An Instability of Mature Cold Fronts

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  • 1 Atmospheric Physics ETH, Zürich, Switzerland
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Abstract

An instability of a cold front possessing a low-level warm-band precursor is examined within an f-plane, quasi- and semigeostrophic framework. The basic frontal state is taken to be two-dimensional and of uniform potential vorticity. Theoretical considerations pinpoint the source and nature of the instability to be a vortex interaction effect acting across the thermal maximum of the warm band. They also reveal that the most unstable mode associated with this flow configuration has a mixed barotropic-baroclinic character and possesses some features (e.g., a wavelength ∼1000 km and a doubling time ∼1½ days) that are akin to surface frontal waves. In addition nonlinear numerical model simulations demonstrate that the frontal wave growth equilibrates rapidly but not before the establishment of a rich mesoscale flow structure.

A brief assessment is presented of the relationship of this diagnosed instability to realized atmospheric flow patterns and to other mechanisms that have been postulated for the growth of frontal waves.

Abstract

An instability of a cold front possessing a low-level warm-band precursor is examined within an f-plane, quasi- and semigeostrophic framework. The basic frontal state is taken to be two-dimensional and of uniform potential vorticity. Theoretical considerations pinpoint the source and nature of the instability to be a vortex interaction effect acting across the thermal maximum of the warm band. They also reveal that the most unstable mode associated with this flow configuration has a mixed barotropic-baroclinic character and possesses some features (e.g., a wavelength ∼1000 km and a doubling time ∼1½ days) that are akin to surface frontal waves. In addition nonlinear numerical model simulations demonstrate that the frontal wave growth equilibrates rapidly but not before the establishment of a rich mesoscale flow structure.

A brief assessment is presented of the relationship of this diagnosed instability to realized atmospheric flow patterns and to other mechanisms that have been postulated for the growth of frontal waves.

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