The Use of Nested Models for Air Pollution Studies: An Application of the EURAD Model to a SANA Episode

Abstract

A multiple-nesting version of the European Acid Deposition Model (EURAD) has been developed in order to increase the horizontal resolution in a region of enhanced pollution, namely the former German Democratic Republic. This new technique allows the ability to simulate large-scale features together with the development of smaller-scale structures in the nested regions. This multiple-nesting approach was applied to a case that occurred in October 1990, the so-called SANA 1 episode. SANA is a German acronym that stands for ?scientific program for the assessment of the air pollution situation in the former German Democratic Republic.? The SANA program was established to observe the rapid change in composition of air pollutants and their concentration levels over the eastern part of Germany due to political and economical changes. Thus, within the SANA program there is a unique chance to observe and control the effect of air quality strategies. Two nested areas are embedded in a coarse domain that covers the main parts of Europe. The second nested domain is nested within the first nested domain. For EURAD, the nesting is two-way for the meteorological part and one-way for the chemistry transport module (CTM). This means that the meteorological variables that coincide with the boundaries in the nested domain are interpolated by a monotone flux-corrected transport method from the next coarser domain and provide a feedback from the nested domain to the next coarser domain. For the CTM the inflow boundary conditions are dynamically determined by interpolation from coarse-grid results. The outflow conditions are specified by continuous advection at the boundary in order to eliminate numerical reflections. The results of the simulations indicate a strong dependence of the horizontal distribution of both meteorological quantities and atmospheric constituents on increased spatial resolution. They exhibit a much more realistic structure, especially for the simulations in the nested domain with highest resolution (8.89-km horizontal grid distance). Comparisons for the predicted SO2 concentrations with observations from selected stations clearly demonstrate that the simulations agree best with observations in the nested domain of highest horizontal resolution.