ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-5-1975-2005 Spectral actinic flux in the lower troposphere: measurement and 1-D simulations for cloudless, broken cloud and overcast situations Kylling A. 1 Webb A. R. 2 Kift R. 2 Gobbi G. P. 3 Ammannato L. 3 Barnaba F. 3 Bais A. 4 Kazadzis S. 4 Wendisch M. 5 Jäkel E. 5 Schmidt S. 5 Kniffka A. 6 Thiel S. 7 Junkermann W. 7 Blumthaler M. 8 Silbernagl R. 8 Schallhart B. 8 Schmitt R. 9 Kjeldstad B. 10 Thorseth T. M. 10 Scheirer R. 11 Mayer B. 11 Norwegian Institute for Air Research, Kjeller, Norway; now at St. Olavs Hospital, Trondheim University Hospital, Norway Physics Department, University of Manchester Institute of Science and Technology, Manchester, UK Istituto di Scienze dell’Atmosfera e del Clima-CNR, Roma, Italy Laboratory of Atmospheric Physics Aristotle University of Thessaloniki, Greece Leibniz-Institut für Troposphärenforschung, Leipzig, Germany Institut für Meteorologie, Universität Leipzig, Leipzig Italy Institut für Meteorologie und Klimaforschung, Garmisch-Partenkirchen, Germany Institute of Medical Physics, University of Innsbruck, Innsbruck, Austria Meteorologie Consult GmbH, Germany Department of Physics, Norwegian University of Science and Technology, Trondheim Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Wessling, Germany 03 08 2005 5 7 1975 1997 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/5/1975/2005/acp-5-1975-2005.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/5/1975/2005/acp-5-1975-2005.pdf

In September 2002, the first INSPECTRO campaign to study the influence of clouds on the spectral actinic flux in the lower troposphere was carried out in East Anglia, England. Measurements of the actinic flux, the irradiance and aerosol and cloud properties were made from four ground stations and by aircraft. The radiation measurements were modelled using the uvspec model and ancillary data. For cloudless conditions, the measurements of the actinic flux were reproduced by 1-D radiative transfer modelling within the measurement and model uncertainties of about ±10%. For overcast days, the ground-based and aircraft radiation measurements and the cloud microphysical property measurements are consistent within the framework of 1-D radiative transfer and within experimental uncertainties. Furthermore, the actinic flux is increased by between 60-100% above the cloud when compared to a cloudless sky, with the largest increase for the optically thickest cloud. Correspondingly, the below cloud actinic flux is decreased by about 55-65%. Just below the cloud top, the downwelling actinic flux has a maximum that is seen in both the measurements and the model results. For broken clouds the traditional cloud fraction approximation is not able to simultaneously reproduce the measured above-cloud enhancement and below-cloud reduction in the actinic flux.