The Causes of Severe Convective Outbreaks in Alberta. Part II: Conceptual Model and Statistical Analysis

Abstract

An intercomparison of all 11 Limestone Mountain Experiment case days provided the basis for a conceptual model of severe convective outbreaks in Alberta. It is proposed that most severe convective events result when upper-level cooling, associated with an advancing, synoptic-scale trough, occurs in phase with strong surface heating over the Alberta foothills. The deep destabilization over the elevated topography acts to amplify the mountain-plain circulation and to generate mesoscale upslope moisture transport. Concurrently, the surface synoptic pressure gradient gives rise to east-northerly winds that advect the moisture-rich air of the eastern plains into the lower branch of the mountain-plain circulation. In this manner, the plains moisture is permitted to reach the convectively active foothills through underrunning of the capping lid. The end product is the initiation of well-organized, severe convective storms that move eastward with the westerly component of the midtropospheric winds. A statistical analysis based on archived hail data furnished additional evidence for the key synoptic-scale features of the conceptual model.