Estimating seasonal variations in cloud droplet number concentration over the boreal forest from satellite observations R. H. H. Janssen1, L. N. Ganzeveld1, P. Kabat1, M. Kulmala2, T. Nieminen2, and R. A. Roebeling3 1Earth System Science and Climate Change, Wageningen University and Research Centre, Wageningen, The Netherlands 2Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland 3Royal Netherlands Meteorological Institute, De Bilt, The Netherlands
Abstract. Seasonal variations in cloud droplet number concentration (NCD) 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
NCD, 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 NCD 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
NCD 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.
Citation: Janssen, R. H. H., Ganzeveld, L. N., Kabat, P., Kulmala, M., Nieminen, T., and Roebeling, R. A.: Estimating seasonal variations in cloud droplet number concentration over the boreal forest from satellite observations, Atmos. Chem. Phys., 11, 7701-7713, doi:10.5194/acp-11-7701-2011, 2011.