This experiment comprises three atmosphere-only simulations conducted with version 2.6.6 of the ICON model as part of WP2 of the EU Horizon 2020 project PolarRES. All simulations follow an AMIP-style setup and are forced with boundary conditions from ERA5, representing present-day climate conditions. The experiments differ only in their horizontal resolution over the polar regions.
PD_ERA_UNREF was performed using a globally uniform R3B5 grid, corresponding to a horizontal resolution of approximately 52.6 km. The two refined simulations, PD_ERA_REF_ARCTIC and PD_ERA_REF_ANTARC, are based on the same R3B5 base grid but include two nested domains over the Arctic and Antarctic, respectively. Each includes an intermediate-resolution nest (R3B6, ~26.3 km) north or south of 50° latitude, and a high-resolution nest (R3B7, ~13.2 km) beyond 57°N or 57°S. Two-way nesting was used to allow feedback from the refined domains to the global domain. All simulations used 90 vertical levels with a model top at approximately 75 km. The time step was halved with each nesting level to ensure numerical stability. All physical parameterisations were applied consistently across domains and chosen to perform robustly across the resolution range.
The simulations were initialised on 1 January 1984 using ERA5 data and integrated for 31 years. The first year is discarded as spin-up, yielding 30 years of output (1985–2014). Sea surface temperatures and sea ice concentrations were prescribed daily from ERA5 for the period 1984–2014. While sea ice concentration is prescribed, the model calculates sea ice thickness prognostically. Time-varying greenhouse gas concentrations (CO₂, CH₄, N₂O, and CFCs), ozone, and aerosols follow CMIP6 historical forcings.
All simulations used the ecRad radiation scheme (Hogan et al., 2018), a single-moment cloud microphysics scheme following Doms et al. (2011) and Seifert and Beheng (2008), and a convection scheme based on Tiedtke (1989) and Bechtold et al. (2008). Turbulent processes are represented by a prognostic TKE-based turbulence scheme (Raschendorfer, 2001). Orographic drag is parameterised following Lott and Miller (1997), and non-orographic gravity wave drag is based on Orr et al. (2010). The land surface is represented using the TERRA component for soil-vegetation-atmosphere transfer (Schrodin and Heise, 2001), with topography derived from the GLOBE dataset (Hastings et al., 1999) at ~1 km native resolution.
Post-processing included horizontal interpolation of selected variables to a regular 0.5° × 0.5° lat-lon grid for the global domain (UNREF), and to 0.125° × 0.125° for the high resolution refined domains. Vertical interpolation to 19 pressure levels was applied to data on native model levels.