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; now at St. Olavs Hospital, Trondheim University Hospital, 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 Italy 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 11Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Wessling, Germany
Abstract. 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
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., 5, 1975-1997, doi:10.5194/acp-5-1975-2005, 2005.