COMPARISON OF PRECIPITATION OBSERVED OVER THE CONTINENTAL UNITED-STATES TO THAT SIMULATED BY A CLIMATE MODEL

This study compares monthly average frequency, intensity, and amount o f hourly precipitation simulated by National Center for Atmospheric Re search Community Climate Model version 2/Biosphere-Atmosphere Transfer Scheme to the smoothed estimates of those observed locally over the c ontinental United States. The observed monthly intensities vary from l ess than 1 to 5 mm h(-1), with lowest values in the winter in northern midcontinent and highest around the Gulf Coast in summer. Model inten sities are on the average 3-4 times less when drizzle is excluded and an order of magnitude less when drizzle is included. As might be antic ipated, relative frequencies are much too high and intensities much to o low in the model fields, compared to those observed. The spatial pat tern of these quantities and the total precipitation are more realisti c. The study also compares extreme events and diurnal and seasonal var iations and finds that, in general, the simulated precipitation has la rger spatial variability, larger diurnal variation, and longer maximum continuous wet and dry periods than those observed. These discrepanci es are largely due to the different nature of model (area averaged) pr ecipitation and observed (local) precipitation. Therefore, the use of GCM output to directly represent local values of intensities and frequ encies may lead to large errors in the coupling of the land surface to the atmosphere above it.