UNCORRECTED PROOF Grid geometry effects on convection in ocean climate models: a conceptual study Sven Titz a, * , Till Kuhlbrodt b , Ulrike Feudel c a Institute for Physics, University of Potsdam, PF 601553, 14415 Potsdam, Germany b Potsdam Institute for Climate Impact Research, PF 60 12 03, 14412 Potsdam, Germany c Institute for Chemistry and Biology of the Marine Environment, PF 2503, 26111, Oldenburg, Germany Received 7 February 2003; received in revised form 14 July 2003; accepted 12 August 2003 Abstract 10 Ocean convection is a highly non-linear and local process. Typically, a small-scale phenomenon of this 11 kind entails numerical problems in the modelling of ocean circulation. One of the tasks to solve is the 12 improvement of convection parameterization schemes, but the question of grid geometry also plays a 13 considerable role. Here, this question is studied in the context of global ocean models coupled to an at- 14 mosphere model. Such ocean climate models have mostly structured, coarsely resolved grids. 15 Using a simple conceptual two-layer model, we compare the discretization effects of a rectangular grid 16 with those of a grid with hexagonal grid cells, focussing on average properties of the ocean. It turns out that 17 systematic errors tend to be clearly smaller with the hexagonal grid. In a hysteresis experiment with the 18 atmospheric boundary condition as a hysteresis parameter, the spatially averaged behaviour shows non- 19 negligible artificial steps for quadratic grid cells. This bias is reduced with the hexagonal grid. The same 20 holds for the directional sensitivity (or horizontal anisotropy) which is found for different angles of the 21 advection velocity. The grid with hexagonal grid cells shows much more isotropic results. From the limited 22 viewpoint of these test experiments, it seems that the hexagonal grid (i.e. icosahedral–hexagonal grids on 23 the sphere) is recommendable for ocean climate models. 24 Ó 2003 Published by Elsevier Ltd. 25 Keywords: Oceanic convection; Ocean modelling; Parameterization; Model grid * Corresponding author. E-mail address: sven@agnld.uni-potsdam.de (S. Titz). 1463-5003/$ - see front matter Ó 2003 Published by Elsevier Ltd. doi:10.1016/j.ocemod.2003.08.001 Ocean Modelling xxx (2003) xxx–xxx www.elsevier.com/locate/omodol OCEMOD 100 No. of Pages 18, DTD = 4.3.1 28 August 2003 Type ARTICLE IN PRESS