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First Edition Preface Second Edition Preface Acknowledgments
Section G G index 131-139 of 425 terms
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geostrophic adjustment—The process by which an unbalanced atmospheric flow field is modified to geostrophic equilibrium, generally by a mutual adjustment of the atmospheric wind and pressure fields depending on the initial horizontal scale of the disturbance.
geostrophic advection—The transport of atmospheric properties by and in the direction of the geostrophic wind.
geostrophic approximation—1. The assumption that the horizontal wind may be represented by the geostrophic wind. 2. Same as quasigeostrophic approximation.
geostrophic balance—Describes a balance between Coriolis and horizontal pressure-gradient forces. Explicitly, and in which f is the Coriolis parameter, u and v the zonal and meridional components of velocity, x and y the zonal and meridional coordinates, p the pressure, and the density.
geostrophic coordinates—The coordinate system used in the semigeostrophic equations. Geostrophic coordinates (X, Y, Z, T) are related to physical space coordinates (x, y, z, t) by where ug and vg are the eastward and northward geostrophic winds and f is the Coriolis parameter. Through this coordinate transform the semigeostrophic equations arise. The geostrophic approximation is not used as frequently in the semigeostrophic equations as in the quasigeostrophic equations; thus the semigeostrophic equations produce a more accurate representation of certain phenomena, such as fronts and jets.
geostrophic current—A current in which the balance in the horizontal components of the equations of motion is between the horizontal pressure gradient and the Coriolis force. The vertical component is in hydrostatic balance and the pressure increases with depth in proportion to the mass of water above. If pressure is mapped on a level surface (geopotential), then geostrophic flow is parallel to the isobars, with high pressure to the right (left) of the flow in the Northern (Southern) Hemisphere. For the geostrophic balance to hold, the flow must be steady, very weak, large-scale, and friction-free.
geostrophic departure—(Also called geostrophic deviation, ageostrophic wind.) The vector (or sometimes only the magnitude) difference between the real (or observed) wind and the geostrophic wind. In the atmospheric boundary layer, winds can be subgeostrophic due to turbulent drag against the surface. This causes a steady-state geostrophic departure, written here as separate Cartesian components (V − Vg) and (U − Ug), or where (U, V) are horizontal wind components, CD is a drag coefficient, M is total wind speed, (Ug, Vg) are geostrophic winds components, fc is the Coriolis parameter, and zi is depth of the boundary layer. The equations above are only approximate (they assume a slab boundary layer with no entrainment), but illustrate the effect.
geostrophic deviation—Same as geostrophic departure, ageostrophic wind.
geostrophic dividers—An instrument, based on the principle of the geostrophic wind scale, for determining the geostrophic wind from data on a weather map.
geostrophic drag coefficient—The ratio of the Reynolds stress at the surface to the square of the geostrophic wind velocity.
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