SL-AV (Semi-Lagranginan Absolute Vorticity) is a global atmospheric model. Its latest version SL-AV20 provides global operational medium-range weather forecast with 20km resolution over Russia. The lower resolution configurations of SL-AV20 are being tested for seasonal prediction and climate modeling.The article presents the model dynamical core. Its main features are vorticity-divergence formulation at the unstaggered grid, high-order finite-difference approximations, semi-Lagrangian semi-implicit discretization and the reduced latitude-longitude 5 grid with variable resolution in latitude.The accuracy of SL-AV20 numerical solutions using reduced lat-lon grid and the variable resolution in latitude is tested with two idealized testcases. The results agree well with other published model solutions. It is shown that the use of the reduced grid having up to 25% less grid points than the regular grid does not significantly affect the accuracy. Variable resolution in latitude allows to improve the accuracy of solution in the region of interest.
10Geosci. Model Dev. Discuss., Shashkin and Tolstykh (2014), there is no one that would put all the developments together. This article describes the present state of the SL-AV20 dynamical core.While developing a dynamical core, we want it to have a given accuracy with respect to test solutions combined with a given wall-clock time and minimum number of processors necessary to calculate these solutions. We call this computational efficiency. Furthermore, as the dynamical core can be used at a range of horizontal and vertical resolutions typical for numerical 5 weather prediction and climate simulations, it is desirable to maintain dynamical core computational efficiency at maximum possible range of resolutions. This is somewhat contradictory requirement. Indeed, using a global dynamical core at maximum possible resolution of first kilometers ultimately requires dynamical core to use efficiently up to tens of thousand cores. This is not so easy to achieve if a semi-implicit time integration scheme is used.Given typical applications of SL-AV20 model mentioned above but also limited computer resources available, our approach 10 to the dynamical core design is based on the following choices. We use semi-implicit (SI) time stepping scheme (Robert et al., 1985) and semi-Lagrangian (SL) treatment of advection (a review -Staniforth and Côté, 1991). This combination allows to circumvent Courant-Friedrichs-Lewy stability limitation for both wind speed and inertia-gravity wave phase speed.Practically, this means that the time-step is much larger than with the Eulerian treatment of advection and/or explicit timestepping scheme, however, at the cost of solving Helmholtz equation at each time-step and having larger communication 15 pattern in parallel implementation. The unstaggered grid is used, i.e. scalar and vector variables are stored at the same grid points. Therefore, only one set of upstream trajectories needs to be computed for SL advection scheme (C-grid requires 3 sets).Also, th...