1. Interannual Variability of Warm-Season Rainfall over the US Great Plains in NCAR/CAM and NASA/NSIPP Simulations: Intercomparisons for NAME Alfredo Ruiz–Barradas and Sumant Nigam University of Maryland Abstract Interannual variability of summer rainfall and moisture fluxes from two state-of-the-art AMIP model simulations (NCAR/CAM2.0 and NSIPP/ARIES) are analyzed over the US Great Plains in order to advance the goals of the North American Monsoon Intercomparison Project (NAMIP). The simulations are produced using the observed 1958-1998 lower boundary conditions. The retrospective U.S. and Mexican precipitation data sets, and the NCEP and 40-year ECMWF reanalysis are used as targets for the simulations. The simulations are in some disagreement with each other and with nature in portrayal of rainfall variability over the US Great Plains and its linkages over southern United States and Mexico. Notable Great Plains precipitation anomalies are linked with vertically integrated, stationary moisture flux anomalies from the Gulf of Mexico in nature, but not in the simulations. Models do produce significant rainfall anomalies over central U.S. but these are likely generated from other processes (e.g., local evaporation). The Great Plains precipitation is found linked to Pacific SSTs in both simulations, but the NCAR structure is closer to the observed pattern. Figure 1: Standard deviation of monthly precipitation during June-August. Box marks the Great Plains region used in defining an index. Contour interval is 0.4 mm/day, and values greater than 0.8 are shaded. Figure 2: Standard deviation of monthly summer precipitation in the NCAR/CAM2.0 and NSIPP/ ARIES AMIP simulations. Contour interval and shading as above. Figure 3: The Great Plains precipitation anomalies in observations; thick line shows the 13-month running mean. JJA correlations are based on unsmoothed data. Figure 4: The Great Plain precipitation index in ERA-40 is split up into its convective and large-scale condensation components. Summary Observational data sets are in some agreement in their description of US Great Plains precipitation variability. NCEP and ERA-40 reanalyses however exhibit lesser accord, with ERA-40 having higher correlations with observations. AMIP simulations are in considerable disagreement both with each other and with verifying observations of hydroclimate variability over the Great Plains. Great Plains precipitation variability is linked with rather different distributions of large-scale moisture fluxes in models and observations. Great Plains precipitation is linked with Pacific SSTs in AMIP simulations, somewhat as in observations. Figure 5: The Great Plains precipitation anomalies in AMIP simulations; uncorrelated with observations; the ensemble-mean simulation could exhibit higher correlation, though (a calculation in progress). Figure 6: JJA Regressions of Moisture Flux Convergence on Great Plains precipitation (unsmoothed index); moisture-flux vector scale is indicated; convergent regions are green. Figure 7: SST regressions on the smoothed Great Plains precipitation index during June-August. Top panel shows regressions with observations.