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Rodger A. Brown

Abstract

One of the distinguishing characteristics of a supercell thunderstorm is the presence of a rotating updraft. During the past 30 years, various hypotheses have been proposed to explain the initiation and maintenance of rotation. However, attempts to verify the initiation process have been frustrated by the lack of multiple-Doppler radar measurements at the time that the first rotating updraft appears. Discussed in this paper are dual-Doppler radar measurements that successfully captured the initiation and evolution of rotation in the Agawam, Oklahoma, storm of 6 June 1979, which occurred during the storm-scale phase of the Severe Environmental Storms and Mesoscale Experiment (SESAME). The process leading to updraft rotation appears to follow that proposed in 1968 by Fujita and Grandoso, whereby a middle-altitude vorticity couplet formed on the downwind flanks of a strong nonrotating updraft, with cyclonic vertical vorticity on the right-forward flank and anticyclonic vertical vorticity on the left-forward flank. With low-altitude flow approaching the storm from the right, a new updraft developed on the rightward-propagating gust front located along the right edge of the storm beneath the cyclonic vorticity region. The growing updraft acquired cyclonic rotation at middle altitudes by entraining and stretching the ambient vertical vorticity. Subsequent right-flank updrafts in the Agawam storm appear to have developed middle-altitude rotation in the same manner. Based on observations made within the Agawam storm and its immediate environment, the conventional hypothesis that employs low-altitude vertical shear of the horizontal wind as the vorticity source did not likely play a significant role in establishing updraft rotation.

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K. A. Browning

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Keith A. Browning

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No abstract available.

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Keith A. Browning

Abstract

A life cycle classification for severe local storms is proposed, which adds a fourth stage, the quasi-steady SR Mature Stage, to the well-known Byers-Braham classification. A combined analysis of the surface weather and radar echoes from three neighboring severe local storms reveals that each storm went through a similar, well-defined metamorphosis at the onset of its SR Mature Stage. The distinctive character of this stage is attributed to the rotational properties of the updraft, which in the storms studied developed when the updrafts had become vigorous enough to produce giant hail.

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J. A. Brown Jr.

Abstract

The hydrodynamic instability characteristics of planetary zonal flows are investigated through use of a quasi-geostrophic numerical model of high spatial resolution. An initial-value technique is employed to obtain solutions of the linear problem.

Certain zonal flows containing both vertical and lateral shears, which are representative of those observed in the earth's atmosphere, are found to be unstable with respect to the large-scale quasi-geostrophic disturbances. Westerly currents, each characterized by a latitudinally symmetric jet containing absolute vorticity extrema at various latitudes, amplify perturbations of some scales through a dominating baroclinic mechanism, and amplify perturbations of other scales through a dominating barotropic mechanism. For these flows, the unstable perturbations of relatively short zonal wavelength convert zonal available potential energy into perturbation energy and simultaneously strengthen the zonal kinetic energy of the basic flow. On the other hand, the unstable perturbations of relatively long zonal wavelength reduce both the zonal kinetic and available potential energies of the basic flow, with the former reduction dominating. For certain flows, these combinations produce two distinct wavelengths of maximum instability. Flows which are similar but contain no vanishing meridional gradient of absolute vorticity are found to produce baroclinically unstable perturbations with a tendency toward barotropic damping.

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J. A. Brown Jr.

Abstract

The numerical model of Part I is used in certain experiments designed to reveal special instability effects caused by the vertical walls which specify the lateral boundary conditions at the northern and southern boundaries of the atmosphere. In these examples the walls suppress instability of the barotropically dominated perturbations and have little influence on the westward progressions of the unstable waves.

Small-scale eddy momentum and heat diffusion processes are simulated in an example of a basic westerly wind field containing absolute vorticity extrema. The inclusion of these mechanisms is found to inhibit instabilities of all zonal wavelengths, with major effects noted for short shallow waves. The significant modifying influence is attributed to large effects of drag at the ground.

The behavior of an unstable wave interacting with the zonal current is obtained through nonlinear numerical calculations. The equilibrium state approached in the presence of a time-independent diabatic heating differential oscillates about a steady state. The several energy conversion rates vary in time in such a way as to minimize the time-rates-of-change of the different types of energies. The equilibrium energy levels appear to be governed by the required baroclinic process, and the resulting period of the oscillating regime is dictated by the barotropic mechanism.

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Rodger A. Brown

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K. A. Browning

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RODGER A. BROWN

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Rodger A. Brown

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