Atmospheric Absorption in ARESE II: Observations and Models

Observations of the absorption of solar radiation by the atmosphere in clear and cloudy skies are compared to several calculations performed with different radiative transfer models. It is found that the calculated absorption differs from the observed value by a significant amount in cloudy conditions, with models underpredicting the absorption. Most interestingly it is also found that the models, all using identical input, differ among themselves significantly, e.g. yielding absorptance estimates ranging from 15% to 19% for the column of atmosphere from the surface up to 7 km. Models predict clear sky absorptance in agreement with the observations within experimental uncertainties. These results are from ARESE II; a comparison with ARESE I results shows agreement within observational uncertainties. As in ARESE I, visible absorptance is present on some days at levels of up to 2.5%, which can be attributed to the presence of aerosols.