ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-4-1797-2004 Modelling tracer transport by a cumulus ensemble: lateral boundary conditions and large-scale ascent Salzmann M. 1 Lawrence M. G. 1 Phillips V. T. J. 2 Donner L. J. 2 Max-Planck-Institute for Chemistry, Department of Atmospheric Chemistry, PO Box 3060, 55020 Mainz, Germany Geophysical Fluid Dynamics Laboratory, NOAA, Princeton University, PO Box 308, Princeton, NJ 08542, USA 13 09 2004 4 7 1797 1811 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.net/4/1797/2004/acp-4-1797-2004.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/4/1797/2004/acp-4-1797-2004.pdf

The vertical transport of tracers by a cumulus ensemble at the TOGA-COARE site is modelled during a 7 day episode using 2-D and 3-D cloud-resolving setups of the Weather Research and Forecast (WRF) model. Lateral boundary conditions (LBC) for tracers, water vapour, and wind are specified and the horizontal advection of trace gases across the lateral domain boundaries is considered. Furthermore, the vertical advection of trace gases by the large-scale motion (short: vertical large-scale advection of tracers, VLSAT) is considered. It is shown that including VLSAT partially compensates the calculated net downward transport from the middle and upper troposphere (UT) due to the mass balancing mesoscale subsidence induced by deep convection. Depending on whether the VLSAT term is added or not, modelled domain averaged vertical tracer profiles can differ significantly. Differences between a 2-D and a 3-D model run were mainly attributed to an increase in horizontal advection across the lateral domain boundaries due to the meridional wind component not considered in the 2-D setup.