The cold rain‐on‐snow event of June 2013 in the Canadian Rockies — characteristics and diagnosis

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, and a diagnosis may help to understand future high discharge‐producing hydrometeorological events in mountainous cold regions. The alpine hydrology of the flood was simulated using a physically based model created with the modular cold regions hydrological modelling platform. The event was distinctive in that, although 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 as a result of low wind speeds, cloud cover and cool temperatures. However, at the basin scale, melt volumes increased during the event as a result of 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. This early summer, high precipitation ROS event was distinctive for the impact of decreased solar irradiance in suppressing melt rates, the contribution of ground heat flux to basin scale snowmelt after precipitation turned to snowfall, the transition from slow sub‐surface to fast overland flow runoff as the sub‐surface storage saturated and streamflow volumes that exceeded precipitation. These distinctions show that summer, mountain ROS events should be considered quite distinct from winter ROS and can be important contributors to catastrophic events. Copyright © 2016 John Wiley & Sons, Ltd.

Analysis and Predictability of the Hydrological Response of Mountain Catchments to Heavy Rain on Snow Events: A Case Study in the Spanish Pyrenees

“…The reason may be the increase in downwelling long-wave radiation from a cloudier atmosphere, which compensates for the reduced energy input from lower air temperature and reduced solar radiation. This highlights the remarks of Pomeroy et al [79] on the unsuitability of simplistic temperature index models to simulate snow ablation and rain on snow episodes. These type of models can perform very satisfactorily when there are hardly any data available (e.g., [80,81]), but are more limited for application ROS events, as shown in lower right panel of Figure 5.…”

Rainwater propagation through snowpack during rain-on-snow sprinkling experiments under different snow conditions

“…ROS events have initiated severe floods in the past in many European countries such as Germany (HND Bayern, 2011;Sui and Koehler, 2001), Switzerland (Badoux et al, 2013;Rössler et al, 2014) and the Czech Republic (Čekal et al, 2011), as well as in North America (Ferguson, 2000;Kattelmann, 1997;Marks et al, 1998;McCabe et al, 2007;Pomeroy et al, 2016). Rainwater also affects snowpack stability which can initiate formation of wet snow avalanches (Ambach and Howorka, 1966;Baggi and Schweizer, 2008;Conway and Raymond, 1993) or trigger slushflows (Hestnes and Sandersen, 1987;Nyberg, 1989;Onesti, 1987).…”

Analysis and Predictability of the Hydrological Response of Mountain Catchments to Heavy Rain on Snow Events: A Case Study in the Spanish Pyrenees

“…The reason may be the increase in downwelling long-wave radiation from a cloudier atmosphere, which compensates for the reduced energy input from lower air temperature and reduced solar radiation. This highlights the remarks of Pomeroy et al [79] on the unsuitability of simplistic temperature index models to simulate snow ablation and rain on snow episodes.…”

“…This approach uses an empirical calibration, which is not ideal to simulate extreme events. For future work it would be advisable to get enough information on land cover and soil properties to implement different runoff models that are less sensitive to calibration (for example [79]). …”