Atmospheric wave
Encyclopedia
An atmospheric wave is a periodic disturbance in the fields of atmospheric variables (like surface pressure
or geopotential height
, temperature
, or wind velocity) which may either propagate (traveling wave) or not (standing wave
). Atmospheric waves range in spatial and temporal
scale from large-scale planetary waves (Rossby wave
s) to minute sound waves. Atmospheric waves with periods which are harmonics of 1 solar day (e.g. 24 hours, 12 hours, 8 hours... etc.) are known as atmospheric tide
s.
effects, for example the obstruction of the flow by mountain ranges like the Rocky Mountains
in the U.S.
or the Alps
in Europe
. Heating effects can be small-scale (like the generation of gravity wave
s by convection
) or large-scale (the formation of Rossby waves by the temperature contrasts between continents and oceans in the Northern hemisphere
winter).
Atmospheric waves transport momentum
, which is fed back into the background flow as the wave dissipates. This wave forcing of the flow is particularly important in the stratosphere
, where this momentum deposition by planetary scale Rossby waves gives rise to sudden stratospheric warming
s and the deposition by gravity waves gives rise to the quasi-biennial oscillation
.
In the mathematical description of atmospheric waves, spherical harmonics
are used. When considering a section of a wave along a latitude
circle, this is equivalent to a sinusoidal shape.
s acting on the air (which is often thought of in terms of air parcel
s when considering wave motion), the types of waves and their propagation characteristics vary latitudinally, principally because the Coriolis effect
on horizontal flow is maximal at the pole
s and zero at the equator
.
The different wave types are:
These are longitudinal or compression waves
. The sound wave propagates in the atmosphere though a series of compressions and expansions parallel to the direction of propagation.
At the equator, mixed Rossby-gravity and Kelvin wave
s can also be observed.
Surface pressure
Surface pressure is the atmospheric pressure at a location on Earth's surface. It is directly proportional to the mass of air over that location....
or geopotential height
Geopotential height
Geopotential height is a vertical coordinate referenced to Earth's mean sea level — an adjustment to geometric height using the variation of gravity with latitude and elevation. Thus it can be considered a "gravity-adjusted height"...
, temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, or wind velocity) which may either propagate (traveling wave) or not (standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
). Atmospheric waves range in spatial and temporal
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
scale from large-scale planetary waves (Rossby wave
Rossby wave
Atmospheric Rossby waves are giant meanders in high-altitude winds that are a major influence on weather.They are not to be confused with oceanic Rossby waves, which move along the thermocline: that is, the boundary between the warm upper layer of the ocean and the cold deeper part of the...
s) to minute sound waves. Atmospheric waves with periods which are harmonics of 1 solar day (e.g. 24 hours, 12 hours, 8 hours... etc.) are known as atmospheric tide
Atmospheric tide
Atmospheric tides are global-scale periodic oscillations of the atmosphere. In many ways they are analogous to ocean tides. Atmospheric tides can be excited by:*The regular day/night cycle in the insolation of the atmosphere...
s.
Causes and effects
The mechanism for the forcing of the wave, for example the generation of the initial or prolonged disturbance in the atmospheric variables, can vary. Generally, waves are either excited by heating or dynamicDynamics (mechanics)
In the field of physics, the study of the causes of motion and changes in motion is dynamics. In other words the study of forces and why objects are in motion. Dynamics includes the study of the effect of torques on motion...
effects, for example the obstruction of the flow by mountain ranges like the Rocky Mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...
in the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
or the Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....
in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
. Heating effects can be small-scale (like the generation of gravity wave
Gravity wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
s by convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
) or large-scale (the formation of Rossby waves by the temperature contrasts between continents and oceans in the Northern hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
winter).
Atmospheric waves transport momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
, which is fed back into the background flow as the wave dissipates. This wave forcing of the flow is particularly important in the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
, where this momentum deposition by planetary scale Rossby waves gives rise to sudden stratospheric warming
Sudden stratospheric warming
A sudden stratospheric warming is an event where the polar vortex of westerly winds in the winter hemisphere abruptly slows down or even reverses direction, accompanied by a rise of stratospheric temperature by several tens of kelvins...
s and the deposition by gravity waves gives rise to the quasi-biennial oscillation
Quasi-biennial oscillation
The quasi-biennial oscillation is a quasi-periodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of 28 to 29 months. The alternating wind regimes develop at the top of the lower stratosphere and propagate downwards at...
.
In the mathematical description of atmospheric waves, spherical harmonics
Spherical harmonics
In mathematics, spherical harmonics are the angular portion of a set of solutions to Laplace's equation. Represented in a system of spherical coordinates, Laplace's spherical harmonics Y_\ell^m are a specific set of spherical harmonics that forms an orthogonal system, first introduced by Pierre...
are used. When considering a section of a wave along a latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
circle, this is equivalent to a sinusoidal shape.
Types of waves
Because the propagation of the wave is fundamentally caused by an imbalance of the forceForce
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
s acting on the air (which is often thought of in terms of air parcel
Air parcel
In fluid dynamics, within the framework of continuum mechanics, a fluid parcel is a very small amount of fluid, identifiable throughout its dynamic history while moving with the fluid flow. As it moves, the mass of a fluid parcel remains constant, while—in a compressible flow—its volume may...
s when considering wave motion), the types of waves and their propagation characteristics vary latitudinally, principally because the Coriolis effect
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...
on horizontal flow is maximal at the pole
Geographical pole
A geographical pole is either of the two points—the north pole and the south pole—on the surface of a rotating planet where the axis of rotation meets the surface of the body...
s and zero at the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
.
The different wave types are:
- sound waves (usually eliminated from the atmospheric equations of motion due to their high frequency)
These are longitudinal or compression waves
Longitudinal wave
Longitudinal waves, as known as "l-waves", are waves that have the same direction of vibration as their direction of travel, which means that the movement of the medium is in the same direction as or the opposite direction to the motion of the wave. Mechanical longitudinal waves have been also...
. The sound wave propagates in the atmosphere though a series of compressions and expansions parallel to the direction of propagation.
- internal gravity waveGravity waveIn fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
s (require stable stratification of the atmosphere) - inertio-gravity waves (also include a significant Coriolis effect as opposed to "normal" gravity waves)
- Rossby waveRossby waveAtmospheric Rossby waves are giant meanders in high-altitude winds that are a major influence on weather.They are not to be confused with oceanic Rossby waves, which move along the thermocline: that is, the boundary between the warm upper layer of the ocean and the cold deeper part of the...
s (can be seen in the troughs and ridges of 500 hPaHPA-Organizations:*Halifax Port Authority, a port authority in Canada*Hamburg Port Authority, the port authority for the Port of Hamburg, Germany*Health Protection Agency, a health organization in the United Kingdom...
geopotential caused by midlatitude cycloneCycloneIn meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth. This is usually characterized by inward spiraling winds that rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale...
s and anticycloneAnticycloneAn anticyclone is a weather phenomenon defined by the United States' National Weather Service's glossary as "[a] large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere"...
s)
At the equator, mixed Rossby-gravity and Kelvin wave
Kelvin wave
A Kelvin wave is a wave in the ocean or atmosphere that balances the Earth's Coriolis force against a topographic boundary such as a coastline, or a waveguide such as the equator. A feature of a Kelvin wave is that it is non-dispersive, i.e., the phase speed of the wave crests is equal to the...
s can also be observed.
Further reading
- Holton, James R.: An Introduction to Dynamic Meteorology, 2004. ISBN 0-12-354015-1