Global simulations of the atmosphere at 1.45 km grid-spacing with the Integrated Forecasting System

Abstract

<p>Global simulations with 1.45 km grid-spacing are presented that were performed with the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). Simulations are uncoupled (without ocean, sea-ice or wave model), using 62 or 137 vertical levels and the full complexity of weather forecast simulations including recent date initial conditions, real-world topography, and state-of-the-art physical parametrizations and diabatic forcing including shallow convection, turbulent diffusion, radiation and five categories for the water substance (vapour, liquid, ice, rain, snow). Simulations are evaluated with regard to computational efficiency and model fidelity. Scaling results are presented that were performed on the fastest supercomputer in Europe - Piz Daint (Top 500, Nov 2018). Important choices for the model configuration at this unprecedented resolution for the IFS are discussed such as the use of hydrostatic and non-hydrostatic equations or the time resolution of physical phenomena which is defined by the length of the time step. </p><p>Our simulations indicate that the IFS model — based on spectral transforms with a semi-implicit, semi-Lagrangian time-stepping scheme in contrast to more local discretization techniques — can provide a meaningful baseline reference for O(1) km global simulations.</p>

A Baseline for Global Weather and Climate Simulations at 1 km Resolution

“…While a detailed description of the parametrization package can be found in the IFS documentation available online (ECMWF, 2020), IFS has the turbulent diffusion and exchange with the surface represented by the Monin‐Obukhov similarity theory in the surface layer and an Eddy‐Diffusivity Mass‐Flux (EDMF) framework above the surface layer and includes a mass‐flux shallow‐convection; a multilayer, multitiled land‐surface scheme (HTESSEL); a five‐species cloud microphysics model; and a shortwave and longwave radiation scheme including cloud radiation interactions. Orography and land use fields are specially prepared for the 1.4 km resolution (Dueben et al, 2020). The subgrid‐scale orographic and nonorographic GW drag parametrizations are both designed to reduce with increase in horizontal resolution and have zero contribution at 1.4 km grid spacing.…”

“…The scalability of the IFS atmospheric model has been tested on Summit CPUs and compared with results on PizDaint (Dueben et al, 2020), the fastest European supercomputer (Top500, November 2019). This is shown in Figure 1.…”

“…In addition to Miyamoto et al (2013), some short simulations at these resolutions have already started to emerge. For example, Fuhrer et al (2018) performed 10 day simulations of the idealized baroclinic life cycle with the Consortium for Small‐Scale Modeling (COSMO) model at a subkilometer grid spacing over a domain spanning 80° N to 80° S. Using the European Centre for Medium Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS), Dueben et al (2020) performed a set of short 48 hr simulations at 1.4 km grid spacing. However, until recently, it had not been possible to perform a full‐complexity global seasonal simulation with a grid spacing below 10 km.…”

“…Although it is a common belief in dynamic meteorology that the hydrostatic assumption should break down at such a high resolution as is used here, the nonhydrostatic configuration of IFS is simply too computationally expensive for performing a seasonal simulation. However, the hydrostatic configuration of IFS still performs well even at 1.4 km grid spacing (Dueben et al, 2020). The importance of nonhydrostatic effects remains an open question, and we hope the simulation presented here can provide a baseline (building on over 40 years of NWP experience) against which future nonhydrostatic simulations can be compared.…”

The Added Value of Large-eddy and Storm-resolving Models for Simulating Clouds and Precipitation

“…Studies going back more than twenty years (Weisman et al 1997) have been demonstrating the ability of models to explicitly resolve convection using grid meshes on the order of a few kilometers. These approaches (see also the review by Guichard and Couvreaux, 2017) have been so successful, see e.g., Miura et al (2007) and Miyamoto et al (2013), that in many countries operational weather prediction systems now incorporate them (e.g., Lean et al, 2008, Baldauf et al, 2011, Hirahara et al 2011, and have begun testing systems capable of resolving convective storms, globally (Weber and Mass 2019; Düben et al 2019). This success has likewise motivated initiatives -such as the UK CASCADE project, a forerunner of HD(CP) 2to use realistically configured kilometer-scale large-domain simulations to study the interaction of convection with large-scale circulations (e.g., Holloway et al, 2012, Marsham et al, 2013, and given new impetus to stormresolving studies of regional climate (Prein et al, 2015, Kendon et al, 2017, Leutwyler et al, 2017.…”