Water Cycle Experiment Continental-Scale International Research Articles

Archival precipitation data set for the Mississippi River Basin: Evaluation

[1] The goals of the Global Energy and Water Cycle Experiment Continental-Scale International Project (GCIP) point to the need for high resolution data sets on all elements of the land surface and atmospheric hydrologic cycle. A high resolution precipitation data set has been derived from radar reflectivity observations taken from the National Weather Service WSR-88D radars in the continental U.S. To evaluate the product the authors provide several case studies of radar-rain gauge comparisons at locations throughout the Mississippi River Basin. They present bias, root mean square difference, fractional standard difference, and correlation coefficient statistics for radar-rain gauge comparisons for the hourly, daily, monthly, yearly and warm season temporal scale. These point (gauge) and pixel (radar) comparisons show large discrepancies at the hourly scale, on the order of 600–800%. An evaluation of the differences associated with temporally integrated estimates shows marked reduction in these discrepancies. At the long-term (warm season), these reduce to about 10%. An estimate of the difference in the comparison of the long term accumulation of gridded gauge based estimates and radar estimates at 0.25° × 0.25° shows values in the range of 20% but decrease to about 15% after applying filtering techniques in the basin-wide comparisons.

NCEP–NCAR and ECMWF Reanalysis Surface Water and Energy Budgets for the Mississippi River Basin

Surface water and energy budgets from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) and European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses are compared here with each other and with available observations over the Mississippi River basin, which is a focus of the Global Energy and Water Cycle Experiment Continental-Scale International Project. There are a number of noticeable differences and similarities in the large-scale basin averages. The NCEP–NCAR reanalysis seasonal precipitation and runoff are larger than the available observations; presumably, evaporation and surface water variations also are too large. The ECMWF reanalysis precipitation is much closer to the observations, whereas the corresponding surface runoff and seasonal surface water variations are too small. The NCEP–NCAR and ECMWF reanalysis seasonal energy components are more similar to each other. The NCEP–NCAR and ECMWF interannual variations also are comparable, indicating that these reanalyses probably can be used to begin to study interannual variations. Nonetheless, improved land surface parameterizations are needed to depict surface water and energy processes and, in particular, variations in seasonal surface water and runoff better.

Evaluating NCEP Eta Model–Derived Data against Observations

Data derived at the National Centers for Environmental Prediction via four-dimensional data assimilation using the Eta Model were evaluated against surface observations from two observational arrays, one located in the semihumid, continental climate of Oklahoma and Kansas and the second in the semiarid climate of southern Arizona. Comparison was made for the period of the Global Energy Water-cycle Experiment Continental-scale International Project’s ‘‘GIST’’ dataset in 1994 and their ‘‘ESOP-95’’ dataset in 1995, and for the months of March and May in 1996. Coding errors in the Eta Model’s postprocessor used to diagnose near-surface temperature and humidity are shown to have compromised the GIST and ESOP-95 near-surface data. A procedure was devised to correct the GIST and ESOP-95 near-surface fields by mimicking the corrected code used in the Eta Model since January 1996. Comparison with observations revealed that modeled surface solar radiation is significantly overestimated except in clear-sky conditions. This discrepancy in cloudy-sky solar radiation was altered little by the substantial January 1996 revisions to Eta Model physics, but the revisions are shown to have greatly improved the model’s ability to capture daily and seasonal variations in near-surface air temperature, specific humidity, and wind speed. The poorly modeled surface radiation complicates evaluation of modeled surface energy fluxes, but comparison with observations suggests that the modeled daytime Bowen ratio may be systematically high. This study clearly demonstrates the strong sensitivity of model-calculated, near-surface variables to the physics used to describe surface interactions in the data assimilation model. To mitigate against this and to aid intercomparisons between other data, it is recommended that model-derived data always include sufficient information to allow potential users to recalculate the extrapolation to the surface using a user-defined model of surface‐atmosphere exchanges.

GDAS’s GCIP Energy Budgets

Abstract The National Centers for Environmental Prediction’s operational global data assimilation system’s (GDAS) atmospheric and surface thermodynamic energy cycles are presented for the Mississippi River basin where the Global Energy and Water Cycle Experiment Continental-Scale International Project (GCIP) is under way. At the surface, during the winter, incoming solar radiation is balanced by longwave cooling. During the summer, latent and sensible cooling are equally important. In the atmosphere, thermodynamic energy convergence is also important, especially during the winter. In most places, precipitation is largely balanced by thermodynamic energy divergence. Anomalously high surface temperatures appear to be mainly related to decreased surface evaporation. Anomalously high (low) precipitation variations may also be related to anomalously high thermodynamic energy divergence (convergence). Unfortunately, residual terms, which are slightly noticeable for the GCIP climatological balances, are especial...