The influence of altitude on wet deposition comparison between field measurements at great dun fell and the predictions of a seeder-feeder model

The influence of topography on rainfall rate and chemical composition has been investigated at Great Dun Fell in northern England. The measurements at eight different altitudes between 250 and 850 m above sea-level on the western slopes of Great Dun Fell (GDF) show, in the presence of a cap cloud on the hill and a west or southwest wind flow, a marked increase in both rainfall amount and concentrations of major ions with altitude. Three case studies of the variation in rainfall rate and chemical composition with altitude when the seeder-feeder mechanism was operating are described and compared with model predictions. Associated measurements of the cap cloud microphysics and chemistry were also made. The case studies show close agreement between model predictions and measured values for changes in chemical composition and wet deposition with altitude. This pattern of wet deposition is a frequent occurrence at GDF, but exceptions do occur, for example when the upstream flow is blocked or the wind speed is small, no increase in the concentration or rainfall amount was observed. © 1988.