The nonhydrostatic global IFS / ARPEGE : model formulation and testing

In preparation for global applications at horizontal scale s finer than about 10 km, where nonhydrostatic dynamics becomes important, the efficacy and stability of th e nonhydrostatic model developed by the ALADIN group and made available by Météo-France in the globa l IFS/ARPEGE model are assessed. The main attraction of this nonhydrostatic dynamical core is its alg orithmic similarity to the existing hydrostatic IFS (H-IFS). The performance of the nonhydrostatic model (NH-I FS) is assessed for a wide range of scales and for a set of canonical test cases relevant to atmospheric flow s. The results obtained for a range of idealised nonhydrostatic flow problems compare satisfactorily to Car tesian-domain analytic solutions, where available, and to the nonhydrostatic research code EULAG. At hydr ostatic scales (for grid-sizes upto 10 km) the NH-IFS gives very similar forecasts to the operational hydr ostatic IFS, and can be run stably with the rather long timesteps used with the latter model. However, the comp utational cost of the NH-IFS per timestep is substantially larger than with the H-IFS (double at 10 km r esolution). It is concluded that the NH-IFS dynamical core is a possible choice for future, globally-un iform high resolution applications at ECMWF, provided its cost can be reduced.

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