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Abstract. Earth system models are essential tools for understandingthe impacts of a warming world, particularly on the contribution of polarice sheets to sea level change. However, current models lack full couplingof the ice sheets to the ocean and are typically run at a coarse resolution(1∘ grid spacing or coarser). Coarse spatial resolution isparticularly a problem over Antarctica, where sub-grid-scale orography iswell-known to influence precipitation fields, and glacier models requirehigh-resolution atmospheric inputs. This resolution limitation has beenpartially addressed by regional climate models (RCMs), which must be forcedat their lateral and ocean surface boundaries by (usually coarser) globalatmospheric datasets, However, RCMs fail to capture the two-way couplingbetween the regional domain and the global climate system. Conversely,running high-spatial-resolution models globally is computationallyexpensive and can produce vast amounts of data. Alternatively, variable-resolution grids can retain the benefits of highresolution over a specified domain without the computational costs ofrunning at a high resolution globally. Here we evaluate a historicalsimulation of the Community Earth System Model version 2 (CESM2)implementing the spectral element (SE) numerical dynamical core (VR-CESM2)with an enhanced-horizontal-resolution (0.25∘) grid over theAntarctic Ice Sheet and the surrounding Southern Ocean; the rest of theglobal domain is on the standard 1∘ grid. We compare it to1∘ model runs of CESM2 using both the SE dynamical core and thestandard finite-volume (FV) dynamical core, both with identical physics andforcing, including prescribed sea surface temperatures (SSTs) and sea ice concentrations fromobservations. Our evaluation reveals both improvements and degradations inVR-CESM2 performance relative to the 1∘ CESM2. Surface massbalance estimates are slightly higher but within 1 standard deviation ofthe ensemble mean, except for over the Antarctic Peninsula, which isimpacted by better-resolved surface topography. Temperature and windestimates are improved over both the near surface and aloft, although theoverall correction of a cold bias (within the 1∘ CESM2 runs) hasresulted in temperatures which are too high over the interior of the icesheet. The major degradations include the enhancement of surface melt aswell as excessive cloud liquid water over the ocean, with resultant impactson the surface radiation budget. Despite these changes, VR-CESM2 is avaluable tool for the analysis of precipitation and surface mass balanceand thus constraining estimates of sea level rise associated with theAntarctic Ice Sheet. 

Abstract. Earth system models (ESMs) allow us to explore minimally observed components of the Antarctic Ice Sheet (AIS) climate system, both historically andunder future climate change scenarios. Here, we present and analyze surface climate output from the most recent version of the National Center forAtmospheric Research's ESM: the Community Earth System Model version 2 (CESM2). We compare AIS surface climate and surface mass balance (SMB) trendsas simulated by CESM2 with reanalysis and regional climate models and observations. We find that CESM2 substantially better represents the mean-state AIS near-surface temperature, wind speed, and surface melt compared with its predecessor, CESM1. This improvement likely results from theinclusion of new cloud microphysical parameterizations and changes made to the snow model component. However, we also find that grounded CESM2 SMB(2269 ± 100 Gt yr−1) is significantly higher than all other products used in this study and that both temperature andprecipitation are increasing across the AIS during the historical period, a trend that cannot be reconciled with observations. This study provides acomprehensive analysis of the strengths and weaknesses of the representation of AIS surface climate in CESM2, work that will be especially useful inpreparation for CESM3 which plans to incorporate a coupled ice sheet model that interacts with the ocean and atmosphere.