Atmos. Chem. Phys. Discuss., 5, 1421-1467, 2005
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Spectral actinic flux in the lower troposphere: measurement and 1-D simulations for cloudless, broken cloud and overcast situations
A. Kylling1,*, A. R. Webb2, R. Kift2, G. P. Gobbi3, L. Ammannato3, F. Barnaba3, A. Bais4, S. Kazadzis4, M. Wendisch5, E. Jäkel5, S. Schmidt5, A. Kniffka6, S. Thiel7, W. Junkermann7, M. Blumthaler8, R. Silbernagl8, B. Schallhart8, R. Schmitt9, B. Kjeldstad10, T. M. Thorseth10, R. Scheirer11, and B. Mayer11
1Norwegian Institute for Air Research, Kjeller, Norway
2Physics Department, University of Manchester Institute of Science and Technology, Manchester, UK
3Istituto di Scienze dell’Atmosfera e del Clima-CNR, Roma, Italy
4Laboratory of Atmospheric Physics Aristotle University of Thessaloniki, Greece
5Leibniz-Institut für Troposphärenforschung, Leipzig, Germany
6Institut für Meteorologie, Universität Leipzig, Leipzig, Germany
7Institut für Meteorologie und Klimaforschung, Garmisch-Partenkirchen, Germany
8Institute of Medical Physics, University of Innsbruck, Innsbruck, Austria
9Meteorologie Consult GmbH, Germany
10Department of Physics, Norwegian University of Science and Technology, Trondheim, Norway
11Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Wessling, Germany
*now at: St. Olavs Hospital, Trondheim University Hospital, Norway

Abstract. 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.

Citation: Kylling, A., Webb, A. R., Kift, R., Gobbi, G. P., Ammannato, L., Barnaba, F., Bais, A., Kazadzis, S., Wendisch, M., Jäkel, E., Schmidt, S., Kniffka, A., Thiel, S., Junkermann, W., Blumthaler, M., Silbernagl, R., Schallhart, B., Schmitt, R., Kjeldstad, B., Thorseth, T. M., Scheirer, R., and Mayer, B.: Spectral actinic flux in the lower troposphere: measurement and 1-D simulations for cloudless, broken cloud and overcast situations, Atmos. Chem. Phys. Discuss., 5, 1421-1467, doi:10.5194/acpd-5-1421-2005, 2005.
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