A Study of a Mesoscale Solitary Wave in the Atmosphere Originating near a Region of Deep Convection

Yuh-Lang Lin Department of Physics and Atmospheric Science, Drexel University, Philadelphia, Pennsylvania

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R. Craig Goff National Severe Storms Laboratory, Norman, Oklahoma

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Abstract

An analysis of a mesoscale wave which occurred over the eastern United States on 6 March 1969 is presented. Its origin is attributed to the perturbation of a midtropospheric inversion by a squall line. After the wave formed, it propagated in the direction of the mid- and upper-level tropospheric flow and expanded as an arc with the center located near the region of convective activity. The wave travelled from western Tennessee and Kentucky over a distance of more than 1000 kilometers to Nova Scotia with a mean speed of 55 m s−1. The event was quite obvious on surface barographs throughout this region. Pressure traces showed an obvious V-shaped pressure drop of 3 to 6 mb over much of the wave path. The wave persisted for about 9 hours with little change in character.

The synoptic situation is analysed and compared with similar atmospheric conditions in other studies. The generation mechanism of an internal gravity wave of depression is investigated by a simple model. It is found that wave of depression or elevation are described by a nondimensional number M = Nz1/gH which is related to the depth and strength of the inversion, the stratification above the inversion, and the position of the heat source. The effects of nonlinearity and dispersion are determined to be the same order of magnitude. A theory of Gear and Grimshaw is used to describe the basic character and to calculate the phase speed and wavelength of the solitary wave. The theoretical description corresponding to the lowest eigenvalue mode describes the basic character of the disturbance as a wave of depression with a preferred propagation in the direction of the basic flow. Both characteristics are verified by observation. The phase speed calculated by the theory compare favorably with the observed values.

Abstract

An analysis of a mesoscale wave which occurred over the eastern United States on 6 March 1969 is presented. Its origin is attributed to the perturbation of a midtropospheric inversion by a squall line. After the wave formed, it propagated in the direction of the mid- and upper-level tropospheric flow and expanded as an arc with the center located near the region of convective activity. The wave travelled from western Tennessee and Kentucky over a distance of more than 1000 kilometers to Nova Scotia with a mean speed of 55 m s−1. The event was quite obvious on surface barographs throughout this region. Pressure traces showed an obvious V-shaped pressure drop of 3 to 6 mb over much of the wave path. The wave persisted for about 9 hours with little change in character.

The synoptic situation is analysed and compared with similar atmospheric conditions in other studies. The generation mechanism of an internal gravity wave of depression is investigated by a simple model. It is found that wave of depression or elevation are described by a nondimensional number M = Nz1/gH which is related to the depth and strength of the inversion, the stratification above the inversion, and the position of the heat source. The effects of nonlinearity and dispersion are determined to be the same order of magnitude. A theory of Gear and Grimshaw is used to describe the basic character and to calculate the phase speed and wavelength of the solitary wave. The theoretical description corresponding to the lowest eigenvalue mode describes the basic character of the disturbance as a wave of depression with a preferred propagation in the direction of the basic flow. Both characteristics are verified by observation. The phase speed calculated by the theory compare favorably with the observed values.

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