Gravity wave

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In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two mediums (e.g. the atmosphere or ocean) which has the restoring force of gravity or buoyancy.

When a fluid parcel is displaced on an interface or internally to a region with a different density, gravity restores the parcel toward equilibrium resulting in an oscillation about the equilibrium state. Gravity waves on an air-sea interface are called surface gravity waves or surface waves while internal gravity waves are called internal waves. Ocean waves and tsunamis are examples of gravity waves.

These waves are generated by wind in the oceans over areas known as the fetch. Gravity waves generally have a period of between 1 and 30 seconds (0.033 to 1 Hz). Alternatively, intragravity waves generally have a period between 30 seconds to 5 minutes (0.05 to 0.005 Hz). Infragravity waves can be felt rather than seen as they have a much greater wave energy.

Atmosphere dynamics on Earth

Since the fluid is a continuous medium, a traveling disturbance will result. In the earth's atmosphere, gravity waves are important for transferring momentum from the troposphere to the mesosphere. Gravity waves are generated in the troposphere by frontal systems or by airflow over mountains. At first waves propagate through the atmosphere without affecting its mean velocity. But as the waves reach more rarefied air at higher altitudes, their amplitude increases, and nonlinear effects cause the waves to break, transferring their momentum to the mean flow.

This process plays a key role in controlling the dynamics of the middle atmosphere.

The clouds in gravity waves can look like Altostratus undulatus clouds, and are sometimes confused with them, but the formation mechanism is different.

References

• Dr. Steven Koch, Hugh D. Cobb, III and Neil A. Stuart, "Notes on Gravity Waves - Operational Forecasting and Detection of Gravity Waves Weather and Forecasting". NOAA, Eastern Region Site Server.
• Gill, A. E., "Gravity wave". Atmosphere Ocean Dynamics, Academic Press, 1982.

See also

• Cloud street
• Lee waves
• Infragravity waves