OROGRAPHIC ENHANCEMENT OF WET DEPOSITION IN THE UNITED-KINGDOM - CASE-STUDIES AND MODELING

Two field experiments to observe the detailed response of wet depositi on to orography in a polluted environment are reported. Rain events we re classed as frontal, convective or mixed on the basis of meteorologi cal data. Analysis of the deposition enhancement and cap cloud composi tion confirmed that for the frontal events the seeder-feeder effect (s cavenging of cap cloud by rain drops) dominates. The greater concentra tion of ions in the water scavenged from the cap cloud than in the rai n means that deposition is enhanced for all ions. For marine ions the scavenged water was found to be between five and six times as concentr ated as the rain and for anthropogenically produced ions it was about twice as concentrated. A computational model of rainfall incorporating the seeder-feeder effect has been broadly successful in predicting en hancement although some details of the observed pattern remain to be e xplained. Convective events were only important in the deposition of m arine ions although this may not be the case in the summer months. Con vective events were found not to be subject to the seeder-feeder effec t.