Uncertainties in quantification of the first indirect aerosol effect

The first indirect aerosol effect is remaining one of the greatest uncertainties of anthropogenic climate change. This effect, also called Towmey effect, describes the increase of cloud albedo due to more aerosols in the boundary layer. This can be observed e.g. as ship tracks in maritime stratocumulus fields.

To quantify the first indirect aerosol effect globally from satellite, the retrieval of two key quantities is necessary:

• The cloud droplet number concentration
• The geometrical extent of the cloud

For the retrieval several assumptions are necessary. This includes a model about the vertical stratification within the cloud. The basic assumption in literature here is the adiabatic cloud model. Therein it is assumed, that cloud droplets grow due to condensation of water vapor without entrainment. The liquid water content is increasing linearly up to the cloud top. The profile depends of liquid water and droplet radius depends on the droplet number concentration and environment variables such as pressure and temperature.

Within the graduate school for clouds, radiation and aerosols (LGS-CAR) the validity of the idealized adiabatic cloud model is investigated, as well as the influence of aerosols on the microphysics and cloud albedo.