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A Diagnostic Model for Equatorial Wave Disturbances: The Role of Vertical Shear of the Mean Zonal Wind

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  • 1 Dept. of Atmospheric Sciences, University of Washington, Seattle
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Abstract

A model is developed to diagnose the response of the atmosphere to a known distribution of diabatic heating. The linearized primitive equations in spherical coordinates are reduced to a single partial differential equation relating the perturbation geopotential to the diabatic heating pattern. The model diagnoses the atmospheric perturbation due to a heating pattern of specified zonal wavenumber, frequency, and distribution in the meridional plane.

The model is used to test the hypothesis that the observed westward propagating wave disturbances in the equatorial Pacific are Rossby waves driven by the latent heat release in the cloud clusters embedded within the waves. For a diabatic heating pattern designed to model the heating in cloud clusters the model duplicates many features of the observed waves. The computed perturbation meridional wind field has maxima in the upper and lower troposphere, separated by a relatively undisturbed region in the mid-troposphere. The structure of the disturbance is quite sensitive to vertical shear of the mean zonal wind. In particular, with westerly shear in the lower troposphere the precipitation occurs to the east of the surface trough, but with easterly shear the precipitation zone is west of the trough. These features are all in qualitative agreement with observations in the western and central Pacific.

Abstract

A model is developed to diagnose the response of the atmosphere to a known distribution of diabatic heating. The linearized primitive equations in spherical coordinates are reduced to a single partial differential equation relating the perturbation geopotential to the diabatic heating pattern. The model diagnoses the atmospheric perturbation due to a heating pattern of specified zonal wavenumber, frequency, and distribution in the meridional plane.

The model is used to test the hypothesis that the observed westward propagating wave disturbances in the equatorial Pacific are Rossby waves driven by the latent heat release in the cloud clusters embedded within the waves. For a diabatic heating pattern designed to model the heating in cloud clusters the model duplicates many features of the observed waves. The computed perturbation meridional wind field has maxima in the upper and lower troposphere, separated by a relatively undisturbed region in the mid-troposphere. The structure of the disturbance is quite sensitive to vertical shear of the mean zonal wind. In particular, with westerly shear in the lower troposphere the precipitation occurs to the east of the surface trough, but with easterly shear the precipitation zone is west of the trough. These features are all in qualitative agreement with observations in the western and central Pacific.

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