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Possible Mechanisms of Clear-Air Turbulence in Strongly Anticyclonic Flows

John A. KnoxDepartment of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin

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Abstract

The forecasting of clear-air turbulence (CAT) continues to be a challenging problem despite progress made in the understanding of vertical shear (Kelvin–Helmholtz) instabilities. The possible connections between horizontal anticyclonic flows and CAT are addressed. Analytical expressions are derived to show that current CAT diagnostics do not correctly account for the dynamics of strongly anticyclonic situations. In gradient-balanced anticyclonic flows, nonfrontogenetical enhancement of vertical shear may lead to CAT. A review of observations, theory, and modeling is presented to support the claim that strong anticyclonic relative vorticity can also lead to CAT through the generation of gravity wave activity by geostrophic adjustment and inertial instability. CAT diagnostics are then discussed in light of these claims. Observational work is in progress to investigate the possibility of inertial instability-triggered CAT.

Corresponding author address: Dr. John A. Knox, Columbia University and NASA/Goddard Institute for Space Studies, Armstrong Hall, 2880 Broadway, New York, NY 10025.

Email: jknox@giss.nasa.gov

Abstract

The forecasting of clear-air turbulence (CAT) continues to be a challenging problem despite progress made in the understanding of vertical shear (Kelvin–Helmholtz) instabilities. The possible connections between horizontal anticyclonic flows and CAT are addressed. Analytical expressions are derived to show that current CAT diagnostics do not correctly account for the dynamics of strongly anticyclonic situations. In gradient-balanced anticyclonic flows, nonfrontogenetical enhancement of vertical shear may lead to CAT. A review of observations, theory, and modeling is presented to support the claim that strong anticyclonic relative vorticity can also lead to CAT through the generation of gravity wave activity by geostrophic adjustment and inertial instability. CAT diagnostics are then discussed in light of these claims. Observational work is in progress to investigate the possibility of inertial instability-triggered CAT.

Corresponding author address: Dr. John A. Knox, Columbia University and NASA/Goddard Institute for Space Studies, Armstrong Hall, 2880 Broadway, New York, NY 10025.

Email: jknox@giss.nasa.gov

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