The dark side of the cloud

Markus Petters, PhD candidate

University of Wyoming

Clouds strongly modify the Earths' radiation budget, primarily by reflecting solar radiation. The amount of sunlight reflected by clouds (albedo) depends on cloud macrophysical and microphysical properties, most importantly cloud droplet number concentration (CDNC) and their size distribution. In the atmosphere, cloud droplets come onstage by condensation of water on cloud condensation nuclei (CCN).  Thus, Twomey (1977) speculated that a change in the CCN leads to a change in cloud albedo, which has also become known as the first indirect effect of aerosol on climate.

To quantify this effect, it is essential to predict changes in CDNC and size spectra resulting from changes in aerosol physicochemical properties. This work shows that CCN activation can be predicted from aerosol size and chemical composition data, and that the probability distribution function of CDNC in stratus clouds can be predicted accurately from updraft and the CCN activation spectrum. Furthermore, this work evaluates the degree of certainty to which the CCN activation spectrum can be known and thus CDNC can be predicted. This includes fitting uncertainties and the assumed functional dependence of the CCN activation spectrum. Furthermore, uncertainties about updraft velocities, observed droplet concentrations and assumed parameters about droplet growth are discussed.