MODELS OVERESTIMATE DIFFUSE CLEAR-SKY SURFACE IRRADIANCE - A CASE FOREXCESS ATMOSPHERIC ABSORPTION

Radiative transfer models consistently overestimate surface diffuse do wnward irradiance in cloud-free atmospheres by 9 to 40% at two low alt itude sites while correctly calculating direct-normal Solar irradiance . For known systematic and random measurement errors and for realistic aerosol optical properties, the discrepancy can be resolved by a redu ction in the vertical aerosol optical thickness (AOT) inferred from su nphotometric measurements by an average 0.02 +/- 0.01 for 32 cases exa mined, together with a compensating increase in a continuum-like atmos pheric absorptance over the solar spectrum of similar to 5.0% +/- 3.0% . This phenomenon is absent at two high altitude sites, where models a nd measurements agree to within their mutual uncertainties. Examinatio n of apparent AOT at several locations around the globe also indicates presence of such excess atmospheric absorption. The proposed absorpti on and corresponding reduction in AOT would have important consequence s for climate prediction and remote sensing.