Significant summer rainfall in the Canadian Prairie Provinces: modes and mechanisms 2000–2004

Abstract

AbstractThis study analysed nearly 1000 significant (≥10 mm in 24 h) summer rain events that occurred within the boundaries of the Canadian Prairie Provinces from 2000 to 2004. The objective of this examination was to identify the mode of each event (i.e. solely or partially convective versus non‐convective), and its primary forcing mechanism (i.e. source of lift). Daily rainfall and lightning maps revealed that most of the significant rain events (79%) were solely or partially convective (i.e. lightning was recorded during the event), while 88% of the total rain area, a measure of the impact of each event, was from events with moist deep convection. Average monthly percentages varied. In June, 74% of the events were solely or partially convective, in July 85%, and in August 79%. In June, 84% of the rain area was from events that were solely or partially convective, in July 93%, and in August 86%.For significant rain events with convection, the most frequent forcing mechanisms were meso‐scale processes (28%), surface low‐pressure centres (16%), surface troughs (14%), and warm and cold fronts (both 14%). While meso‐scale forcing mechanisms were responsible for 28% of the significant rain events with convection, they generated just 11% of the rain area with convection. Surface low‐pressure centres, responsible for just 16% of the events with convection, produced 25% of the rain area with convection. The most frequent forcing mechanisms for significant rain events without convection were surface low‐pressure centres (25%), surface troughs (22%), cold fronts (19%), and cold lows (12%). The portion of the significant rain area without convection attributed to each of the synoptic forcing mechanisms was generally consistent with its relative frequency of occurrence. Copyright © 2008 Royal Meteorological Society