Atmospheric Gravity Waves

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To be moved to the learners area

The atmosphere is basically a fluid that is being acted upon by a force due to the Earth's gravity. Under the influence of gravity, the background gas density decreases exponentially with increasing height, and so does the background gas pressure. The amplitude of the gravity wave increases exponentially with height. The physical interpretation is that in maintaining the vertical flux of wave energy constant, it offsets the decrease of background gas density.

The mechanism of the gravity waves is that when the force of Earth's gravity and the stabilizing restoring force (produced by the atmospheric density gradients) become comparable with compressibility forces, the resultant waves are gravity waves.The waves may be termed internal or surface waves according to whether the vertical wave number is pure real or pure imaginary, respectively. For internal waves, in the high-frequency limit, it behaves like simple sound waves, so it is called acoustic gravity wave. In the low-frequency or high period range (from several minutes to several hours), it is termed as internal gravity wave.

Gravity waves can be generated by many sources like jet streams, tidal waves, tropical cyclones, hurricanes, earthquakes, volcanic eruptions, nuclear explosions and thunderstorms. They propagate vertically and horizontally, dissipate, interact non linearly, and profoundly influence the momentum, energy, and the constituents in the atmosphere (see Fig. 4.10). They can help the mixing of chemical reactions in the atmosphere. Some gravity waves are observed to propagate horizontally for thousands of kilometers. Gravity waves may alter their environment and profoundly affect the circulation of the middle atmosphere on a global basis.

Basically, gravity waves act as a vehicle for energy and momentum transport into the stratosphere and mesosphere. Gravity waves can act to transfer mean horizontal momentum from the ground to levels aloft in the atmosphere or from one layer of the atmosphere to another. Flow over topography can generate stationary gravity waves that break non linearly in the troposphere and lower stratosphere. Such waves transfer momentum from the breaking region to the Earth's surface, and this process is thought to act as a significant drag on the eastward mean winds in the mid latitude troposphere. Other processes (such as convection, jet stream, instabilities, etc.) can produce gravity waves with nonzero horizontal phase speeds and which act to transfer mean momentum between the troposphere and the stratosphere/mesosphere.

The gravity wave-critical layer interaction is known to have several properties which are important in the dynamics of the atmosphere (Hines 1968; Hamilton1998; Nappo 2002). These are the strong coupling between a gravity wave and the mean flow occurring at a critical layer, and the tendency for a gravity wave to develop large amplitudes and small vertical wavelengths near a critical layer. The interaction of a gravity wave with the mean flow near the critical layer results in a severe gravity wave attenuation with much of its energy and momentum being absorbed by the mean flow (Kim et al. 2003

Source: K. Mohanahumar, Stratosphere Troposphere Interactions