Geoscientific Model Development Discussions www.geosci-model-dev-discuss.net 1991-9611 1991-962X 2 2 2009 10.5194/gmdd-2-709-2009 http://www.geosci-model-dev-discuss.net/2/709/2009/ http://www.geosci-model-dev-discuss.net/2/709/2009/gmdd-2-709-2009.html http://www.geosci-model-dev-discuss.net/2/709/2009/gmdd-2-709-2009.pdf 709 762 2009-07-03 The Lagrangian chemistry and transport model ATLAS: validation of transport and mixing I. Wohltmann ingo.wohltmann@awi.de M. Rex Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany We present a new global Chemical Transport Model (CTM) with full stratospheric chemistry and Lagrangian transport and mixing called ATLAS (Alfred Wegener InsTitute LAgrangian Chemistry/Transport System). Lagrangian (trajectory-based) models have several important advantages over conventional Eulerian (grid-based) models, including the absence of spurious numerical diffusion, efficient code parallelization and no limitation of the largest time step by the Courant-Friedrichs-Lewy criterion. The basic concept of transport and mixing is similar to the approach in the commonly used CLaMS model. Several aspects of the model are different from CLaMS and are introduced and validated here, including a different mixing algorithm which is less diffusive and agrees better with observations with the same mixing parameters. In addition, values for the vertical and horizontal stratospheric bulk diffusion coefficients are inferred and compared to other studies. This work focusses on the description of the dynamical part of the model and the validation of the mixing algorithm. 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