ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-9-22865-2009 Effects of resolution on the relative importance of numerical and physical diffusion in atmospheric composition modelling D'Isidoro M. 1 Maurizi A. 1 Tampieri F. 1 CNR-ISAC, via Gobetti 101, 40129 Bologna, Italy 28 10 2009 9 5 22865 22881 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/9/22865/2009/acpd-9-22865-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/9/22865/2009/acpd-9-22865-2009.pdf

Numerical diffusion induced by advection has a large influence on concentration of substances in atmospheric composition models. At coarse resolutions numerical effects dominate, whereas at increasing model resolutions a description of physical diffusion is needed. The effects of changing resolution and Courant number are investigated for the WAF advection scheme (used in BOLCHEM), evidencing a sub-diffusive process. The spreading rate from an instantaneous source is compared with the physical diffusion necessary to simulate unresolved turbulent motions. The time at which the physical diffusion process overpowers the numerical spreading is estimated, and is shown to reduce as the resolution increases, and to increase with wind velocity.

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