| Section B | B index | 371-379 of 517 terms |
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boundary layer radarA type of wind profiler specially designed to study the lower part of the troposphere. Because clear-air reflectivities in the boundary layer are usually orders of magnitude higher than in the upper troposphere, these profilers can be smaller, lower powered, and less expensive than those profilers designed to cover higher regions of the atmosphere (see, e.g.,, MST radars). Boundary layer radars are generally characterized by short pulse lengths (less than 100 m) and the ability to make measurements starting at 100–200 m above the radar and extending to at least 2–4 km under typical atmospheric conditions. To satisfy these requirements, boundary layer radars usually operate in the UHF radar band.
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boundary layer separationA condition that occurs at sufficiently high Reynolds numbers in which the surface streamlines break away from the surface. Separation is due to the presence of a solid boundary, at which the no-slip condition—that is, the velocity of the fluid particles in contact with the surface is the velocity of that surface—is satisfied and vorticity is generated. Separation of a steady boundary layer at a plane or rounded rigid wall occurs whenever the velocity of the fluid just outside of the boundary layer decreases in the mean flow direction sufficiently rapidly and by a sufficient amount. This can be accomplished by the imposition of an opposing pressure gradient in the direction of flow.
Batchelor, G. K., 1967: Fluid Dynamics, Cambridge University Press, p. 325.
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boundary layer1. The layer of fluid near a boundary that is affected by friction against that boundary surface, and possibly by transport of heat and other variables across that surface. In meteorology, this is the atmospheric boundary layer. 2. In a physical or mathematical system, a region over which some property or term in the equations varies rapidly, that is, over its full range; conversely, a region outside of which certain terms may be neglected.
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boundary mixingMixing occurring on sloping topography on the ocean margins or on seamounts primarily as the result of breaking internal waves. Boundary mixing is thought to play a major role in the vertical transport of heat.
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boundary of saturationThe surface between the saturated and unsaturated zones in a soil. The boundary of saturation is the top of the capillary fringe.
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boundary-value problemA physical problem completely specified by a differential equation in an unknown, valid in certain information (boundary conditions) about the unknown given on the boundaries of that region. The information required to determine the solution depends completely and uniquely on the particular problem. A great variety of meteorological problems are formulated as boundary-value problems. See also initial-value problem.
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bounded weak echo region(Abbreviated BWER.) A nearly vertical channel of weak radar echo, surrounded on the sides and top by significantly stronger echo. The BWER, sometimes called a vault, is related to the strong updraft in a severe convective storm that carries newly formed hydrometeors to high levels before they can grow to radar- detectable sizes. BWERs are typically found at midlevels of convective storms, 3–10 km above the ground, and are a few kilometers in horizontal diameter. See also weak echo region.
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Bourdon tubeA closed curved tube of elliptical cross section used in some thermometers and barometers. The Bourdon-tube thermometer consists of a Bourdon tube completely filled with liquid. The expansion of the liquid due to a temperature change causes an increase in the radius of curvature of the tube. The curvature may then be measured by the travel of the tip of the tube. The Bourdon- tube barometer consists of an evacuated Bourdon tube and operates in a similar manner. In both cases the curvature is a measure of the difference between the pressure inside the tube and that outside.
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