Abstract. We present a nonhydrostatic finite-volume global atmospheric model
formulation for numerical weather prediction with the Integrated Forecasting
System (IFS) at ECMWF and compare it to the established operational
spectral-transform formulation. The novel Finite-Volume Module of the IFS
(henceforth IFS-FVM) integrates the fully compressible equations using
semi-implicit time stepping and non-oscillatory forward-in-time (NFT)
Eulerian advection, whereas the spectral-transform IFS solves the hydrostatic
primitive equations (optionally the fully compressible equations) using a
semi-implicit semi-Lagrangian scheme. The IFS-FVM complements the
spectral-transform counterpart by means of the finite-volume discretization
with a local low-volume communication footprint, fully conservative and
monotone advective transport, all-scale deep-atmosphere fully compressible
equations in a generalized height-based vertical coordinate, and flexible
horizontal meshes. Nevertheless, both the finite-volume and
spectral-transform formulations can share the same quasi-uniform horizontal
grid with co-located arrangement of variables, geospherical
longitude–latitude coordinates, and physics parameterizations, thereby
facilitating their comparison, coexistence, and combination in the IFS. We highlight the advanced semi-implicit NFT finite-volume integration of the
fully compressible equations of IFS-FVM considering comprehensive
moist-precipitating dynamics with coupling to the IFS cloud parameterization
by means of a generic interface. These developments – including a new
horizontal–vertical split NFT MPDATA advective transport scheme, variable
time stepping, effective preconditioning of the elliptic Helmholtz solver in
the semi-implicit scheme, and a computationally efficient implementation of
the median-dual finite-volume approach – provide a basis for the efficacy of
IFS-FVM and its application in global numerical weather prediction. Here,
numerical experiments focus on relevant dry and moist-precipitating
baroclinic instability at various resolutions. We show that the presented
semi-implicit NFT finite-volume integration scheme on co-located meshes of
IFS-FVM can provide highly competitive solution quality and computational
performance to the proven semi-implicit semi-Lagrangian integration scheme of
the spectral-transform IFS.
Multigrid and Runge-Kutta time stepping applied to the uniformly non-oscillatory scheme for conservation laws
