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Title: The cold rain-on-snow event of June 2013 in the Canadian Rockies – characteristics and diagnosis

item POMEROY, JOHN - University Of Saskatchewan
item FANG, XING - University Of Saskatchewan
item Marks, Daniel

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2016
Publication Date: 5/15/2016
Citation: Pomeroy, J., Fang, X., Marks, D.G. 2016. The cold rain-on-snow event of June 2013 in the Canadian Rockies – characteristics and diagnosis. Hydrological Processes. 30:2899–2914. doi: 10.1002/hyp.10905.

Interpretive Summary: In June, 2013, the a devastating flood resulting from a rain-on-snow (ROS) event, occurred in the Canadian Rockies, causing loss of life, extensive property damage to homes, structures, highways and rail lines, and devastation to streams, rivers and natural systems in the region. Such an event had not occurred in historical time, and the region was unprepared for the consequences. In this paper we describe the event, and use a physics-based snow model to simulate the processes that were responsible for damage. We show that because the region was cooler than ROS events reported from more southern regions in the US, while snow melt did occur, most of the flooding was the direct result of rain. This was because the snow-covered area (SCA) was limited at the beginning of the event. Snowmelt during the event provided about 20% of the surface flow that became streamflow during the event. The anomalous nature of this flood is indicative of what can be expected in the Canadian Rockies as the climate continues to warm.

Technical Abstract: The June 2013 flood in the Canadian Rockies featured rain-on-snow (ROS) runoff generation at alpine elevations that contributed to the high streamflows observed during the event. Such a mid-summer ROS event has not been diagnosed in detail, but may be typical of high discharge producing hydrometeorological events in mountainous cold regions. The alpine hydrology of the flood was diagnosed using a physically based model created with the modular Cold Regions Hydrological Modelling Platform that included new algorithms such as psychrometric precipitation phase calculation and runoff flow through hillslope snowpacks. The event was distinctive in that though at first, relatively warm rain fell onto existing snowdrifts inducing ROS melt, the rainfall turned to snowfall as the air mass cooled and so increased snowcover and snowpacks in alpine regions, which then melted rapidly from ground heat fluxes in the latter part of the event. Melt rates of existing snowpacks were substantially lower during the ROS than during the relatively sunny periods preceding and following the event, due to low wind speeds, cloud cover and cool temperatures. However at the basin scale, melt volumes increased during the event, due to increased snowcover from the fresh snowfall and consequent large ground heat contributions to melt energy, causing snowmelt to enhance rainfall-runoff by one-fifth. Flow pathways also shifted during the event, from relatively slow sub-surface flow prior to the flood to an even contribution from sub-surface and fast overland flow during and immediately after the event. Sub-surface storage dynamics played an important role in translating surface inputs into streamflow discharge.