Assessment of land-surface energy budgets from regional and global models

The surface energy budgets estimated from the 0- to 12-hour forecasts of th ree operational model suites and the National Centers for Environmental Pre diction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis are analyzed at local to continental scales. The models are (1) the Eta model, (2) the Mesoscale Analysis and Prediction System (MAPS), and (3) the Globa l Environmental Multiscale (GEM) model. The first two are regional, while t he third one is global with a variable grid with a resolution over North Am erica that is equivalent to that of the regional models. This assessment of one summer month (August 1997) and one winter month (January 1998) has the purpose of estimating the reliability of the surface energy budgets within the context of the Global Energy and Water Cycle Experiment (GEWEX) Contin ental-Scale International Project (GCIP) goals. Satellite estimates were us ed to evaluate the downward short wave radiation at the surface, while meas urements from the southern Great Plains region were used to evaluate the mo del computed surface energy budget estimates. The results show that the sur face short wave radiation biases of the models are widespread and of the or der of 25-50 W m(-2) and averaging over larger areas does not help reduce t he differences. These biases are compensated by the other long and short wa ve radiation terms so that the resulting errors in the net radiation are sm aller. During August 1997, continental east-west gradients of latent heat f lux and Bowen ratio were surprisingly dissimilar among models. Still, the B owen ratio estimated from the Eta and GEM models was close to observations over the southern Great Plains region, while both the Reanalysis and MAPS h ad ratios that at least doubled the observed ones. In the case of MAPS a re vised latent heat flux formulation was introduced in fall 1997, and, subseq uently, for January 1998, estimates were closer to the other models' estima tes. However, during January 1998 all models had difficulties reproducing t he Bowen ratios from observations. Further, daily time series showed that m odels' estimates also tended to miss the amplitude of the day-to-day variab ility. It is conceivable that this may be the result of difficulties in par ameterizing the total cloud cover, and, particularly, attenuation by clouds may still be insufficient.