MICROPHYSICS OF CLOUDS - MODEL VS MEASUREMENTS

In order to study the relation between the initial aerosol particle sp ectrum at cloud base and the resulting droplet spectrum in cloud for t he ''Ground-based Cloud Experiment'' field campaign at the Great Dun F ell in 1993 numerical model simulations have been performed. The dropl et spectra were calculated from a microphysical model coupled to a dyn amic air flow model. The resulting droplet spectra were compared with cloud droplet spectra measured with a forward scattering spectrometer probe. The size distribution and chemical composition of the initial a erosol population were derived from a combination of size distribution and size-segregated chemical measurements below cloud base. From this we concluded that the aerosol particles consisted almost entirely of an inorganic salt. As part of the sensitivity studies two different mi crophysical models were used, as well as the dynamic flow fields from two different air flow models. As in previous studies we found that th e measured droplet spectra were broader and contained larger drops tha n the modelled spectra. From the sensitivity studies we identified flu ctuations in the dynamics as the most likely explanation for these dif ferences. (C) 1997 Elsevier Science Ltd.