The unexplained solar absorption and atmospheric H2O: a direct test using clear-sky data

This paper is motivated by several recent studies that have shown that observations of atmosphericsolar absorption systematically and significantly exceed model estimates. This papertests whether uncertainties in the physics of water vapor absorption in clear skies are directlyresponsible for this unexplained excess absorption. Radiative transfer calculations of clear-skysolar fluxes are compared to measurements in the tropical Pacific at the surface and the tropopause.We find that the atmospheric absorption in excess of that predicted by radiative transfermodels, if it exists, is less than the experimental uncertainty of 7 W m^-2 (diurnally averaged).Furthermore, the difference between observed and modeled absorption is essentially independentof water vapor amounts between 35 and 51 kg m^-2. A more direct test of the accuracy ofmodeled water vapor absorption is conducted with two independent multi-spectral radiometersat the Atmospheric Radiation Measurement site in Oklahoma, each providing over 16 000surface measurements of direct solar radiation in the 0.94 mm region. These spectral data confirmstate-of-the-art radiation model computations of water vapor line absorption to within 5% forthe wavelength region tested. The model-observation agreement for both tropical andOklahoma data strongly suggests that uncertainties in the physics of water vapor absorptionin clear skies are not a source for any significant excess solar absorption, thus narrowing thesearch to other atmospheric constituents or water vapor in cloudy skies.