A Study of the Global Structure of the Migrating Diurnal Tide Using Generalized Hough Modes

David A. Ortland Northwest Research Associates, Bellevue, Washington

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Abstract

The theory of generalized Hough modes developed in a companion paper is applied here to investigate the structure of the migrating diurnal tide in the mesosphere and lower thermosphere. A series of simple examples illustrate how equatorial and midlatitude jets affect the horizontal structure and vertical wavelength of the tidal modes. Complex damping coefficients are used to model the effects of diffusion and gravity wave forcing acting on the tide. It is shown that the imaginary part of the damping coefficient can have significant effects that cannot be adequately represented by real damping coefficients alone. An exact solution of the linearized equations obtained with realistic mean flow and diffusion values representing solstice conditions is compared to the approximate generalized Hough mode solution. It is found that excellent agreement is obtained when expanding to first order, where the corrections at this order account for the observed hemispheric asymmetry in tidal winds during solstice. It is concluded that the asymmetry arises as the result of strong vertical shear in the mean winds that result from the reversal of the sign of the summer and winter jets in the upper mesosphere.

Corresponding author address: David Ortland, Northwest Research Associates, 14508 NE 20th St., Bellevue, WA 98006. Email: ortland@nwra.com

Abstract

The theory of generalized Hough modes developed in a companion paper is applied here to investigate the structure of the migrating diurnal tide in the mesosphere and lower thermosphere. A series of simple examples illustrate how equatorial and midlatitude jets affect the horizontal structure and vertical wavelength of the tidal modes. Complex damping coefficients are used to model the effects of diffusion and gravity wave forcing acting on the tide. It is shown that the imaginary part of the damping coefficient can have significant effects that cannot be adequately represented by real damping coefficients alone. An exact solution of the linearized equations obtained with realistic mean flow and diffusion values representing solstice conditions is compared to the approximate generalized Hough mode solution. It is found that excellent agreement is obtained when expanding to first order, where the corrections at this order account for the observed hemispheric asymmetry in tidal winds during solstice. It is concluded that the asymmetry arises as the result of strong vertical shear in the mean winds that result from the reversal of the sign of the summer and winter jets in the upper mesosphere.

Corresponding author address: David Ortland, Northwest Research Associates, 14508 NE 20th St., Bellevue, WA 98006. Email: ortland@nwra.com

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