Abstract
The response of the Greenland Ice Sheet (GrIS) to a warmer climate is uncertain on long time scales. Climate models, such as those participating in the Coupled Model Intercomparison Project phase 6 (CMIP6), are used to assess this uncertainty. The Community Earth System Model version 2.1 (CESM2) is a CMIP6 model capable of running climate simulations with either one‐way coupling (fixed ice sheet geometry) or two‐way coupling (dynamic geometry) to the GrIS. The model features prognostic snow albedo, online downscaling using elevation classes, and a firn pack to refreeze percolating melt water. Here we evaluate the representation of the GrIS surface energy balance and surface mass balance in CESM2 at 1° resolution with fixed GrIS geometry. CESM2 agrees closely with ERA‐Interim reanalysis data for key controls on GrIS SMB: surface pressure, sea ice extent, 500 hPa geopotential height, wind speed, and 700 hPa air temperature. Cloudsat‐CALIPSO data show that supercooled liquid‐containing clouds are adequately represented, whereas comparisons to Moderate Resolution Imaging Spectroradiometer and CM SAF Cloud, Albedo, and Surface Radiation data set from Advanced Very High Resolution Radiometer data second edition data suggest that CESM2 underestimates surface albedo. The seasonal cycle and spatial patterns of surface energy balance and surface mass balance components in CESM2 agree well with regional climate model RACMO2.3p2, with GrIS‐integrated melt, refreezing, and runoff bracketed by RACMO2 counterparts at 11 and 1 km. Time series of melt, runoff, and SMB show a break point around 1990, similar to RACMO2. These results suggest that GrIS SMB is realistic in CESM2, which adds confidence to coupled ice sheet‐climate experiments that aim to assess the GrIS contribution to future sea level rise.
Highlights
The Greenland Ice Sheet (GrIS) is the Earth's second largest freshwater reservoir, with the potential to raise global mean sea level by about 7.4 m were it to melt completely (Morlighem et al, 2017)
Runoff, and surface mass balance (SMB) show a break point around 1990, similar to RACMO2. These results suggest that GrIS SMB is realistic in Community Earth System Model version 2.1 (CESM2), which adds confidence to coupled ice sheet-climate experiments that aim to assess the GrIS contribution to future sea level rise
The polar vortex is slightly weaker in CESM2, with its central geopotential height overestimated by 2 dams (Figure 1f), the southward expansion of the polar vortex appears exaggerated
Summary
The Greenland Ice Sheet (GrIS) is the Earth's second largest freshwater reservoir (after the Antarctic Ice Sheet), with the potential to raise global mean sea level by about 7.4 m were it to melt completely (Morlighem et al, 2017). Representing ice sheet surface processes in global climate models, such as those participating in the Coupled Model Intercomparison Project phase 6 (CMIP6, Eyring et al, 2016), could reduce this uncertainty and improve our understanding of feedbacks, for example, with ocean circulation (Fyke et al, 2018; Little et al, 2016). Dynamic feedbacks become important, and ice sheet volume and extent must be modeled with a dynamical ice sheet model (Levermann & Winkelmann, 2016; Le clec'h et al, 2019). This is recognized by the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), which provides protocols for coupled ice sheet-climate model experiments (Nowicki et al, 2016). The evolution of a dynamical ice sheet model is sensitive to the applied SMB (Khan et al, 2015), underscoring the need for a realistic representation of ice sheet surface climate and snow/firn properties in climate models
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