COMPUTER MODELING OF CLOUDS AT KLEINER-FELDBERG

The airflow, cloud microphysics and gas- and aqueous-phase chemistry o n Kleiner Feldberg have been modelled for the case study of the evenin g of 1 November 1990, in order to calculate parameters that are not ea sily measured in the cloud and thus to aid the interpretation of the G CE experimental data-set. An airflow model has been used to produce th e updraught over complex terrain for the cloud model, with some care r equired to ensure realistic modelling of the strong stable stratificat ion of the atmosphere. An extensive set of measurements has been made self-consistent and used to calculate gas and aerosol input parameters for the model. A typical run of the cloud model has calculated a peak supersaturation of 0.55% which occurs about 20 s after entering cloud where the updraught is 0.6 m s(-1). This figure has been used to calc ulate the efficiency with which aerosol particles were scavenged; it i s higher than that calculated by other methods, and produces a cloud w ith slightly too many droplets. A broad cloud droplet size spectrum ha s been produced by varying the model inputs to simulate turbulent mixi ng and fluctuations in cloud parameters in space and time, and the abi lity of mixing processes near cloud-base to produce a lower peak super saturation is discussed. The scavenging of soluble gases by cloud drop lets has been observed and departures from Henry's Law in bulk cloud-w ater samples seen to be caused by variation of pH across the droplet s pectrum and the inability of diffusion to adjust initial distributions of highly soluble substances across the spectrum in the time availabl e. Aqueous-phase chemistry has been found to play a minor role in the cloud as modelled, but circumstances in which these processes would be more important are identified.