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This paper analyzes the ability of the North American Regional Climate Change Assessment Program (NARCCAP) ensemble of regional climate models to simulate extreme monthly precipitation and its supporting circulation for regions of North America, comparing 18 years of simulations driven by the National Centers for Environmental Prediction (NCEP)--Department of Energy (DOE) reanalysis with observations. The analysis focuses on the wettest 10%% of months during the cold half of the year (October--March), when it is assumed that resolved synoptic circulation governs precipitation. For a coastal California region where the precipitation is largely topographic, the models individually and collectively replicate well the monthly frequency of extremes, the amount of extreme precipitation, and the 500-hPa circulation anomaly associated with the extremes. The models also replicate very well the statistics of the interannual variability of occurrences of extremes. For an interior region containing the upper Mississippi River basin, where precipitation is more dependent on internally generated storms, the models agree with observations in both monthly frequency and magnitude, although not as closely as for coastal California. In addition, simulated circulation anomalies for extreme months are similar to those in observations. Each region has important seasonally varying precipitation processes that govern the occurrence of extremes in the observations, and the models appear to replicate well those variations. [ABSTRACT FROM AUTHOR]

This paper examines the accuracy of high-resolution nested mesoscale model simulations of surface climate. The nesting capabilities of the atmospheric fifth-generation Pennsylvania State University (PSU)–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) were used to create high-resolution, 5-yr climate simulations (from 1 October 1994 through 30 September 1999), starting with a coarse nest of 20 km for the western United States. During this 5-yr period, two finer-resolution nests (5 and 1.7 km) were run over the Yampa River basin in northwestern Colorado. Raw and bias-corrected daily precipitation and maximum and minimum temperature time series from the three MM5 nests were used as input to the U.S. Geological Survey’s distributed hydrologic model [the Precipitation Runoff Modeling System (PRMS)] and were compared with PRMS results using measured climate station data. The distributed capabilities of PRMS were provided by partitioning the Yampa River basin into hydrologic response units (HRUs). In addition to the classic polygon method of HRU definition, HRUs for PRMS were defined based on the three MM5 nests. This resulted in 16 datasets being tested using PRMS. The input datasets were derived using measured station data and raw and bias-corrected MM5 20-, 5-, and 1.7-km output distributed to 1) polygon HRUs and 2) 20-, 5-, and 1.7-km-gridded HRUs, respectively. Each dataset was calibrated independently, using a multiobjective, stepwise automated procedure. Final results showed a general increase in the accuracy of simulated runoff with an increase in HRU resolution. In all steps of the calibration procedure, the station-based simulations of runoff showed higher accuracy than the MM5-based simulations, although the accuracy of MM5 simulations was close to station data for the high-resolution nests. Further work is warranted in identifying the causes of the biases in MM5 local climate simulations and developing methods to remove them. [ABSTRACT FROM AUTHOR]

For 14 alternative climate futures, water yield and snow water equivalent (SWE) throughout the contiguous United States (CONUS) were projected over the twenty-first century using the Variable Infiltration Capacity model (VIC). The futures correspond to climate projections from seven CMIP5 models each forced by two representative concentration pathways for greenhouse gases (RCP4.5 and RCP8.5). With both RCPs, decreases in water yields are projected for roughly two-thirds of the CONUS, and in 60% of that area-mainly at more northern latitudes, where the greatest temperature increases are expected-this occurs despite projected increases in precipitation. The greatest relative decreases in yield are projected for the southern Great Plains and the Southwest, where temperature and precipitation changes combine to decrease yield. Snow accumulation is projected to decrease almost everywhere by the latter half of the century, with the time of peak SWE in some basins projected to occur up to 2 months earlier than it now does. These changes, should they come to pass, will challenge the adaptation capacity of future water management. [ABSTRACT FROM AUTHOR]

Drought indices are often used for monitoring interannual variability in macroscale hydrology. However, the diversity of drought indices raises several issues: 1) which indices perform best and where; 2) does the incorporation of potential evapotranspiration (PET) in indices strengthen relationships, and how sensitive is the choice of PET methods to such results; 3) what additional value is added by using higher-spatial-resolution gridded climate layers; and 4) how have observed relationships changed through time. Standardized precipitation index, standardized precipitation evapotranspiration index (SPEI), Palmer drought severity index, and water balance runoff (WBR) model output were correlated to water-year runoff for 21 unregulated drainage basins in the Pacific Northwest of the United States. SPEI and WBR with time scales encompassing the primary precipitation season maximized the explained variance in water-year runoff in most basins. Slightly stronger correlations were found using PET estimates from the Penman-Monteith method over the Thornthwaite method, particularly for time periods that incorporated the spring and summer months in basins that receive appreciable precipitation during the growing season. Indices computed using high-resolution climate surfaces explained over 10% more variability than metrics derived from coarser-resolution datasets. Increased correlation in the latter half of the study period was partially attributable to increased streamflow variability in recent decades as well as to improved climate data quality across the interior mountain watersheds. [ABSTRACT FROM AUTHOR]

Hourly surface precipitation type (Ptype) grids (a total of 408 h from 1 December 2005 through April 20, 2006) were generated by mapping the elevation of the radar-detected brightband height (BBH) to terrain elevation during the 2005/06 observation period of the western Hydrometeorology Testbed (HMT-West) in the North Fork American River basin. BBH Ptype grids were compared to those derived by the standard National Weather Service (NWS) temperature threshold method. In this method, a fixed threshold tem-perature separating rain and snow was applied to hourly 4-km gridded temperature data. The BBH Ptype grids agreed well (>90%) with the temperature threshold-based grids below an elevation of 1524 m. The agreement dropped to below 60% above this elevation, and BBH Ptype produced more rainfall than the temperature-based Ptype. Continuous hourly streamflow simulations were generated using spatially lumped and distributed hydrologie models with and without the BBH Ptype data from 1 October 2005 through 30 September 2006. Simple insertion of BBH Ptype data did not always improve streamflow simulations for the 11 events examined relative to corresponding simulations using temperature threshold-derived precipitation type, possibly because of the use of the models calibrated with the temperature-based Ptype. The simple sensitivity test indicated simulations of both peak flows from midwinter storms and spring snowmelt runoff are affected by errors in precipitation type estimates. [ABSTRACT FROM AUTHOR]

An atmospheric general circulation model simulation for 1948-97 of the water budgets for the MacKenzie, Mississippi, and Amazon River basins is presented. In addition to the water budget, passive tracers are included to identify the geographic sources of water for the basins, and the analysis focuses on the mechanisms contributing to precipitation recycling in each basin. While each basin's precipitation recycling has a strong dependency on evaporation during the mean annual cycle, the interannual variability of the recycling shows important relationships with the atmospheric circulation. The MacKenzie River basin recycling has only a weak interannual correspondence with evaporation, where the variations in zonal moisture transport from the Pacific Ocean can affect the basin water cycle. On the other hand, the Mississippi River basin precipitation and recycling have strong interannual correlation on evaporation. The evaporation is related to the moist and shallow planetary boundary layer that provides moisture for convection at the cloud base. At global scales, high precipitation recycling is also found to be partly correlated to warm SSTs in the tropical Pacific Ocean. The Amazon River basin evaporation exhibits small interannual variations, so the interannual variations of precipitation recycling are related to atmospheric moisture transport from the tropical South Atlantic Ocean. Increasing SSTs over the 50-yr period are causing increased easterly transport across the basin. As moisture transport increases, the Amazon precipitation recycling decreases (without real-time varying vegetation changes). In addition, precipitation recycling from a bulk diagnostic method is compared to the passive tracer method used in the analysis. While the mean values of the different recycling methods are different, the interannual variations are comparable between each method. The methods also exhibit similar relationships to the terms of the basin-scale water budgets. [ABSTRACT FROM AUTHOR]

ABSTRACT This study used the atmospheric Fifth-Generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5) and the University of Washington Distributed Hydrology-Soil-Vegetation Model (DHSVM) for the simulation of a complex rain-on-snow flood event that occurred from 28 December 1996 to 3 January 1997 on the 1560-km[sup 2] Snoqualmie River watershed in western Washington. Three control simulations were created with MM5 applied at 36-, 12-, and 4-km horizontal spacing and DHSVM at a horizontal spacing of 100 m. Results showed that the accuracy of the atmospheric fields increased with higher horizontal resolution, although underforecasting of precipitation was evident for all three resolutions. Simulated river flows captured 67% (36 km), 75% (12 kin), and 72% (4 km) of the total flow and 52% (36 km), 58% (12 kin), and 62% (4 km) of the event peak flow. Several sensitivity simulations of the modeling system (4-km spacing only) were conducted to improve on the control simulations. Adjusting the MM5 precipitation using observations led to a streamflow forecast that captured 90% of the total flow. Reduction of the model high-wind speed bias improved the simulated snowmelt, although the resulting effects on streamflow were relatively small. A sensitivity experiment that included the precipitation from an intense rainband that was not captured by MM5 revealed the importance of this high-intensity, short-lived feature; simulated streamflow from this experiment captured 93% of the total flow and over 82% of the peak flow, with a 4-h timing error. A final set of sensitivity simulations, using both a higher- and... [ABSTRACT FROM AUTHOR]