11.5.3.3 Temperature and Precipitation Extremes

Several RCM studies focused particularly on changes in extreme temperature events. Bell et al. (2004) examine changes in temperature extremes in their simulations centred on California. They find increases in extreme temperature events, both as distribution percentiles and threshold events, prolonged hot spells and increased diurnal temperature range. Leung et al. (2004) examine changes in extremes in their RCM simulations of the western USA; in general, they find increases in diurnal temperature range in six sub-regions of their domain in summer. Diffenbaugh et al. (2005) find that the frequency and magnitude of extreme temperature events changes dramatically under SRES A2, with increases in extreme hot events and a decrease in extreme cold events.

In a study of precipitation extremes over California, Bell et al. (2004) find that changes in precipitation exceeding the 95th percentile followed changes in mean precipitation, with decreases in heavy precipitation in most areas. Leung et al. (2004) find that extremes in precipitation during the cold season increase in the northern Rockies, the Cascades, the Sierra Nevada and British Columbia by up to 10% for 2040 to 2060, although mean precipitation was mostly reduced, in accord with earlier studies (Giorgi et al., 2001a). In a large river basin in the Pacific Northwest, increases in rainfall over snowfall and rain-on-snow events increased extreme runoff by 11%, which would contribute to more severe flooding. In their 25-km RCM simulations covering the entire USA, Diffenbaugh et al. (2005) find widespread increases in extreme precipitation events under SRES A2, which they determine to be significant.