Reply on RC2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Earth System Models are essential tools for understanding the impacts of a warming world, particularly on the contribution of polar ice sheets to sea level change. However, current models lack full coupling of the ice sheets to the ocean, and are typically run at a coarse resolution (1 degree grid spacing or coarser) to save on computational expense. Coarse spatial resolution is particularly a problem over Antarctica, where sub-gridscale orography is well-known to influence precipitation fields. This resolution limitation has been partially addressed by regional climate models (RCMs), which must be forced at their lateral and ocean surface boundaries by (usually coarser) global atmospheric datasets, However, RCMs fail to capture the coupling between the regional domain and the global climate system. Conversely, running high spatial resolution models globally is computationally expensive, and can produce vast amounts of data. Alternatively, variable-resolution, nested grids are a promising way forward, as they can retain the benefits of high resolution over a specified domain without the computational costs of running at a high resolution globally. Here we evaluate a historical simulation of the Community Earth System Model, version 2, (CESM2) implementing the spectral element (SE) numerical dynamical core with an enhanced-horizontal-resolution (0.25&deg;) grid over the Antarctic Ice Sheet and the surrounding Southern Ocean; the rest of the global domain is on the standard 1&deg; grid. We compare it to a 1&deg; model run of CESM2 using the standard finite-volume dynamical core with identical physics and forcing, including prescribed SSTs and sea ice concentrations from observations. Our evaluation indicates both improvements and degradations in VR-CESM2 performance relative to the 1&deg; CESM2. Surface mass balance estimates are slightly higher, but within one standard deviation of the ensemble mean, except for over the Antarctic Peninsula, which is impacted strongly by better-articulated surface topography. Temperature and wind estimates are improved over both the near-surface and aloft, although the overall correction of a cold bias (within the 1&deg; CESM2 runs) has resulted in temperatures which are too high over the interior of the ice sheet. The major degradations include the enhancement of surface melt as well as excessive liquid cloud water over the ocean, with resultant impacts on the radiation balance. Despite these changes, VR-CESM2 is a valuable tool for the analysis of future estimates of precipitation and surface mass balance, and thus constraining estimates of sea level rise.