EVALUATING NCEP ETA MODEL-DERIVED DATA AGAINST OBSERVATIONS

Data derived at the National Centers for Environmental Prediction via four-dimensional data assimilation using the Era Model were evaluated against surface observations from two observational arrays, one locate d in the semihumid, continental climate of Oklahoma and Kansas and the second in the semiarid climate of southern Arizona. Comparison was ma de for the period of the Global Energy Water-cycle Experiment Continen tal-scale International Project's ''GIST'' dataset in 1994 and their ' 'ESOP-95'' dataset in 1995, and for the months of March and May in 199 6. Coding errors in the Eta Model's postprocessor used to diagnose nea r-surface temperature and humidity are shown to have compromised the G IST and ESOP-95 near-surface data. A procedure was devised to correct the GIST and ESOP-95 near-surface fields by mimicking the corrected co de used in the Era Model since January 1996. Comparison with observati ons revealed that modeled surface solar radiation is significantly ove restimated except in clear-sky conditions. This discrepancy in cloudy- sky solar radiation was altered little by the substantial January 1996 revisions to Era Model physics, but the revisions are shown to have g reatly improved the model's ability to capture daily and seasonal vari ations in near-surface air temperature, specific humidity, and wind sp eed. The poorly modeled surface radiation complicates evaluation of mo deled 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 inter actions in the data assimilation model. To mitigate against this and t o aid intercomparisons between other data, it is recommended that mode l-derived data always include sufficient information to allow potentia l users to recalculate the extrapolation to the surface using a user-d efined model of surface-atmosphere exchanges.