| Section S | S index | 241-249 of 1376 terms |
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self-healing of atmosphere—Effect in which depletion of ozone at high altitudes is partially compensated for by an increased penetration of solar radiation to lower altitudes, thus increasing the rate of ozone production at the lower altitudes.
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selsyn—1. Designation for a self-synchronous motor system, consisting of a driver motor and one or more remote followers (or repeaters) with armatures remaining in synchronization with that of the driver. 2. Informal Navy designation for a wind-measuring system consisting of a wind vane and a bridled-cup anemometer, both of which are coupled to selsyn drivers and remotely record or indicate by means of the repeaters of the selsyn system.
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semantic network—A labeled, directed graph with nodes representing physical or conceptual objects and labeled arcs representing relations between objects. This permits the use of generic rules, inheritance, and object-oriented programming, which in turn allow the development of meteorological expert systems that can be adapted to more than one location.
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semi-implicit method—A finite difference approximation in which some terms producing time change are specified at an earlier time level. The approximation (fn + 1 − fn − 1)/2Δt = g(fn + 1) + h(fn) (where superscript n denotes a point in time, separated by step Δt from the prior [n − 1] and subsequent [n + 1] discrete time level) is an example of a semi-implicit approximation to df/dt = g(f) + h(f). Semi-implicit approximations may increase computational efficiency when g produces relatively higher frequencies or more rapid time changes in f than does h. See implicit time difference.
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semi-Lagrangian method—A physically motivated numerical technique for solving the advection (transport) equation. In the advective form, DQ/Dt = 0, where D( )/Dt is the total derivative, and “mixing ratio” Q is an invariant along a flow trajectory. By tracing (along the flow trajectory) backward in time to the “departure point”, the value at the “arrival point” can be obtained by an interpolation or a remapping procedure (between the fixed Eulerian grid and a time-dependent distorted Lagrangian grid). Because of the discrete particle–like approach, total mass is generally not conserved. To ensure mass conservation, the semi-Lagrangian method can be formulated with the conservative flux form. The singular particle discretization is replaced by a finite control-volume discretization. Analogous to an Eulerian flux-form formulation, total flux from the upstream direction, computed in the Lagrangian fashion, is used for the prediction of the volume-averaged quantity, which can be the density or a density-weighted mixing ratio–like quantity. Because the size of the time step is not limited by the CFL condition, both the advective-form and the flux-form semi-Lagrangian methods are computationally efficient, particularly in spherical geometry.
Staniforth, A., and J. Cote, 1991: Semi-Lagrangian integration schemes for atmospheric models – A review. Mon. Wea. Rev., 119, 2206–2223.
Lin, S.-J., and R. B. Rood, 1996: Multidimensional flux-form semi-Lagrangian transport schemes. Mon. Wea. Rev., 124, 2046–2070.
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