Development of daily precipitation projections for the United States based on probabilistic downscaling

Projections of mid and late 21st century precipitation for 963 stations across the contiguous United States are derived using probabilistic downscaling of 10 coupled atmosphere-ocean general circulation models (AOGCMs). The projections are constructed by downscaling the statistical parameters describing precipitation occurrence and intensity, using a first-order Markov chain and two-parameter gamma distribution, respectively. Future downscaled values of the parameters are used to derive projections of wet day probability, wet day precipitation intensity and its distribution, and total seasonal precipitation for the cold season (November through March) and the warm season (May through September). Downscaled results for the 10 AOGCMs indicate several robust features of possible changes in the U. S. regional precipitation climatology. Cold season projections are characterized by increases in precipitation in the northwest and northeast regions, decreases in precipitation in the southwest region, and smaller or inconsistent changes in other regions. With the exception of the northeast region, warm season projections reflect drier conditions overall resulting primarily from fewer wet days. In both the cold and warm seasons, changes in both the occurrence and intensity processes contribute to changes in total precipitation. Changes in total precipitation, and the relative roles of the occurrence and intensity processes, are found to be sensitive to the change in the distribution of wet day precipitation intensities. Regions with increasing seasonal precipitation totals are characterized by disproportionate increases in large precipitation events, while those with decreasing seasonal precipitation totals are characterized by the largest fractional decreases in small precipitation events.