Time step size as a function of radius for different time step criteria. Because of the polar geometry of the grid, the innermost cells become narrower and thus require shorter time steps. The purple dashed horizontal line denotes the time step size of δt = 8 s that is used in the 2D simulation. The dashed vertical line marks the radius below which this step size is larger than a time step size according to Eq. (1) with a CFL number of 50. The gray shaded area indicates the convective region.