Further developments in closure experiments for surface diffuse irradiance under cloud-free skies at a continental site

Direct and diffuse surface irradiances measured with the shaded and ventilated Black and White Eppley pyranometer under cloud-free skies at a continental site are compared with radiative transfer model calculated values. For the measurements in 2002, calculations of irradiance utilizing surface-measured aerosol single scattering albedo (SSA) agree with that measured to within the mutual uncertainties. For the days in 2001 and 2003, the modeled irradiance values agree with that measured only if highly absorbing aerosols of SSA similar to0.7, well below surface-measured SSA, are used. Modeled diffuse/direct irradiance ratios continue to exceed those measured in the visible and near-IR using silicon detector-based radiometers, suggesting problems with measurements and/or model estimates of atmospheric, most likely, aerosol, absorption. A detailed study is therefore required to establish the role of aerosol absorption in shortwave radiative transfer.