Geostrophic wind

The geostrophic wind is the theoretical wind that would result from an exact balance between the Coriolis force and the pressure gradient force. This condition is called "geostrophic balance." The geostrophic wind is directed parallel to isobars (lines of constant pressure at a given height). This balance seldom holds exactly in nature. The true wind almost always differs from the geostrophic wind due to other forces such as friction from the ground or the centrifugal force from curved fluid flow. Thus, the actual wind would equal the geostrophic wind only if there were no friction and the isobars were perfectly straight. Despite this, much of the atmosphere outside the tropics is close to geostrophic flow much of the time and it is a valuable first approximation.

Origin

Air naturally moves from areas of high pressure to areas of low pressure, due to the pressure gradient force. As soon as the air starts to move, however, the Coriolis force deflects it due to the rotation of the earth. The wict|deflection is to the right in the northern hemisphere, and to the left in the southern hemisphere. As the air moves from the high pressure area, its speed increases, and so does the deflection from the Coriolis force. The deflection increases until the Coriolis and pressure gradient forces are in geostrophic balance, at which point the air is no longer moving from high to low pressure, but instead moves along an isobar, a line of equal pressure (note that this explanation assumes that the atmosphere starts in a geostrophically unbalanced state and describes how such a state would evolve into a balanced flow. In practice, the flow is nearly always balanced. The geostrophic approximation has no predictive value since it does not contain any expression for change: it is purely diagnostic). The geostrophic balance helps to explain why low pressure systems spin counterclockwise and high pressure systems spin clockwise in the northern hemisphere (and the opposite in the southern hemisphere).

Geostrophic currents

Flow of ocean water is also largely geostrophic. Just as multiple weather balloons that measure pressure as a function of height in the atmosphere are used to map the atmospheric pressure field and infer the geostrophic wind, measurements of density as a function of depth in the ocean are used to infer geostrophic currents. Satellite altimeters are also used to measure sea surface height anomaly, which permits a calculation of the geostrophic current at the surface. Geostrophic flow in air or water is a zero-frequency inertial wave.

Limitations of the Geostrophic approximation

The effect of friction, between the air and the land, breaks the geostrophic balance. Friction slows the flow, lessening the effect of the Coriolis force. As a result, the pressure gradient force has a greater effect and the air still moves from high pressure to low pressure, though with great deflection. This explains why high pressure system winds radiate out from the center of the system, while low pressure systems have winds that spiral inwards.

The geostrophic wind neglects frictional effects, which is usually a good approximationFact|date=June 2008 for the synoptic scale instantaneous flow in the midlatitude mid-troposphere. Although ageostrophic terms are relatively small, they are essential for the time evolution of the flow and in particular are necessary for the growth and decay of storms.

Governing formula

Assuming geostrophic balance, the geostrophic wind components (u_g,v_g) on a constant-pressure surface can be derived as:

: u_g = - {g over f} {partial Z over partial y}

: v_g = {g over f} {partial Z over partial x}

where "g" is the acceleration due to gravity (9.81 m.s-2), "f" is the Coriolis parameter (approximately 10−4 s−1, varying with latitude) and "Z" is the geopotential height of the constant pressure surface. The validity of this approximation depends on the local Rossby number. It is invalid at the equator, because "f" is equal to zero there, and therefore generally not used in the tropics.

Other variants of the equation are possible; for example, the geostrophic wind vector can be expressed in terms of the gradient of the geopotential height Φ on a surface of constant pressure:

: overrightarrow{V_g} = {hat{k} over f} imes abla_p Phi

See also

*Geostrophic current
*Thermal wind
*Gradient wind
*Prevailing winds

External links

* [http://atmos.nmsu.edu/education_and_outreach/encyclopedia/geostrophic.htm Geostrophic approximation]
* [http://nsidc.org/arcticmet/glossary/geostrophic_winds.html Definition of geostrophic wind]
* [http://www.met.tamu.edu/class/ATMO151/tut/windpres/wind8.html Geostrophic wind description]


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • geostrophic wind — noun a) Meteorology. A wind whose direction and speed are determined by a balance of the horizontal pressure gradient force and the force due to the earths rotation to the left in the northern hemisphere and to the right in the southern… …   Wiktionary

  • geostrophic wind —   wind blowing parallel to isobars because of deflection of the pressure gradient force by the Coriolis Force …   Geography glossary

  • geostrophic wind — noun : a wind whose direction and speed are determined by a balance of the pressure gradient force and the force due to the earth s rotation * * * a wind whose velocity and direction are mathematically defined by the balanced relationship of the… …   Useful english dictionary

  • geostrophic wind — a wind whose velocity and direction are mathematically defined by the balanced relationship of the pressure gradient force and the Coriolis force: conceived as blowing parallel to isobars. Cf. gradient wind. [1915 20] * * * …   Universalium

  • geostrophic wind — That horizontal wind velocity for which the Coriolis acceleration exactly balances the horizontal pressure or gradient force. Pressure gradient force causes air parcel to accelerate. Coriolis begins deflecting air to the right. Coriolis increases …   Aviation dictionary

  • geostrophic wind speed — The speed of a geostrophic wind calculated from the pressure gradient, air density, rotational velocity of the earth, and latitude. The calculation ignores the curvature of the wind’s path. A geostrophic wind is proportional to the pressure… …   Aviation dictionary

  • geostrophic wind level — noun (Meteorology). The lowest level at which the wind becomes geostrophic. In practice, the geostrophic wind level is between 1.2 kilometers (3,928 feet) and 1.6 kilometers (5,238 feet). This wind level probably marks the upper limit of… …   Wiktionary

  • geostrophic wind speed — /dʒiəˈstɒfɪk/ (say jeeuh stofik) noun the speed of the wind calculated from the pressure gradient, the air density, the rotational velocity of the earth, and the latitude, but neglecting the curvature of the path of the air …   Australian English dictionary

  • Wind shear — Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere. Wind shear can be broken down into vertical and horizontal components, with… …   Wikipedia

  • Wind — For other uses, see Wind (disambiguation). Wind, from the …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.