and this oscillation between displacement and the corrective force generates the wave. Atmospheric gravity waves occur in both the troposphere and the stratosphere and are generated in three distinct ways: by convection, by terrain features, or by the geostrophic adjustment of the flow. The oscillation frequency of these atmospheric gravity waves cannot take just any value; it is bounded by the Coriolis frequency (marking the low-frequency limit) and the Brunt-Väisälä frequency (marking the high-frequency limit).

One of the three generation mechanisms—terrain features—gives rise to mountain waves; these occur when horizontal airflow strikes and is disturbed by the topography. If the flow striking the terrain is stable and of moderate intensity, the resulting disturbance splits into two types of waves: some gravity waves remain trapped near the surface, forming so-called lee waves, while others propagate vertically upward, moving away from the mountain that generated them.

Vertically propagating waves do not maintain a constant amplitude; as air density decreases with altitude, the wave amplitude must increase to conserve energy. Eventually, the amplitude grows to the point where the isentropic surfaces deformed by the wave fold over on themselves. At this stage, the wave breaks, depositing the momentum it carried into that region of the atmosphere; this momentum deposition causes mountain wave drag—a process that numerical models must parameterize and which slows down the wind jets passing through that same region. This momentum deposition—compounded by the multitude of atmospheric gravity waves breaking at various locations and altitudes—makes atmospheric gravity waves the primary mechanism for transporting momentum from the troposphere to the stratosphere and mesosphere. Such momentum transport is critical to the momentum balance of the middle atmosphere's mean circulation; without atmospheric gravity waves depositing momentum as they break, this mean circulation would lose an essential component that maintains its equilibrium.

Precisely because atmospheric gravity waves are so vital to this momentum balance, the inability to accurately represent them in global models poses a serious problem: their horizontal scale—ranging from ten to one hundred kilometers—falls below the typical resolution of global forecasting models. Consequently, no global model resolves them directly; instead, they must all be parameterized, much like the drag caused by breaking mountain waves. Thus, atmospheric gravity waves—driven by restoring buoyancy; generated by convection, orography, or geostrophic adjustment; and capable of breaking and depositing momentum far from their source—remain, despite their small scale, an indispensable element of the momentum balance across the entire middle atmosphere.

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