The ICON dynamical core project: modelling strategies and preliminary results Marco Giorgetta, Luis Kornblueh, Erich Roeckner, Hui Wan Max Planck Institut f¨ ur Meteorologie Bundesstr. 53, 20146, Hamburg, Germany e-mail: giorgetta@dkrz.de, kornblueh@dkrz.de, roeckner@dkrz.de, wan@dkrz.de Thomas Heinze, Detlev Majewski, Pilar R´ ıpodas Deutscher Wetterdienst Kaiserleistr. 42, 63067, Offenbach, Germany e-mail: thomas.heinze@dwd.de, detlev.majewski@dwd.de, maria-pilar.ripodas@dwd.de Luca Bonaventura MOX - Politecnico di Milano P.zza Leonardo da Vinci 32, 20133, Milano, Italy e-mail: luca.bonaventura@polimi.it The ICON project is a joint development effort of MPI-M and DWD to achieve a unified climate and NWP model using geodesic grids with local grid refinement. The model under development in the ICON project will use the fully elastic, nonhydrostatic Navier-Stokes equations, which provide a framework that is sufficiently general for meteorological applications on most scales relevant to numerical weather prediction and climate simulation. As an intermediate step, a semi-implicit discretization for the hydrostatic primitive equations is being developed. The proposed horizontal discretiza- tion uses the triangular Delaunay cells of the icosahedral grid as control volumes. It achieves mass and potential enstrophy conservation, thus repli- cating the results of [4] for standard rectangular C grids. Vector radial ba- sis function interpolation is used to reconstruct a uniquely defined velocity field from the velocity components normal to the cell sides, which are the discrete model variables along with the cell averaged values of the mass vari- ables like pressure, temperature or geopotential height. A full description of the horizontal discretization can be found in [1], [2]. One result obtained with a preliminary shallow water implementation on an idealized test case (see [5]) is shown in figure 1(a). The difference in the geopotential height field after 15 days of simulation between the model using 327680 triangles (approximately 40 km horizontal resolution) and a reference field obtained with resolution T426 of a slightly modified NCAR spectral shallow water model [3] are presented. Application of the same technique to a hydrostatic model with local grid refinement option is currently being investigated. Fig- ure 1(b) and (c) show two possible computational grids, with different local refinement strategies. 1