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Section S S index 761-769 of 1376 terms
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specification—See perfect prognosis method.
spectral diffusivity—A form of eddy viscosity or eddy diffusivity that varies with eddy size. For a spectrum of turbulent eddy sizes there is a spectrum of diffusivities. By integrating over all eddy sizes and diffusivities, the total effect of turbulence can be approximated. This is a form of nonlocal turbulence closure and can be shown to be related to transilient turbulence theory.
spectral function—(Or spectrum.) The Fourier representation of a given function, that is, the Fourier transform if the given function is aperiodic, or the set of coefficients of the Fourier series if the given function is periodic. The existence of the spectral function depends on the mathematical behavior of the given function. If it exists, the spectral function will in general be a complex function, having both amplitude and phase. See also continuous spectrum, discrete spectrum.
spectral gap—A wavenumber, wavelength, or frequency band within a Fourier energy spectrum that has a relative minimum of spectral energy. Much of the theoretical development of turbulence in the atmosphere is based on the assumption of a spectral gap between larger-wavelength motions (called mean motions) and small- scale motions (called turbulence). However, a growing body of experimental evidence indicates that there is often not a spectral gap in the atmospheric boundary layer, thereby raising questions about the Reynolds averaging approach that has formed the basis for turbulence theory for the past century.
spectral hygrometer—(Also called optical hygrometer.) A hygrometer that determines the amount of precipitable moisture in a given region of the atmosphere by measuring the attenuation of radiant energy caused by the absorption bands of water vapor. The instrument consists of a collimated energy source, separated by the region under investigation from a detector that is sensitive to those frequencies that correspond to the absorption bands of water vapor. The basis for determining the water vapor concentration is Beer's Law: I/ I0 = exp(−kx), where I is the light intensity after passing through the sample, I0 is the incident intensity, x is the pathlength reduced to some absolute standard like STP, and k is the absorption coefficient. The most useful regions of the electromagnetic spectrum for this purpose lie in the ultraviolet and infrared regions. The most widespread application is the monitoring of very-high- frequency variations in humidity, as the time constant of a spectral hygrometer is typically just a few milliseconds. The use of spectral hygrometers remains mostly restricted to research applications. See Krypton hygrometer, Lyman-alpha hygrometer, differential absorption hygrometer.
spectral interval—(Or spectral band, spectral channel.) The width, generally expressed in wavelength, wavenumber, or frequency, of a particular portion of the electromagnetic spectrum. A given sensor is designed to measure or be sensitive to energy received at the satellite from that part of the spectrum.
spectral irradiance—The irradiance per unit wavelength or wavenumber interval. Units are typically W m−2 μm−1 or W m−2(cm−1)−1.
spectral line—A bright, or dark, line found in the spectrum of some radiant source. The wavelength location of the line is controlled by the physics of the emission (bright line) or absorption (dark line) process involved. See absorption line, emission line.
spectral method—A numerical method for solving differential equations in which the dependent variables are expanded as series of orthogonal basis functions and the original equations are reduced to a set of algebraic equations for the modal amplitudes.
spectral model—A model in which the prognostic field variables are represented as sums of a finite set of spectral modes rather than at gridpoints. The spectral modes may be Fourier modes in the one-dimensional case or double Fourier modes or spherical harmonics in the two-dimensional case. The advantage of a spectral model is that horizontal derivatives can be calculated exactly for the spectral modes represented in the model and thus the model error is confined only to the unrepresented higher spectral modes beyond the model's spectral truncation.
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