Compressible EULAG Dynamical Core in COSMO: Convective-Scale Alpine Weather Forecasts MICHAL Z. ZIEMIA  NSKI, a DAMIAN K. WÓJCIK, a BOGDAN ROSA, a AND ZBIGNIEW P. PIOTROWSKI a a Institute of Meteorology and Water Management–National Research Institute, Warsaw, Poland (Manuscript received 21 September 2020, in final form 10 August 2021) ABSTRACT: This paper presents the semi-implicit compressible EULAG as a new dynamical core for convective-scale numerical weather prediction. The core is implemented within the infrastructure of the operational model of the Consortium for Small-Scale Modeling (COSMO), forming the NWP COSMO-EULAG model (CE). This regional high- resolution implementation of the dynamical core complements its global implementation in the Finite-Volume Module of ECMWF’s Integrated Forecasting System. The paper documents the first operational-like application of the dynamical core for realistic weather forecasts. After discussing the formulation of the core and its coupling with the host model, the paper considers several high-resolution prognostic experiments over complex Alpine orography. Standard verification experi- ments examine the sensitivity of the CE forecast to the choice of the advection routine and assess the forecast skills against those of the default COSMO Runge–Kutta dynamical core at 2.2-km grid size showing a general improvement. The skills are also compared using satellite observations for a weak-flow convective Alpine weather case study, showing favorable results. Additional validation of the new CE framework for partly convection-resolving forecasts using 1.1-, 0.55-, 0.22-, and 0.1-km grids, designed to challenge its numerics and test the dynamics–physics coupling, demonstrates its high robustness in simulating multiphase flows over complex mountain terrain, with slopes reaching 858, and the flow’s realistic representation. KEYWORDS: Convective-scale processes; Mesoscale models; Model evaluation/performance; Nonhydrostatic models; Numerical weather prediction/forecasting 1. Introduction High-resolution convection-permitting numerical weather pre- diction (NWP) models have become standard tools for operational regional applications (see, e.g., a review by Yano et al. 2018). The models undergo intensive development with a perspective of subkilometer horizontal grid size, also for ensemble applications. Their resolutions allowing for explicit representation of convective processes bring new scientific challenges to these efforts, as dis- cussed by Yano et al. (2018, see also references therein). In response to these challenges, the Science Plans of the Consortium of Small-Scale Modeling (COSMO) for 2010–14 (Arpagaus et al. 2010) and 2015–20 (Ziemia nski et al. 2015) formulated a scientific strategy for all elements of the NWP production chain. The requirements for the dynamics and numerics pointed at the high accuracy of the numerical schemes, numerical stability, especially over steep orography, and computational effi- ciency. The strategy considered potential advantages of conserva- tive formulations of the numerical schemes for treating strong gradients and discontinuities, pointed, for example, by LeVeque (2002, 3–5). Following that discussion, the Consortium decided to test an already existing conservative EULAG dynamical core for its possible implementation within the COSMO model. EULAG has a relatively long history as a nonhydrostatic multiscale flow solver successfully used for research purposes, including representations of atmospheric, oceanic, stellar, and planetary processes (see, e.g., a review in Prusa et al. 2008). It was developed as a soundproof model based on the anelastic Lipps and Hemler (1982) equations. While the model also works as a semi-Lagrangian solver, its conservative flux-form Eulerian ver- sion was only considered within the COSMO project. Its results for the casting of the legacy EULAG FORTRAN 77 code (Prusa et al. 2008) into the FORTRAN 90/95 form followed by tests for idealized dry and moist flow configurations were published by Wójcik et al. (2011), Rosa et al. (2011), and Kurowski et al. (2011), respectively. The results for semirealistic and realistic flows over the Alps were published in Ziemia nski et al. (2011) and Baldauf et al. (2013), respectively. The operationalized COSMO model employing the EULAG dynamical core, the COSMO-EULAG (CE), was presented in Kurowski et al. (2016). In the meantime, a compressible EULAG dynamical core was developed (see Smolarkiewicz et al. 2014; Kurowski et al. 2014, 2015; Smolarkiewicz et al. 2016a). Its semi-implicit version allows for time steps restricted by the CFL stability condition for mete- orological flows’ speed (and not the sound’s speed). That devel- opment led in the ECMWF to the construction of the finite-volume module of the global IFS model (IFS-FVM) (Smolarkiewicz et al. 2016b; Kühnlein et al. 2019). In COSMO, the work focused on the local-area convective-scale application of the core, and this publi- cation documents the resulting CE prototype. The novelty of the work lies in the successful implementation of the implicit-compressible formulation of the EULAG nu- merics within an operational regional convective-scale weather Denotes content that is immediately available upon publica- tion as open access. Corresponding author: Michal Z. Ziemia nski, michal.ziemianski@ imgw.pl Publisher’s Note: This article was revised on 21 October 2021 to designate it as open access. OCTOBER 2021 ZIEMIA  NSKI ET AL. 3563 DOI: 10.1175/MWR-D-20-0317.1 Ó 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 01/28/23 02:49 PM UTC