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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |  |
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First Edition Preface Second Edition Preface Acknowledgments
Section S S index 311-319 of 1376 terms
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shear–gravity waveA combination of gravity waves and a Helmholtz wave on a surface of discontinuity of density and velocity. If the densities of the lower and upper layers respectively are and ′ and the velocities U and U′, the phase speed c of the shear–gravity wave is where g is the acceleration of gravity and L the wavelength. The motion is unstable if and only if the bracketed quantity is negative; the density difference thus contributes to stability and the velocity difference to instability. Applications have been made to atmospheric frontal surfaces and inversions; perhaps the most successful of these is to the phenomenon of billow clouds. Reasonable atmospheric values for the parameters yield stationary wavelengths of the order of 1 km.
Drazin, P. G., and W. H. Reid, 1981: Hydrodynamic Stability, Cambridge University Press, 14–22.
shear hodographSee hodograph.
shear instabilitySee Kelvin–Helmholtz instability.
shear layerA layer in the atmosphere across which there is a change of wind speed or wind direction.
shear lineIn meteorology, a line or narrow zone across which there is an abrupt change in the horizontal wind component parallel to this line; a line of maximum horizontal wind shear.
shear productionThe generation of turbulence kinetic energy caused by wind shear. This mechanical production term S of the turbulence kinetic energy budget equation is (written in Einstein's summation notation) and is almost always positive, that is, a source. In the fair-weather, turbulent atmospheric boundary layer, the terms associated with horizontal wind usually dominate, allowing the shear production term to be approximated as where ( ,
) are the Reynolds stresses (turbulent kinematic momentum fluxes) in the (x, y) directions, (U, V) are the respective wind components, the prime denotes deviation from the mean, and the overbar denotes an average. The ratio of the buoyant to the negative of the shear production terms is also known as the flux Richardson number and is a measure of dynamic stability.
shear ridgeA ridge formed primarily by shear deformation, usually found along the boundary between fast ice and pack ice.
shear vectorThe derivative of any vector in any spatial direction. The particular direction should be clear from the context.
shear wave1. Same as Helmholtz wave. 2. A wave propagating within an elastic solid.
shearThe variation (usually the directional derivative) of a vector field along a given direction in space. The most frequent context for this concept is wind shear.
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