| Section T | T index | 331-339 of 589 terms |
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topography—1. Generally, the disposition of the major natural and man-made physical features of the earth's surface, such as would be entered on a map. This may include forests, rivers, highways, bridges, etc., as well as contour lines of elevation, although the term is often used to denote elevation characteristics (particularly orographic features) alone. 2. The study or process of topographic mapping.
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tornadic vortex signature—(Abbreviated TVS.) The Doppler velocity signature of a tornado or of an incipient tornado-like circulation aloft. As the signature occurs when the radar beam is wider than the vortex, the measured Doppler velocities are weaker than the rotational velocities within the vortex and the apparent core diameter is larger than that of the vortex. The signature, which may extend throughout a considerable vertical depth, is ideally characterized by extreme Doppler velocity values of opposite sign separated in azimuth by the equivalent of one beamwidth. However, since most radars display and record Doppler velocity values at discrete azimuthal intervals, the extreme Doppler velocity values are usually at azimuthally adjacent positions that are roughly one beamwidth apart. If the centers of the radar beam and the vortex coincide, the signature includes a zero Doppler velocity value that separates the extreme values.
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Tornado Alley—A term often used by the media to denote a zone in the Great Plains region of the central United States, often a north–south oriented region centered on north Texas, Oklahoma, Kansas, and Nebraska, where tornadoes are most frequent. Since statistics are variable on all timescales, the term has little scientific value.
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tornado cyclone—A term coined by Brooks (1949) to describe a surface low pressure area in a convective storm that, with its attendant winds, has a radius of about 8–16 km and is associated with, but is larger than, a tornado. Starting with Agee (1976), the tornado cyclone has been redefined as a distinct circulation with a scale larger than that of the tornado but smaller than that of the mesocyclone (although embedded within it and smaller in scale than the mesolow). The intermediate-scale tornado cyclone is sometimes inferred from high-resolution Doppler radar observations, but at other times it is not apparent.
Brooks, E. M., 1949: The tornado cyclone. Weatherwise, 2, 32–33.
Agee, E. M., 1976: Multiple vortex features in the tornado cyclone and the occurrence of tornado families. Mon. Wea. Rev., 104, 552–563.
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tornado echo—A type of precipitation echo observed in connection with large, strong tornadoes located at relatively close ranges from a radar. It appears on PPI displays in the form of a doughnut-shaped echo at the end of a hook echo in a supercell thunderstorm. A quasi-vertical tube having minimum radar reflectivity in the center may be observed on an RHI display or on a vertical sequence of PPI displays.
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tornado outbreak—Multiple tornado occurrences associated with a particular synoptic-scale system. In recent years, Galway (1977) has defined ten or more tornadoes as constituting an outbreak.
Galway, J. G., 1977: Some climatological aspects of tornado outbreaks. Mon. Wea. Rev., 105, 477–484.
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tornado—1. A violently rotating column of air, in contact with the surface, pendant from a cumuliform cloud, and often (but not always) visible as a funnel cloud. When tornadoes do occur without any visible funnel cloud, debris at the surface is usually the indication of the existence of an intense circulation in contact with the ground. On a local scale, the tornado is the most intense of all atmospheric circulations. Its vortex, typically a few hundred meters in diameter, usually rotates cyclonically (on rare occasions anticyclonically rotating tornadoes have been observed) with wind speeds as low as 18 m s−1 (40 mph) to wind speeds as high as 135 m s−1 (300 mph). Wind speeds are sometimes estimated on the basis of wind damage using the Fujita scale. Some tornadoes may also contain secondary vortices (suction vortices). Tornadoes occur on all continents but are most common in the United States, where the average number of reported tornadoes is roughly 1000 per year, with the majority of them on the central plains and in the southeastern states (see Tornado Alley). They can occur throughout the year at any time of day. In the central plains of the United States they are most frequent in spring during the late afternoon. See also supercell tornado, nonsupercell tornado, gustnado, landspout, waterspout. 2. A violent thundersquall in West Africa and adjacent Atlantic waters.
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