Wave-CISK in a Baroclinic Basic State

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  • 1 Center for Meteorology and Physical Oceanography, Massachusetts Institute of Technology, Cambridge, MA 02139
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Abstract

Prefrontal squall lines are mesoscale convective systems that often cannot be linked to any preexisting organizing mechanism. This suggests the possibility that they are manifestations of a mesoscale instability involving the interaction between convective and larger scales. To investigate this hypothesis, a wave-CISK model is developed for two-dimensional disturbances in a baroclinic basic state with constant vertical wind shear. The governing equations are the linearized Boussinesq equations for an inviscid and hydrostatic fluid on an f-plane. The model domain is infinite in the horizontal and consists of two layers in the vertical representing the troposphere and the stratosphere. The model stratosphere has a larger static stability and no wind shear. The convective heating is confined to the troposphere. Normal mode solutions are assumed, and the convective heating is parameterized in the standard simple fashion its vertical structure is specified, and it is set proportional to the low-level vertical velocity. The model allows for arbitrary orientations of the disturbance axis in the horizontal plane.

Results show the existence of two modes: large scale Eady modes, which are amplified slightly by heating, and smaller scale wave-USK model The wave-OSK modes have their maximum growth rates near the symmetric axis, i.e., with disturbance axes approximately parallel to the shear vector. For heating amplitudes that are not unrealistically large, wavelengths of maximum growth are finite and on the mesoscale (on the order of 500 km). Sensitivity experiments for these wave-CISK modes show that the value of the maximum growth rates and the wavelength of maximum growth, are not very sensitive to the form of the vertical heating profile, while other characteristics of the fastest growing mode are. In particular, the orientation of the disturbance axis depends on the heating profile: for maximum heating in the middle troposphere, the disturbance axis is rotated 20°–30° clockwise from the symmetric axis, implying upshear propagation, while for higher levels of maximum heating the disturbance is more nearly aligned with the shear vector. If low level cooling is included in the heating profile, disturbance axes are rotated counterclockwise.

Comparisons with observations of squall lines in the atmosphere show some aspects of the solution, such as its vertical structure, to be in qualitative agreement. The orientation angle of the fastest growing mode, however, is near to observed values only if beating profiles with heating maxima at upper levels and cooling at lower levels are used. Predicted phase speeds at observed orientation angles are then too high by a factor of two to five. Reasons for this failure of the wave-CISK theory are discussed.

Abstract

Prefrontal squall lines are mesoscale convective systems that often cannot be linked to any preexisting organizing mechanism. This suggests the possibility that they are manifestations of a mesoscale instability involving the interaction between convective and larger scales. To investigate this hypothesis, a wave-CISK model is developed for two-dimensional disturbances in a baroclinic basic state with constant vertical wind shear. The governing equations are the linearized Boussinesq equations for an inviscid and hydrostatic fluid on an f-plane. The model domain is infinite in the horizontal and consists of two layers in the vertical representing the troposphere and the stratosphere. The model stratosphere has a larger static stability and no wind shear. The convective heating is confined to the troposphere. Normal mode solutions are assumed, and the convective heating is parameterized in the standard simple fashion its vertical structure is specified, and it is set proportional to the low-level vertical velocity. The model allows for arbitrary orientations of the disturbance axis in the horizontal plane.

Results show the existence of two modes: large scale Eady modes, which are amplified slightly by heating, and smaller scale wave-USK model The wave-OSK modes have their maximum growth rates near the symmetric axis, i.e., with disturbance axes approximately parallel to the shear vector. For heating amplitudes that are not unrealistically large, wavelengths of maximum growth are finite and on the mesoscale (on the order of 500 km). Sensitivity experiments for these wave-CISK modes show that the value of the maximum growth rates and the wavelength of maximum growth, are not very sensitive to the form of the vertical heating profile, while other characteristics of the fastest growing mode are. In particular, the orientation of the disturbance axis depends on the heating profile: for maximum heating in the middle troposphere, the disturbance axis is rotated 20°–30° clockwise from the symmetric axis, implying upshear propagation, while for higher levels of maximum heating the disturbance is more nearly aligned with the shear vector. If low level cooling is included in the heating profile, disturbance axes are rotated counterclockwise.

Comparisons with observations of squall lines in the atmosphere show some aspects of the solution, such as its vertical structure, to be in qualitative agreement. The orientation angle of the fastest growing mode, however, is near to observed values only if beating profiles with heating maxima at upper levels and cooling at lower levels are used. Predicted phase speeds at observed orientation angles are then too high by a factor of two to five. Reasons for this failure of the wave-CISK theory are discussed.

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