The effect of orography on wet deposition in an industrial area

Abstract

Two field experiments were carried out to observe the variation in wet deposition of ions caused by orography in a polluted region of the UK. the site chosen was Winter Hill which lies on the edge of the industrialised Mersey valley. Rain and cloud samples were collected daily and chemically analysed. Synoptic and meteorological data were used to classify individual rain events as frontal or convective. For the frontal events the altitude related increase in wet deposition was found to be entirely caused by the seeder-feeder mechanism (the scavenging of cap cloud by rain from above). the concentration of marine ions in the water scavenged from the cap cloud was found to be between five and six times that in the seeder rain whereas for anthropogenically produced ions it was about twice as great. the efficiency of the seeder-feeder mechanism is correspondingly affected. Convective events accounted for less than a quarter of the rainfall volume. However, this contained over half of the deposition of marine ions in the valley. Deposition of anthropogenically produced ions in convective events was small probably due to a lack of convection over land. A small decrease in deposition with altitude was observed for these events although this is expected to be a highly local effect. It is concluded that reliable predictions of the wet deposition of non-marine ions can be made on the basis of the seeder-feeder effect for northern and western regions of the UK. Elsewhere in the UK and for much of continental Europe wet deposition from convective rain should be taken into account.