In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere and the ocean, which gives rise to wind waves.
When a fluid element is displaced on an interface or internally to a region with a different density, gravity will try to restore it toward equilibrium, resulting in an oscillation about the equilibrium state or wave orbit. Gravity waves on an airâ€"sea interface of the ocean are called surface gravity waves or surface waves, while gravity waves that are within the body of the water (such as between parts of different densities) are called internal waves. Wind-generated waves on the water surface are examples of gravity waves, as are tsunamis and ocean tides.
Wind-generated gravity waves on the free surface of the Earth's ponds, lakes, seas and oceans have a period of between 0.3 and 30 seconds (3 Hz to 0.03 Hz). Shorter waves are also affected by surface tension and are called gravityâ€"capillary waves and (if hardly influenced by gravity) capillary waves. Alternatively, so-called infragravity waves, which are due to subharmonic nonlinear wave interaction with the wind waves, have periods longer than the accompanying wind-generated waves.
Atmosphere dynamics on Earth
In the Earth's atmosphere, gravity waves are a mechanism for the transfer of momentum from the troposphere to the stratosphere. Gravity waves are generated in the troposphere by frontal systems or by airflow over mountains. At first, waves propagate through the atmosphere without appreciable change in mean velocity. But as the waves reach more rarefied (thin) 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 studying 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.
Quantitative description
The phase velocity of a linear gravity wave with wavenumber is given by the formula
where g is the acceleration due to gravity. When surface tension is important, this is modified to
where σ is the surface tension coefficient and Ï is the density.
Since is the phase speed in terms of the angular frequency and the wavenumber, the gravity wave angular frequency can be expressed as
The group velocity of a wave (that is, the speed at which a wave packet travels) is given by
and thus for a gravity wave,
The group velocity is one half the phase velocity. A wave in which the group and phase velocities differ is called dispersive.
Shallow water
Gravity waves traveling in shallow water (where the depth is much less than the wavelength), are nondispersive: the phase and group velocities are identical and independent of wavelength and frequency. When the water depth is h,
The generation of ocean waves by wind
Wind waves, as their name suggests, are generated by wind transferring energy from the atmosphere to the ocean's surface, and capillary-gravity waves play an essential role in this effect. There are two distinct mechanisms involved, called after their proponents, Phillips and Miles.
In the work of Phillips, the ocean surface is imagined to be initially flat (glassy), and a turbulent wind blows over the surface. When a flow is turbulent, one observes a randomly fluctuating velocity field superimposed on a mean flow (contrast with a laminar flow, in which the fluid motion is ordered and smooth). The fluctuating velocity field gives rise to fluctuating stresses (both tangential and normal) that act on the air-water interface. The normal stress, or fluctuating pressure acts as a forcing term (much like pushing a swing introduces a forcing term). If the frequency and wavenumber of this forcing term match a mode of vibration of the capillary-gravity wave (as derived above), then there is a resonance, and the wave grows in amplitude. As with other resonance effects, the amplitude of this wave grows linearly with time.
The air-water interface is now endowed with a surface roughness due to the capillary-gravity waves, and a second phase of wave growth takes place. A wave established on the surface either spontaneously as described above, or in laboratory conditions, interacts with the turbulent mean flow in a manner described by Miles. This is the so-called critical-layer mechanism. A critical layer forms at a height where the wave speed c equals the mean turbulent flow U. As the flow is turbulent, its mean profile is logarithmic, and its second derivative is thus negative. This is precisely the condition for the mean flow to impart its energy to the interface through the critical layer. This supply of energy to the interface is destabilizing and causes the amplitude of the wave on the interface to grow in time. As in other examples of linear instability, the growth rate of the disturbance in this phase is exponential in time.
This Milesâ€"Phillips Mechanism process can continue until an equilibrium is reached, or until the wind stops transferring energy to the waves (i.e., blowing them along) or when they run out of ocean distance, also known as fetch length.
See also
- Horizontal convective rolls
- Lunitidal interval
- Lee waves
- Morning Glory cloud
- Rayleighâ€"Taylor instability
- Rogue wave
- Orrâ€"Sommerfeld equation
Notes
- ^ Lighthill, James (2001), Waves in fluids, Cambridge University Press, p. 205, ISBN 9780521010450Â
- ^ Bromirski, Peter D.; Sergienko, Olga V.; MacAyeal, Douglas R. (2010), "Transoceanic infragravity waves impacting Antarctic ice shelves", Geophysical Research Letters (American Geophysical Union) 37 (L02502), Bibcode:2010GeoRL..3702502B, doi:10.1029/2009GL041488.Â
- ^ Phillips, O. M. (1957), "On the generation of waves by turbulent wind", J. Fluid Mech. 2 (5): 417â€"445, Bibcode:1957JFM.....2..417P, doi:10.1017/S0022112057000233Â
- ^ Miles, J. W. (1957), "On the generation of surface waves by shear flows", J. Fluid Mech. 3 (2): 185â€"204, Bibcode:1957JFM.....3..185M, doi:10.1017/S0022112057000567Â
References
- Gill, A. E., "Gravity wave". Atmosphere Ocean Dynamics, Academic Press, 1982.
Further reading
- Nappo, Carmen J. (2012). An Introduction to Atmospheric Gravity Waves, Second Ed. Waltham, Massachusetts: Elsevier Academic Press (International Geophysics Volume 102). ISBNÂ 978-0-12-385223-6.Â
External links
- Koch, Steven; Cobb, III, Hugh D.; Stuart, Neil A., Notes on Gravity Waves â€" Operational Forecasting and Detection of Gravity Waves Weather and Forecasting, NOAA, retrieved 2010-11-11Â
- Gallery of cloud gravity waves over Iowa, retrieved 2010-11-11Â
- Time-lapse video of gravity waves over Iowa, retrieved 2010-11-11Â
- Water Waves Wiki, retrieved 2010-11-11Â
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