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Glossary Boundary Layer: The layer of a fluid adjacent to a physical boundary in which the fluid motion is affected by the boundary and has a mean velocity less than the free-stream value. Bright Band: The enhanced radar echo of snow as it melts to rain. The bright band is observed primarily in stratiform cloud systems. Clutter: Echoes that interfere with observation of desired signals on a radar display. Convective Rain: Rain associated with convective clouds or cumuliform clouds characterized by vertical development in the form of rising mounds, domes, or towers. Convergence Lines: A horizontal line along which horizontal convergence of the airflow is occurring. Common forms of convergence lines are sea-breeze fronts, cold-air outflows from thunderstorms and synoptic fronts. Cumuliform: Descriptive of all clouds with vertical development in the form of rising mounds domes, or towers. Decibel (dB): A logarithmic expression for the ratio of two quantities. Mathematically: dB = lOLog(P~/P2) Doppler shift: The change in frequency at a receiver due to the relative motion of the receiver and the energy source. Dowaburst: A strong downdraft that induces an outburst of damaging winds on or near the ground (macrobursts and microbursts are versions of this). Eye Wall: The area of tall cumulonimbus storms surrounding the eye of the storm. Heavy rain and very high winds occur in the eye wall. The "eye of the storm" (hurricane, typhoon) is the roughly circular area of comparatively light winds and fair weather found at the center of a severe tropical cyclone. Freezing Level: The lowest altitude in the atmosphere, over a given location, at which the air temperature is O degrees Celsius. Ground Clutter: The pattern of radar echoes due to fixed ground targets. Hail: Precipitation in the form of balls or irregular lumps of ice produced by convective clouds, usually cumulonimbus. An individual unit of hail is called a hailstone. By convention, hail has a diameter of 5 mm or more, while smaller particles of similar origin may be classed as ice pellets or snow pellets. 59
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60 Assessment of NEXRAD Coverage Hook Echo: A pendants curved-shaped region of reflectivity caused by precipitation being drawn into the cyclonic spiral of a mesocyclone (Davies-Iones, 1985~. The hook echo is a fairly shallow feature, typically extending only up to 4 km in height. Hurricane: Severe tropical cyclone, with winds of 74 mph or greater, occurring in the North Atiantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific off the west coast of Mexico. A tropical cyclone is a general term for a cyclone that originates over the tropical oceans. At maturity, the tropical cyclone is one of the most intense and feared storms of the world. ~.# ~_ Kinematics: Study of the motion of bodies or fluids without reference to the forces producing the motion. In meteorology, the analysis of the motion of isobars and fronts when treated as geometric features. Lake-effect Snow: Localized snow that occurs over and in the lee of lakes that is caused by relatively cold air flowing over warm water. In the United States this phenomenon is most noted along the south and east shores of the Great Lakes during arctic cold-air outbreaks. Macroburst: Similar to a microburst (see below) except the damaging winds extend over an area greater than 4 km and may last for tens of minutes. The term "downburst" includes bow microbursts and macrobursts without reference to scale (Fujita 19851. A. Reflectivity signature Several reflectivity patterns have been associated with macrobursts. One of the most common is the "bow echo," or region in a line of thunderstorms that bulges ahead of the line and is associated with damaging surface winds and occasionally tornadoes. B. Velocity signature The velocity signature of a macroburst shows a broad pattern of approaching and receding velocities associated with the strong divergent storm outflow. Macroscale: Large scale, characteristic of weather systems several hundred to several thousand kilometers in diameter. Median: A measure of central tendency. The middle value in a set of numbers arranged in order from lowest to highest. Melting Level: The altitude at which ice crystals and snowflakes melt as they descend through He atmosphere. Mesocyclone: A horizontal atmospheric rotation on a scale between 4 and 400 km (Fujita, 1981~. Its use here refers to a rotation within a thunderstorm typically surrounding a small area of low pressure. Typically within thunderstorms the rotation is 4-12 km across and 4-10 km in height. Mesoscale: On a scale of 4 km to 400 km. Microburst: A strong downdraft induces an outburst of damaging winds on or near the ground. Damaging winds, either straight or curved, are highly divergent. The damaging winds extend over an area of less than 4 km and typically last for only a few minutes. This scale of diverging winds has been found to be particularly hazardous to aircraft while landing or taking off (Fujita, 19851. Mini-supercell: Contains similar severe weather characteristics to a supercell, but the storm is significantly smaller in height and width. The diameter of the radar detected rotation is ~ to ~ km. This is a relatively new storm type whose existence has been confirmed by data from the new Doppler radars in the eastern half of the United States. Differentiating on scale is useful here because of the greater difficulty in detecting these smaller rotations. Misoscate: On a scale of 40 m to 4 km.
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Glossary 61 Misocyclone: A horizontal atmospheric rotation on a scale between 40 m and 4 km (Fujita, 1981~. Its use here refers to (~) a rotation within a thunderstorm with a horizontal scale < 4 km; and (2) a near-surface rotation along a convergence line wig a horizontal dimension < 4 km. Nautical mile: An international unit equal to 6,076.~15 feet (1,852 meters) used officially in the United States since July i, 1959. ~ nautical mile = 1.15 statute miles = 1.85 kilometers. Nowcast: Statement of currently occurring weather conditions; also used to mean short-term forecast, Mat is, minutes to a few hours. Pre-NEXRAD Network: Network of radars established before the introduction of the WSR-~8D. Reflectivity: A measure of the efficiency of a radar target in intercepting and returning radio energy, including effects of reflection, scattering, and diffraction. Resolution: The smallest increment of a measurement of a parameter. Storm: A disturbed state of the atmosphere strongly implying destructive or unpleasant weather. Stratiform: Descriptive of clouds of extensive horizontal development, as contrasted to the more narrow and vertically developed cumuliform types. Stratiform Rain: Horizontally widespread rain, uniform in character, typically associated with macroscale fronts and pressure systems. Stratiform Snow: Same as for Stratiform rain except precipitation is in the form of snow. Supercell: Potentially the most dangerous convective storm type. It may produce high winds, large hail and long-lived tornadoes over a wide path. In its purest form it consists of a single, quasi- steady, rotating updraft that may have a lifetime of several hours (Weisman and Klemp, 1986~. The radar-identified rotation typically has a diameter of 4 km to 12 km. Thin-line Echo: A narrow, elongated nonprecipitating echo, usually associated with thunderstorm outflow, fronts, or other density discontinuities. Also called a ''fine line." Tornadic Vortex Signature (TVS): A Doppler velocity signature sometimes produced by a large, intense tornado at close range. The signature is a very strong azimuthal shear in a distance 2 km. Tornado: A violently rotating column of air, pendant from a cumulonimbus cloud, and nearly always observable as a "funnel cloud." On a local scale, it is the most destructive of all atmospheric phenomena. Its vortex, commonly several hundreds of meters in diameter, whirls cyclonically with wind speeds estimated at 50 m/s to 150 m/s. 1 Wind Shear: The rate of change of the vector wind in a specified direction normal to the wind direction. Vertical shear is the variation of the horizontal wind in the vertical direction. WSR-~8D System: Combination of the hardware, software, facilities, communications, logistics, staff, operations, and procedures for the collection, processing, analysis, dissemination, and application of data from the WSR-~8D unit. WSR-~8D Unit: Combination of one radar data acquisition, one radar product generator, all associat- ed radar product generator operational positions and principal user processor, and interconnect- ing communications of the WSR-~8D.
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