Projected Changes in Precipitation Extremes for Western Canada based on High-Resolution Regional Climate Simulations

An analysis of changes in precipitation extremes in western Canada is presented, based upon an ensemble of high-resolution regional climate projections. The ensemble is composed of four independent, identically configured Community Earth System Model (CESM) integrations that were dynamically downscaled to 10-km resolution, using the WRF Model in two different configurations. Only the representative concentration pathway 8.5 (RCP8.5) scenario is considered. Changes in extremes are found to generally follow changes in the (seasonal) mean, but changes in mean and extreme precipitation differ strongly between seasons and regions (where extremes are defined as the seasonal maximum of daily precipitation). At the end of the twenty-first century, the highest projected increase in precipitation extremes is approximately 30% in winter away from the coast and in fall at the coast. Changes in winter are consistent between models; however, changes in summer are not: CESM is characterized by a decrease in summer precipitation (and extremes), while one WRF configuration shows a significant increase and another no statistically significant change. Nevertheless, the fraction of convective precipitation (extremes) in summer increases by 20%-30% in all models. There is also evidence that the climate change signal in summer is sensitive to the choice of the convection scheme. A comparison of CESM and WRF shows that higher resolution clearly improves the representation of winter precipitation (extremes), while summer precipitation does not appear to be sensitive to resolution (convection is parameterized in both models). To increase the statistical power of the extreme value analysis that has been performed, a novel method for combining data from climatologically similar stations was employed.