Friday, 7 June 2002: 9:00 AM
Substantial Underestimation of Solar Global and Diffuse Radiation caused by pyranometer thermal offsets
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.