Journal of Climate 2013 ; e-View
The key role of heavy precipitation events in climate model disagreements of future annual precipitation changes in California
David W. Pierce,1,* Daniel R. Cayan,1 Tapash Das,1,6 Edwin P. Maurer,2 Norman L. Miller,3 Yan Bao,3 M.Kanamitsu,1,+ Kei Yoshimura,1 Mark A. Snyder,4 Lisa C. Sloan,4 Guido Franco,5 and Mary Tyree1
Climate model simulations disagree on whether future precipitation will increase or decrease over California, which has impeded efforts to anticipate and adapt to human-induced climate change. This disagreement is explored in terms of daily precipitation frequency and intensity. It is found that divergent model projections of changes in the incidence of rare heavy (> 60 mm/day) daily precipitation events explain much of the model disagreement on annual timescales, yet represent only 0.3% of precipitating days and 9% of annual precipitation volume. Of the 25 downscaled model projections we examine, 21 agree that precipitation frequency will decrease by the 2060s, with a mean reduction of 6-14 days/year. This reduces California’s mean annual precipitation by about 5.7%. Partly offsetting this, 16 of the 25 projections agree that daily precipitation intensity will increase, which accounts for a model average 5.3% increase in annual precipitation. Between these conflicting tendencies, 12 projections show drier annual conditions by the 2060s and 13 show wetter. These results are obtained from sixteen global general circulation models downscaled with different combinations of dynamical methods (WRF, RSM, and RegCM3) and statistical methods (BCSD and BCCA), although not all downscaling methods were applied to each global model. Model disagreements in the projected change in occurrence of the heaviest precipitation days (> 60 mm/day) account for the majority of disagreement in the projected change in annual precipitation, and occur preferentially over the Sierra Nevada and Northern California. When such events are excluded, nearly twice as many projections show drier future conditions.