Abstract
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.
Highlights
Notwithstanding the achievements made over the last decades (Bauer et al, 2015), numerical weather prediction (NWP) faces the formidable challenge of resolving rather than parameterizing essential small-scale forcings and circulations in the multi-scale global flow – most notably processes associated with the surface, convective clouds, gravity waves, and troposphere–stratosphere interaction
While there is a need for advancement in many aspects of global NWP model infrastructures, prerequisites are the ability of the numerical model formulations to accurately predict atmospheric flows throughout the large-scale hydrostatic and small-scale nonhydrostatic regimes and to run efficiently on emerging and future high-performance computing (HPC) architectures
The uncertainties concerning the SISL integration based on the ST method with regard to emerging and future HPC architectures is one of the main reasons for ECMWF and its European partners to look into alternative nonhydrostatic, all-scale global model formulations and discretization schemes to be incorporated in the Integrated Forecasting System (IFS)
Summary
Notwithstanding the achievements made over the last decades (Bauer et al, 2015), numerical weather prediction (NWP) faces the formidable challenge of resolving rather than parameterizing essential small-scale forcings and circulations in the multi-scale global flow – most notably processes associated with the surface, convective clouds, gravity waves, and troposphere–stratosphere interaction. While the viability of the ST method at ECMWF is ensured for the decade, uncertainties concerning the scalability of the non-local high-volume parallel communications in the STs exist in the longer term (Wedi et al, 2013, 2015) These scalability issues could be exacerbated from the time when the increase in horizontal resolution makes the nonhydrostatic formulation based on the fully compressible equations (Bubnová et al, 1995; Bénard et al, 2010) essential. The uncertainties concerning the SISL integration based on the ST method with regard to emerging and future HPC architectures is one of the main reasons for ECMWF and its European partners to look into alternative nonhydrostatic, all-scale global model formulations and discretization schemes to be incorporated in the IFS.
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