The Causes of Severe Convective Outbreaks in Alberta. Part I: A Comparison of a Severe Outbreak with Two Nonsevere Events

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  • 1 Department of Meteorology, McGill University, Montreal, Quebec, Canada
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Abstract

Abstract

Mesoscale and synoptic-scale analyses were carried out for a severe convective outbreak and two nonsevere convective events in central Alberta. High-resolution upper-air and surface observations gathered during the Limestone Mountain Experiment (LIMEX-85) permitted a detailed diagnosis of the evolution of the atmosphere over the Alberta foothills. On the severe day, deep convection was triggered when upper-level cooling, associated with an advancing, synoptic-scale trough, occurred in phase with strong surface heating over the Alberta foothills from 0800 to 1200 local daylight time (LDT). The deep destabilization over the elevated topography acted to amplify the mountain-plain circulation and to generate mesoscale upslope moisture transport. Concurrently, the surface synoptic pressure gradient gave rise to northeasterly winds that advected a tongue of moist plains air into the lower branch of the mountain-plain circulation. The plains moisture was thus permitted to reach the foothills in time to reinforce the initial convection and effectuate a secondary destabilization. On the nonsevere days, the absence of such joint meso-synoptic-scale upslope moisture transport precluded the occurrence of severe convection.

Abstract

Abstract

Mesoscale and synoptic-scale analyses were carried out for a severe convective outbreak and two nonsevere convective events in central Alberta. High-resolution upper-air and surface observations gathered during the Limestone Mountain Experiment (LIMEX-85) permitted a detailed diagnosis of the evolution of the atmosphere over the Alberta foothills. On the severe day, deep convection was triggered when upper-level cooling, associated with an advancing, synoptic-scale trough, occurred in phase with strong surface heating over the Alberta foothills from 0800 to 1200 local daylight time (LDT). The deep destabilization over the elevated topography acted to amplify the mountain-plain circulation and to generate mesoscale upslope moisture transport. Concurrently, the surface synoptic pressure gradient gave rise to northeasterly winds that advected a tongue of moist plains air into the lower branch of the mountain-plain circulation. The plains moisture was thus permitted to reach the foothills in time to reinforce the initial convection and effectuate a secondary destabilization. On the nonsevere days, the absence of such joint meso-synoptic-scale upslope moisture transport precluded the occurrence of severe convection.

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