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Abstract:
In the upper troposphere and lower stratosphere, cosmic rays create Beryllium 7 atoms, which subsequently attach to submicron dust particles, so that wet deposition ultimately removes most 7Be from the troposphere. Because this source is weil known and because there is a large climatological data set for 7Be concentration in surface air and deposition on the surface, simulating 7Be provides a good test of the wet scavenging parameterization in a global climate simulation model, such as ECHAM2, which is the European Center for Medium Range Weather Forecasting (ECMWF) model with new physics introduced by the University of Hamburg. A simple parameterization in which the simulated condensation rate determines the scavenging frequency in each grid cell is used in the tracer transport model GLOMAC1, which is embedded in the meteorological model ECHAM2. In this paper we compare observed and model-calculated values of monthly average and annual average surface concentration at a global network of 79 stations. The average absolute value of the error in simulated surface concentration is 1.4 mBq m−3 compared with an average observed concentration of 3.5 mBq m−3. At most stations and in most regions the simulated surface concentration has about the correct magnitude and seasonal cycle, although there is a bias so that the modeled concentration is high at mountain stations in the tropics and low at sea level in polar regions. There are less climatological deposition data than there are climatological concentration data, but the model basically simulates the correct latitudinal variation of the zonally averaged deposition, which has a peak at the polar front (30°–50°N), although the model also has a peak produced by convective precipitation in the intertropical convergence zone, a peak that is not observed. We think that 7Be, used in conjunction with other species such as 210Pb, provides an excellent test of wet scavenging in a global model.
