ANALYSIS OF DAILY VARIABILITY OF PRECIPITATION IN A NESTED REGIONAL CLIMATE MODEL - COMPARISON WITH OBSERVATIONS AND DOUBLED CO2 RESULTS

We analyze daily mean, variability, and frequency of precipitation in two continuous 3 1/2 year long climate simulations over the continenta l U.S., one for present conditions and one for conditions under double d carbon dioxide concentration, conducted with a regional climate mode l (RegCM) nested in a general circulation model (GCM). The purpose of the work is to analyze model errors and limitations in greater detail than previously done and to calculate quantities that eventually will be used to form climate change scenarios that account for changes in d aily variability of precipitation. The models used are a version of th e NCAR Community Climate Model (CCM) and the climate version (RegCM) o f the NCAR/Penn State mesoscale model (MM4) at 60 km horizontal grid p oint spacing. Model output is compared with a 30-year daily observatio nal data set for mainly two regions of the U.S.: the Northwest, and th e central Great Plains. Statistics compared include mean daily precipi tation, mean daily intensity, frequency, transition probabilities, qua ntiles of precipitation intensity, and interquartile ranges. We discus s how different measures of daily precipitation lead to different conc lusions about the quality of the control run. For example, good agreem ent between model and observed data regarding mean daily precipitation usually results from compensating errors in the intensity and frequen cy fields (too high frequency and too low intensity). We analyze how d etailed topographic features of the RegCM enhance the simulation of da ily precipitation compared to the CCM simulation. In general, errors i n all measures are smallest at the Northwest grid points, and the damp ing of the seasonal cycle of mean daily precipitation from the coast t o inland Oregon is basically well reproduced. However, some errors in the frequency and intensity fields can be traced to inadequate represe ntation of topography, even with a horizontal resolution of 60 km, Dif ferences in the control and doubled CO, runs (for both RegCM and CCM) for these regions are also presented. The most significant changes for the RegCM grid points is increased variability of daily precipitation under doubled CO, conditions. Areas with significant changes (both in creases and decreases) of precipitation frequency and intensity are fo und, There are some areas where frequency decreases, but precipitation mean daily amounts increase. Such changes, which would be masked by m ore traditional analyses of precipitation change, are important from a climate impacts point of view. The limitations on the analyses posed by small sample sizes are discussed.