ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-7701-2011 Estimating seasonal variations in cloud droplet number concentration over the boreal forest from satellite observations Janssen R. H. H. 1 Ganzeveld L. N. 1 Kabat P. 1 Kulmala M. 2 Nieminen T. 2 Roebeling R. A. 3 Earth System Science and Climate Change, Wageningen University and Research Centre, Wageningen, The Netherlands Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland Royal Netherlands Meteorological Institute, De Bilt, The Netherlands 02 08 2011 11 15 7701 7713 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/11/7701/2011/acp-11-7701-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/7701/2011/acp-11-7701-2011.pdf

Seasonal variations in cloud droplet number concentration (<i>N</i><sub>CD</sub>) in low-level stratiform clouds over the boreal forest are estimated from MODIS observations of cloud optical and microphysical properties, using a sub-adiabatic cloud model to interpret vertical profiles of cloud properties. An uncertainty analysis of the cloud model is included to reveal the main sensitivities of the cloud model. We compared the seasonal cycle in <i>N</i><sub>CD</sub>, obtained using 9 yr of satellite data, to surface concentrations of potential cloud activating aerosols, measured at the SMEAR II station at Hyytiälä in Finland. The results show that <i>N</i><sub>CD</sub> and cloud condensation nuclei (CCN) concentrations have no clear correlation at seasonal time scale. The fraction of aerosols that actually activate as cloud droplet decreases sharply with increasing aerosol concentrations. Furthermore, information on the stability of the atmosphere shows that low <i>N</i><sub>CD</sub> is linked to stable atmospheric conditions. Combining these findings leads to the conclusion that cloud droplet activation for the studied clouds over the boreal forest is limited by convection. Our results suggest that it is important to take the strength of convection into account when studying the influence of aerosols from the boreal forest on cloud formation, although they do not rule out the possibility that aerosols from the boreal forest affect other types of clouds with a closer coupling to the surface.

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