Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 2 2005 10.5194/acpd-5-1421-2005 http://www.atmos-chem-phys-discuss.net/5/1421/2005/ http://www.atmos-chem-phys-discuss.net/5/1421/2005/acpd-5-1421-2005.html http://www.atmos-chem-phys-discuss.net/5/1421/2005/acpd-5-1421-2005.pdf 1421 1467 2005-03-10 Spectral actinic flux in the lower troposphere: measurement and 1-D simulations for cloudless, broken cloud and overcast situations A. Kylling A. R. Webb R. Kift G. P. Gobbi L. Ammannato F. Barnaba A. Bais S. Kazadzis M. Wendisch E. Jäkel S. Schmidt A. Kniffka S. Thiel W. Junkermann M. Blumthaler R. Silbernagl B. Schallhart R. Schmitt B. Kjeldstad T. M. Thorseth R. Scheirer B. Mayer Norwegian Institute for Air Research, Kjeller, 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, Germany 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, Norway Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Wessling, Germany now at: St. Olavs Hospital, Trondheim University Hospital, Norway In September 2002 an extensive 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. For cloudless conditions the measurements of the actinic flux were reproduced by a 1-D radiative transfer model within the measurement and model uncertainties of about ±5%. For overcast days 1-D radiative transfer calculations reproduce the overall behaviour of the actinic flux measured by the aircraft. 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. Similarily the below cloud actinic flux is decreased by about 55–65%. Just below the cloud top the downwelling actinic flux has a maximum which 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.