Investigation of the 2013 Alberta flood from weather and climate perspectives

The destructive 2013 Alberta flood was the focus of a recent integrative project at CNRCWP. Twelve members of our team contributed their expertise to this project, including co-investigators from our three research themes. The main findings are summarized as follows.

During 19–21 June 2013 a heavy precipitation event affected southern Alberta and adjoining regions, leading to severe flood damage in numerous communities and resulting in the costliest natural disaster in Canadian history. This flood was caused by a combination of meteorological and hydrological factors, which are investigated from weather and climate perspectives with the fifth generation Canadian Regional Climate Model. Results show that the contribution of orographic ascent to precipitation was important, exceeding 30 % over the foothills of the Rocky Mountains. Another contributing factor was evapotranspiration from the land surface, which is found to have acted as an important moisture source and was likely enhanced by antecedent rainfall that increased soil moisture over the northern Great Plains. Event attribution analysis suggests that human induced greenhouse gas increases may also have contributed by causing evapotranspiration rates to be higher than they would have been under pre-industrial conditions. Frozen and snow-covered soils at high elevations are likely to have played an important role in generating record streamflows. Results point to a doubling of surface runoff due to the frozen conditions, while 25 % of the modelled runoff originated from snowmelt. The estimated return time of the 3-day precipitation event exceeds 50 years over a large region, and an increase in the occurrence of similar extreme precipitation events is projected by the end of the 21st century. Event attribution analysis suggests that greenhouse gas increases may have increased 1-day and 3-day return levels of May–June precipitation with respect to pre-industrial climate conditions. However, no anthropogenic influence can be detected for 1-day and 3-day surface runoff, as increases in extreme precipitation in the present-day climate are offset by decreased snow cover and lower frozen water content in soils during the May–June transition months, compared to pre-industrial climate.

For further information, please consult:

Return times of (a) average May–June evapotranspiration over the northern Great Plains, (b) maximum 1-day and (c) 3-day May–June precipitation over southern Alberta, in present-day (red) and pre-industrial ensembles (blue). Gray horizontal lines show (a) average evapotranspiration during the 14–21 June period, (b) average precipitation on 20 June and (c) average precipitation during the 19–21 June period, for the members of the reference ensemble. Black dashed lines show (b) average precipitation across the region on 20 June and (c) average precipitation during the 19–21 June period, as estimated from CaPA

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