The Influence of a Capping Inversion on the Dynamic and. Convective Instability of a Boundary Layer Model with Shear

Earl E. Gossard Wave Propagation Laboratory, NOAA, Boulder, Colo. 80302

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William R. Moninger Coe College, Cedar Rapids, Iowa 52402

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Abstract

The dynamic instability and the kinematics of a multi-layer, shear model of a convective boundary layer are analyzed. Important features of the model include a capping temperature inversion that may or may not be accompanied by a wind discontinuity, a surface-based superadiabatic layer, and a statically stable upper atmosphere. It is shown that the capping inversion can result in a relatively narrow band of dynamically unstable wavenumbers that depend on shear layer thickness, implying a strong selection of scale in growing disturbances. The influence of the various model parameters on selection of the “most unstable” scales is shown and their corresponding propagation velocities are calculated.

A simple form of the model is also used to examine the characteristics of the convectively unstable modes. It is found that two-dimensional disturbances aligned transverse to the wind shear are most dynamically unstable, whereas two-dimensional disturbances parallel to the wind shear are most convectively unstable.

The vorticity and general kinematics of the disturbances are affected in an important way by the presence of a critical level within the height range occupied by the disturbance.

Abstract

The dynamic instability and the kinematics of a multi-layer, shear model of a convective boundary layer are analyzed. Important features of the model include a capping temperature inversion that may or may not be accompanied by a wind discontinuity, a surface-based superadiabatic layer, and a statically stable upper atmosphere. It is shown that the capping inversion can result in a relatively narrow band of dynamically unstable wavenumbers that depend on shear layer thickness, implying a strong selection of scale in growing disturbances. The influence of the various model parameters on selection of the “most unstable” scales is shown and their corresponding propagation velocities are calculated.

A simple form of the model is also used to examine the characteristics of the convectively unstable modes. It is found that two-dimensional disturbances aligned transverse to the wind shear are most dynamically unstable, whereas two-dimensional disturbances parallel to the wind shear are most convectively unstable.

The vorticity and general kinematics of the disturbances are affected in an important way by the presence of a critical level within the height range occupied by the disturbance.

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