DOWNWELLING LONGWAVE FLUXES AT CONTINENTAL SURFACES - A COMPARISON OFOBSERVATIONS WITH GCM SIMULATIONS AND IMPLICATIONS FOR THE GLOBAL LAND-SURFACE RADIATION BUDGET

Previous work suggests that general circulation (global climate) model s have excess net radiation at land surfaces, apparently due to overes timates in downwelling shortwave Aux and underestimates in upwelling l ongwave flux. Parr of this excess, however, may be compensated for by an underestimate in downwelling longwave flux. Long term observations of the downwelling longwave component at several land stations in Euro pe, the United States, Australia, and Antarctica suggest that climate models (four are used, as in previous studies) underestimate this flux component on an annual basis by up to 10 W m(-2), yet with low statis tical significance. It is probable that the known underestimate in bou ndary-layer air temperature contributes to this, as would low model cl oudiness and neglect of minor gases such as methane, nitrogen oxide, a nd the freons. The bias in downwelling longwave flux, together with th ose found earlier for downwelling shortwave and upwelling longwave flu xes, are consistent with the model bias found previously for net radia tion. All annually averaged fluxes and biases are deduced for global l and as a whole.