Into the Teeth of the Gale: The Remarkable Advance of a Cold Front at Grand Manan

Robert M. Cunningham Lincoln, MA 01773

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Frederick Sanders Marblehead, MA 01945

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Abstract

The afternoon of 30 December 1962 saw the nearly simultaneous arrival at Grand Manan (an island in the Bay of Fundy) of an intense cold front, accompanied by northwesterly gales and snow, and an intense small cyclonic vortex, producing a mild southeasterly gale. Surprisingly, the cold front arrived first, having advanced over the preceding 9 hours against a geostrophic flow increasing from a few meters per second to about 40 m s−1. The mild air returned for about 2 h as the vortex passed across the Grand Manan prior to the final arrival of the mass of cold air. The frontal oscillation was marked by temperature changes from 3°C to −9°C to 3°C to −12°C, within 6 h. There was only a single, strongly defined, low-pressure center.

A simplistic model of the ageostrophic response to geostrophic frontogenetical forcing, neglecting the effects of friction and stratification, shows more than enough ageostrophic flow to account for the observations. Comparison with physically more satisfying models supports the conclusion that this anomalous frontal behavior was the consequence of ageostrophic response to geostrophic forcing acting on an already-intense horizontal temperature gradient, exacerbated by the coincidental approach of an intense cyclone. This comparison also indicates, however, that semigeostrophic theory is not quantitatively reliable when applied to even moderately strong fronts in the real atmosphere.

Abstract

The afternoon of 30 December 1962 saw the nearly simultaneous arrival at Grand Manan (an island in the Bay of Fundy) of an intense cold front, accompanied by northwesterly gales and snow, and an intense small cyclonic vortex, producing a mild southeasterly gale. Surprisingly, the cold front arrived first, having advanced over the preceding 9 hours against a geostrophic flow increasing from a few meters per second to about 40 m s−1. The mild air returned for about 2 h as the vortex passed across the Grand Manan prior to the final arrival of the mass of cold air. The frontal oscillation was marked by temperature changes from 3°C to −9°C to 3°C to −12°C, within 6 h. There was only a single, strongly defined, low-pressure center.

A simplistic model of the ageostrophic response to geostrophic frontogenetical forcing, neglecting the effects of friction and stratification, shows more than enough ageostrophic flow to account for the observations. Comparison with physically more satisfying models supports the conclusion that this anomalous frontal behavior was the consequence of ageostrophic response to geostrophic forcing acting on an already-intense horizontal temperature gradient, exacerbated by the coincidental approach of an intense cyclone. This comparison also indicates, however, that semigeostrophic theory is not quantitatively reliable when applied to even moderately strong fronts in the real atmosphere.

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