- 行业: Weather
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The American Meteorological Society promotes the development and dissemination of information and education on the atmospheric and related oceanic and hydrologic sciences and the advancement of their professional applications. Founded in 1919, AMS has a membership of more than 14,000 professionals, ...
An acoustic radar (sodar) that can determine radial velocity (velocity toward or away from the sodar) of the air by measuring the frequency shift of the returning sound waves that were scattered from regions of turbulent fluctuations of air temperature, compared to the original transmitted frequency. Velocities within individual surface-layer plumes and the bottom of convective thermals can be measured. Most sodar signals are blown away in strong winds, and the sodar also requires acoustic shielding from outside noise such as from roads, towns, and rustling tree leaves. See Doppler effect, acoustic sounder, sodar; compare RASS.
Industry:Weather
A radar that detects and interprets the Doppler effect in terms of the radial velocity of a target. The signal received by a radar from a moving target differs in frequency from the transmitted frequency by an amount that is proportional to the radial component of the velocity relative to the radar. See Doppler frequency shift.
Industry:Weather
A laser radar (lidar) that can determine radial velocity (velocity toward or away from the laser) of the air by measuring the frequency shift of the returning light that was scattered from atmospheric aerosols, compared to the original transmitted frequency. Velocities of individual convective thermals and large-diameter turbulent eddies can be measured. Most lidar signals are blocked by clouds and fog, and are greatly attenuated in very polluted air. See Doppler effect.
Industry:Weather
“The change in the apparent time interval between two events which arises from the motion of an observer together with the finite velocity of information about the events” (Gill 1965). Doppler effect is often used to mean frequency shifts (Doppler shift) of acoustic and electromagnetic waves because of relative motion between sources and observers. The relative magnitude of a Doppler shift is of order of the ratio of a characteristic speed (e.g., speed of a source) to a speed of propagation (e.g., speed of sound, speed of light). A shift to lower frequency (relative to a reference frequency) is sometimes called a red shift whereas a shift to higher frequency is sometimes called a blue shift, although no colorimetric meaning should be attached to these terms. Acoustic waves do not evoke sensations of color, nor do electromagnetic waves outside the visible spectrum, and even Doppler shifts of visible light are so small as to yield no visually perceptible color changes. According to classical theory, there is no frequency shift of electromagnetic radiation for motion of a transmitter perpendicular to the line between receiver and transmitter. But according to relativistic theory, even for this kind of motion there is a Doppler shift (transverse Doppler shift), although it is appreciably smaller than the longitudinal Doppler shift. See also Doppler frequency shift.
Industry:Weather
In general, the change in frequency of a signal reaching a receiver when the receiver and the transmitting source are in motion relative to one another. This phenomenon was first noted for sound waves by the Austrian physicist Christian Johann Doppler (1803–53) in 1842. In meteorology, this effect is successfully employed with remote sensors, such as Doppler radars and Doppler lidars, in which the receiver (collocated with the transmitter) is fixed and only the scatterers (upon which transmitted power impinges and is reradiated) are moving. The frequency shift, f, induced by a scatterer having a radial component of motion vr relative to the radar may be expressed as
where λ is the wavelength of the transmitter, f is positive for motion toward the radar, and, by the usual convention, vr is positive for motion away from the radar. See also Doppler velocity.
Industry:Weather
The error in measurement of the radial velocity of a radar target. The error is due to 1) noise in the measurement system; 2) fluctuations in the signal itself, which are a characteristic of echoes from distributed targets such as precipitation; 3) limited measurement time; and 4) atmospheric refractive effects (usually negligible) that modify the ray path and the speed of light.
Industry:Weather
“The change in the apparent time interval between two events which arises from the motion of an observer together with the finite velocity of information about the events” (Gill 1965). Doppler effect is often used to mean frequency shifts (Doppler shift) of acoustic and electromagnetic waves because of relative motion between sources and observers. The relative magnitude of a Doppler shift is of order of the ratio of a characteristic speed (e.g., speed of a source) to a speed of propagation (e.g., speed of sound, speed of light). A shift to lower frequency (relative to a reference frequency) is sometimes called a red shift whereas a shift to higher frequency is sometimes called a blue shift, although no colorimetric meaning should be attached to these terms. Acoustic waves do not evoke sensations of color, nor do electromagnetic waves outside the visible spectrum, and even Doppler shifts of visible light are so small as to yield no visually perceptible color changes. According to classical theory, there is no frequency shift of electromagnetic radiation for motion of a transmitter perpendicular to the line between receiver and transmitter. But according to relativistic theory, even for this kind of motion there is a Doppler shift (transverse Doppler shift), although it is appreciably smaller than the longitudinal Doppler shift. See also Doppler frequency shift.
Industry:Weather
A nautical term for the equatorial trough, with special reference to the light and variable nature of the winds. Compare horse latitudes; see equatorial air.
Industry:Weather
An area of human activity presumed to contain expertise and knowledge suitable for the basis of an expert or knowledge-based system. Expert system domains should be kept as narrow as possible without becoming trivial. For example, prediction of severe storms rather than general weather prediction, or analysis of cloud type rather than the whole of satellite meteorology.
Industry:Weather
A specialist who possesses knowledge and/or expertise about a particular domain, that is, a source of expertise for developing a knowledge base for use by an expert system.
Industry:Weather
