11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Friday, 7 June 2002: 9:00 AM
Substantial Underestimation of Solar Global and Diffuse Radiation caused by pyranometer thermal offsets
Rolf Philipona, World Radiation Center, Davos Dorf, Switzerland
Climate change perspectives intensified investigations of the radiative balance of the Earth- atmosphere system. At the top of the atmosphere solar irradiance is known with absolute uncertainty of 0.3% and theoretical models agree with albedo measurements. But solar shortwave radiation at the Earth’s surface is measured lower than calculated by radiative- transfer models. This model-observation discrepancy (10 -25 Wm -2 ) led to a decade long controversy on unexplained enhanced absorption of shortwave radiation in clear-sky atmospheres as well as in clouds. Reinvestigations of pyranometer calibration in conjunction with thermal offsets and pyranometer thermal conditioning demonstrate an underestimation of clear-sky solar diffuse as well as global radiation by 8 to 20 Wm -2 , caused by pyranometer differential cooling. Comparisons between unconditioned and thermally conditioned pyranomter measurements suggest that traditional shortwave radiation measurements considerably underestimated global radiation, and this could explain a substantial part of the missing absorption of solar radiation in the atmosphere sought-after in model calculations.

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