Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 4 2004 10.5194/acpd-4-3975-2004 http://www.atmos-chem-phys-discuss.net/4/3975/2004/ http://www.atmos-chem-phys-discuss.net/4/3975/2004/acpd-4-3975-2004.html http://www.atmos-chem-phys-discuss.net/4/3975/2004/acpd-4-3975-2004.pdf 3975 4018 2004-07-28 The two-way nested global chemistry-transport zoom model TM5: algorithm and applications M. Krol krol@phys.uu.nl S. Houweling B. Bregman M. van den Broek A. Segers P. van Velthoven W. Peters F. Dentener P. Bergamaschi Institute for Marine and Atmospheric Research Utrecht, The Netherlands Space Research Organisation Netherlands, Utrecht, The Netherlands Royal Netherlands Meteorological Institute, de Bilt, The Netherlands National Oceanic and Atmospheric Administration, Climate Monitoring and Diagnostics Laboratory, Boulder, USA Joint Research Centre, Institute for Environment and Sustainability, Ispra, Italy This paper describes the global chemistry Transport Model, version 5 (TM5) which allows two-way nested zooming. The model is used for global studies which require high resolution regionally but can work on a coarser resolution globally. The zoom algorithm introduces refinement in both space and time in some predefined regions. Boundary conditions of the zoom region are provided by a coarser parent grid and the results of the zoom area are communicated back to the parent. A case study using ²²²Rn measurements that were taken during the MINOS campaign reveals the advantages of local zooming. As a next step, it is investigated to what extent simulated concentrations over Europe are influenced by using an additional zoom domain over North America. An artificial ozone-like tracer is introduced with a lifetime of twenty days and simplified non-linear chemistry. The concentration differences at Mace Head (Ireland) are generally smaller than 10%, much smaller than the effects of the resolution enhancement over Europe. Thus, coarsening of resolution at some distance of a sampling station seems allowed. However, it is also noted that the budgets of the tracers change considerably due to resolution dependencies of, for instance, vertical transport. Due to the two-way nested algorithm, TM5 therefore offers a consistent tool to study the effects of grid refinement on global atmospheric chemistry issues like intercontinental transport of air pollution.