1. Results: Model Skill Cool season RegCM2 precipitation exhibits negative (positive) bias in the south central U. S. (northwestern U.S.). Warm season precipitation rates greater than 1 mm day -1 are more frequent in RegCM2 than observations, whereas all rates are less frequent in RegCM2 during the cool season. Results: Model Diagnostics We examined subsurface and atmospheric water balances to infer sources of precipitation error. Biases of water balance components are related by:  E =  P - {(P-E) m - R o } + {dW/dt} o  C = {(P-E) m - R o } + {dW/dt} o where subscripts o and m indicate observation and model, and  is bias. Observed P, R o and model output suggest test values for the unobserved {dW/dt} o. SON  E is larger than  C over much of the range of possible error in dW/dt. For DJF, results are mixed. We generated self-organizing maps (SOMs) from RegCM2 and VEMAP monthly precipitation fields. Centroids of frequency distributions for monthly precipitation show the trajectories of simulated and observed annual cycles through the SOM space. SOMs show: Variability of monthly precipitation is largest in the southeastern and northwestern U. S. Precipitation increases in the southeastern U.S. (northwestern U. S.) along the x-axis (y-axis) Trajectories of the annual cycle show: RegCM2 favors patterns of low (high) precipitation in the southeastern U. S. (northwestern U. S.) during fall and winter. A RegCM2 cluster during December-February suggests a preferred winter regime in the model. Centroids map onto SOMs Francis O. Otieno, W. J. Gutowski, Jr., R.W. Arritt, Z. Pan and E. S. Takle Iowa State University, Ames, Iowa, USA Analysis of REGM2 Precipitation Error using Self-Organizing Maps and Land-Atmosphere Water Budgets Francis O. Otieno, W. J. Gutowski, Jr., R.W. Arritt, Z. Pan and E. S. Takle Iowa State University, Ames, Iowa, USA Conclusion RegCM2 precipitation has a persistent negative bias in the south central U.S. during the cool season. The region’s precipitation deficiency in SON appears to result more from evapotranspiration error than moisture flux convergence error. Project Overview We generated 10-year simulations (January 1979 - December 1988) over the U.S. with the Regional Climate Model Version 2 (RegCM2). We compared RegCM2 precipitation with gridded precipitation from the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP). We analyzed subsurface and surface water balances using also a runoff climatology developed at the University of New Hampshire and the Global Runoff Data Center. Model grid: 52 km over the contiguous U.S. and adjacent oceans. Boundary data source: NCEP/NCAR Reanalysis Initial soil moisture: NCEP/NCAR Reanalysis Objectives Evaluate and characterize RegCM2 precipitation errors Explain systematic precipitation error in the south central U. S. Acknowledgement This research was funded by grants from the U.S. National Oceanic and Atmospheric Administration and the Electric Power Research Institute. Bias score versus precipitation threshold [mm/d] Model - observed precipitation [mm/d] Bias score at selected thresholds