Description of the 3D LOTOS model

This report is part of the description of a tentative version of the three-dimensional regional air quality model LOTOS-HPCN. The starting-point of these models is the solution of the continuity equation that describes the change in concentration of a chemical species in the air as a result of transport, emissions, chemistry and some other processes. The physical domain of the model is part of a shell around the earth. In the vertical direction hybrid coordinates are used: the lower boundary is given by the orography of the earth and the upper boundary by a surface having equal pressure. For reasons of computational simplicity such an irregular physical domain is often transformed into a rectangular computational domain. The necessary transformation of the model equation is also described in this report.