Analysis of storms and precipitation across the continental divide in the Canadian Rockies

Abstract:

Storms and precipitation across the continental divide in southwestern Canada are critical aspects of the regional water cycle as this region is the headwater of major rivers flowing into three oceans. Most atmospheric flows and storm tracks are westerly, therefore annual precipitation is typically higher and more persistent on the western side of the divide, but easterly upslope storms can often produce extreme precipitation amounts leading to major floods such as June 2013 in Calgary, Alberta. Given the importance of these events, a field campaign was carried out in April-June 2019 to enhance our knowledge of the contribution of different moisture fluxes to precipitation across this region. Storms and precipitation were monitored simultaneously on both sides of the continental divide with an array of equipment including vertically pointing Doppler radars. Thirteen storms of varying atmospheric conditions, precipitation amounts and types were documented. Four produced >20 mm of precipitation on the eastern side. The western side was warmer and drier than the eastern side, and, regardless of storm type, precipitation on the western side only occurred when precipitation also occurred on the eastern side. Snow mainly fell on the eastern side whereas only rain fell on the western side. All major events were associated with moisture flux flowing westward across the continental divide, which was evident during the event with the highest amount of precipitation. The longest duration event (66 hours) was associated with a more southerly moisture flux along the eastern side of the continental divide and this moisture originated from an atmospheric river that made landfall over the west coast of the United States. Overall, this field experiment is improving our understanding of the processes leading to storms and precipitation across the continental divide in the Canadian Rockies.