On the parameterization of activation spectra from cloud condensation nuclei microphysical properties

A simple parametric relationship is established between factors describing the shape of cloud condensation nuclei (CCN) activation spectra and observa ble properties of the aerosol population they grow on (size distribution an d solubility). This is done independently for maritime and continental aero sol types because of their very different characteristics. The data used fo r the multiple statistical adjustments in the procedure described in this p aper are generated by running a numerical model of aerosol growth coupled t o a simple cloud droplet activation scheme. Each aerosol population (mariti me and continental) is assumed to be of homogeneous chemical composition, l ognormally distributed and with variable solubility. The parameterization i s then evaluated using a large set of aerosol populations with randomized p roperties. Finally, the study presents a preliminary analysis of the most i mportant aerosol properties that influence the shape of the CCN spectra. An idealized scenario of a clean maritime boundary layer cloud perturbed by a nthropogenic emissions (such as the ship track problem) illustrates the cap ability of the parameterization to selectively increase the cloud droplet c oncentration in a partially polluted cloud. The calibration results present ed in this paper are not meant to be the definitive activation spectra prod uced by any lognormally distributed aerosols. These results are indeed a st ep toward an objective initialization of CCN spectra and hence toward the c omputation of cloud droplet concentrations based on measurable multimodal a erosol features, as required by three-dimensional numerical models with a c oupled interactive aerosol module.