Quantifying the Role of Orography in the 2013 Alberta Flood

The 19–21 June 2013 Alberta Flood was the costliest ($6 billion CAD) natural disaster in Canadian history. A multi-faceted extreme meteorological and hydrological event, one of the primary drivers of the Alberta Flood was an extreme rainfall event which resulted in 75–150 mm of precipitation in the foothills west of Calgary. Combined with anomalous late-season snowmelt in the higher elevations, the extreme rainfall event forced record streamflow to move downhill towards Calgary, causing an evacuation of downtown, substantial damage to property, and loss of life. Using high-resolution WRF simulations, the mesoscale dynamics and thermodynamics that contributed to the extreme rainfall event are elucidated. In addition, terrain reduction sensitivity experiments using a Gaussian smoother quantify the importance of the regional orography in producing extreme rainfall. Results indicate that the rainfall event was characterized by quasi-geostrophic (QG) and orographic ascent, which both acted to release convective instability. QG ascent dominated early in the event, while both sources of lift were important during the latter half of the event, in part through feedback mechanisms. The orography helped to sustain and anchor a MSLP lee cyclone in southeastern Alberta, which in turn extended the duration of easterly upslope warm and high-θe advection into the foothills flood region. The role of diabatic heating and associated feedbacks in orographic precipitation enhancement is also explored. Finally, although the magnitudes of ascent and precipitation were smaller in simulations with a 10 and 25 percent reduction in terrain, only a terrain reduction of greater than 25 percent drastically altered the location and magnitude of the heaviest precipitation and the associated physical processes.