Validation of very high cloud droplet number concentrations in air masses transported thousands of kilometres over the ocean

The microstructure of orographic clouds related to the aerosol present was studied during the second Aerosol Characterisation Experiment (ACE-2). Very high cloud droplet number concentrations (almost 3000 cm(-3)) were observe d. These high concentrations occurred when clouds formed on a hill slope at Tenerife in polluted air masses originating in Europe that had transported the order of 1000 km over the Atlantic Ocean. The validity of the measured droplet number concentrations was investigated by comparing with measureme nts of the aerosol upstream of the cloud and cloud interstitial aerosol. Gu ided by distributions of the ratios between the measurements, three criteri a of typically 30% in maximum deviation were applied to the measurements to test their validity. Agreement was found for 88% of the cases. The validat ed data set spans droplet number concentrations of 150 3000 cm(-3). The upd raught velocity during the cloud formation was estimated to 2.2 m s(-1) by model calculations, which is typical of cumuliform clouds. The results of t he present study are discussed in relation to cloud droplet number concentr ations previously reported in the literature. The importance of promoting t he mechanistic understanding of the aerosol/cloud interaction and the use o f validation procedures of cloud microphysical parameters is stressed in re lation to the assessment of the indirect climatic effect of aerosols.