A Cold Rain-on-Snow Event in a Canadian Rockies Alpine Catchment: Characteristics and Modelling
Thursday, 18 December 2014
Over three days in late June 2013, 250 mm of precipitation fell on the partially snow-covered and heavily instrumented Marmot Creek Research Basin, Canadian Rockies, causing the largest recorded flood in the region, the destruction of most gauging stations in the research basin and the most expensive natural disaster in Canadian history. Precipitation was remarkable in that similar depths fell at all elevations from forest-grassland valley bottom to mid-elevation montane forest to alpine ridge-top and started with relatively warm conditions even at high elevations (6 oC), turning to snowfall at the highest elevations towards the end of the event. Snowfall in the alpine zone of the basin is highly redistributed by wind and forms deep drifts on lee slopes and in tree-line forests. These snowdrifts persist into summer and form a critical water supply for the creek. Snow surveys six days before and one day after the event show alpine snowdrift water equivalent declining from 350 to 150 mm and snow-covered area declining from 97% to 55% The snowpack ablation of 120 mm during the heavy precipitation period enhanced water delivery to alpine surfaces from rainfall by 50%. The event was simulated using the modular, physically-based Cold Regions Hydrological Model which was set up here to include a blowing snow model and infiltration to frozen soil model coupled to the SNOBAL energy balance snowmelt model. The results show substantially slower snowmelt rates during the rain-on-snow (ROS) event than before or after the ROS. ROS was characterized by greatly reduced solar irradiance, which was not compensated for by increased net longwave radiation, advected heat from rainfall and small but consistently downward fluxes of sensible and latent heat. Turbulent fluxes did not drive melt as has been suggested for more temperate ROS melts and energy sources can be ranked as K*>L*>Qa>Qh>Qe>>Qg. Nevertheless the combination of rainfall, snowmelt and frozen soils caused alpine snowdrifts to be a ‘hot spot’ for runoff generation during the flood. The distinctive character of this ROS event occurring near summer solstice and with relatively cool atmospheric conditions meant that melt rates were not enhanced by ROS, but snowmelt during ROS was an important contributor to alpine streamflow.