Storms and Precipitation Across the continental Divide Experiment (SPADE)

Abstract

The Canadian Rockies are a triple-continental divide, whose high mountains are drained by major snow-fed and rain-fed rivers flowing to the Pacific, Atlantic and Arctic Oceans. The objective of the April-June 2019 Storms and Precipitation Across the continental Divide Experiment (SPADE) was to determine the atmospheric processes producing precipitation on the eastern and western sides of the Canadian Rockies during springtime, a period when upslope events of variable phase dominate precipitation on the eastern slopes. To do so, three observing sites across the divide were instrumented with advanced meteorological sensors. During the 13 observed events, the western side recorded only 25% of the eastern side’s precipitation accumulation, rainfall occurred rather than snowfall, and skies were mainly clear. Moisture sources and amounts varied markedly between events. An atmospheric river landfall in California led to moisture flowing persistently northward and producing the longest duration of precipitation on both sides of the divide. Moisture from the continental interior always produced precipitation on the eastern side but only in specific conditions on the western side. Mainly slow-falling ice crystals, sometimes rimed, formed at higher elevations on the eastern side (> 3 km MSL), were lifted, and subsequently drifted westward over the divide during non-convective storms to produce rain at the surface on the western side. Overall, precipitation generally crossed the divide in the Canadian Rockies during specific spring-storm atmospheric conditions although amounts at the surface varied with elevation, condensate type, and local and large-scale flow fields.

Precipitation gradients across the Continental Divide in the southern Canadian Rockies

“…Subsequently, precipitation maxima were located above the easternmost Canadian Rockies and the Foothills during July, and then may have migrated even slightly further east during August when few precipitation maxima existed near the Continental Divide. These findings are consistent with greater diurnal heating and lee-side convergence and enhanced precipitation layer growth (Barry 2008;Thériault et al 2022). Large precipitation events in the spring and early summer were associated with easterly flow during the SPADE campaign, which could create precipitation maxima near the Continental Divide due to orographic enhancement (Thériault et al 2022).…”

“…These findings are consistent with greater diurnal heating and lee-side convergence and enhanced precipitation layer growth (Barry 2008;Thériault et al 2022). Large precipitation events in the spring and early summer were associated with easterly flow during the SPADE campaign, which could create precipitation maxima near the Continental Divide due to orographic enhancement (Thériault et al 2022). The switch from larger precipitation frequencies and amounts on the eastern side of the Continental Divide during May to larger frequencies and amounts of precipitation on the western side of the Continental Divide during June could be indicative of a transition from a more easterly flow regime to a more westerly flow regime.…”

“…Precipitation was recorded most frequently at sites on the western side of the Continental Divide in cooler periods during the early morning hours of the 2019 SPADE campaign, and relatively little precipitation was recorded during convective events without synoptic forcing at Nipika (Thériault et al 2022). Precipitation evaporated before it reached the ground on several occasions at this site, with instances of virga identified by both human observers and Micro Rain Radar.…”

“…In addition, higher freezing levels could have allowed areas of higher precipitation growth to penetrate further into the mountain range east of the Continental Divide and migrate into the prairies during the summer months (Foresti et al 2018). Warm, deep layers of moisture and upward air motion can facilitate the transportation of precipitation across mountain barriers such as the Continental Divide (Thériault et al 2022).…”

“…Conversely, precipitation occurred more frequently during warmer periods of the day on the eastern side of the Continental Divide. During the SPADE campaign fast falling hydrometeors and a large condensate flux from the east side of the Continental Divide were necessary for precipitation to occur on both sides of the Continental Divide simultaneously (Thériault et al 2022). A true comparison of evaporation on both sides of the Continental Divide may not be possible, since the lower mean elevation of stations on the western side could mean that greater amounts of precipitation is evaporating at these stations.…”