Abstract
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.
Highlights
The biosphere makes a very large contribution to the levels of atmospheric aerosols and cloud condensation nuclei (Andreae and Rosenfeld, 2008)
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
The results show that NCD and cloud condensation nuclei (CCN) concentrations have no clear correlation at seasonal time scale
Summary
The biosphere makes a very large contribution to the levels of atmospheric aerosols and cloud condensation nuclei (Andreae and Rosenfeld, 2008). Monoterpenes are emitted in large quantities by boreal forests, following a strong seasonal pattern, determined by a pronounced seasonal cycle in temperature, light intensity and vegetation productivity (Hakola et al, 2003; Lappalainen et al, 2009) Once oxidized, these organics condense onto freshly nucleated clusters to grow them to sizes larger than 3 nm, which allows them to survive as individual aerosols (O’Dowd et al, 2002; Cavalli et al, 2006), and contribute to their further growth to a diameter of 50 to 100 nm, which allows them to act as CCN (Tunved et al, 2008). The paper is concluded by a discussion and conclusions Sect. 4
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