Abstract
Abstract. We present a reconstruction of historical (1950–2014) surface mass balance (SMB) of the Greenland ice sheet (GrIS) using a high-resolution regional climate model (RACMO2; ∼11 km) to dynamically downscale the climate of the Community Earth System Model version 2 (CESM2; ∼111 km). After further statistical downscaling to 1 km spatial resolution, evaluation using in situ SMB measurements and remotely sensed GrIS mass change shows good agreement. Comparison with an ensemble of previously conducted RACMO2 simulations forced by climate reanalysis demonstrates that the current product realistically represents the long-term average and variability of individual SMB components and captures the recent increase in meltwater runoff that accelerated GrIS mass loss. This means that, for the first time, climate forcing from an Earth system model (CESM2), which assimilates no observations, can be used without additional corrections to reconstruct the historical GrIS SMB and its recent decline that initiated mass loss in the 1990s. This paves the way for attribution studies of future GrIS mass loss projections and contribution to sea level rise.
Highlights
A common approach to project the future surface mass balance (SMB) of the Greenland ice sheet (GrIS) is to force a regional climate model (RCM), typically running at 5 to 10 km horizontal resolution, at the lateral and top boundaries with the outputs of an Earth system model (ESM; ∼ 100 km) (Van Angelen et al, 2013a; Fettweis et al, 2013; Mottram et al., 2017)
The Community Earth System Model version 2 (CESM2)-forced Regional Atmospheric Climate Model version 2.3p2 (RACMO2).3p2 historical simulation is compared to SMB and individual components from an ensemble of eight previous RACMO2 simulations (Van Angelen et al, 2013a, b; Noël et al, 2015, 2016, 2018, 2019), using different climate forcing (ERA reanalysis or the ESM HadGEM2) at various spatial resolutions (1, 5.5 and 11 km)
Good agreement with observations can be partly attributed to dynamical downscaling in RACMO2, which results in realistic SMB gradients if appropriate climate forcing is prescribed (Noël et al, 2018), and to statistical downscaling, as it minimises SMB bias by enhancing runoff in marginal ablation zones (Noël et al, 2016)
Summary
A common approach to project the future surface mass balance (SMB) of the Greenland ice sheet (GrIS) is to force a regional climate model (RCM), typically running at 5 to 10 km horizontal resolution, at the lateral and top boundaries with the outputs of an Earth system model (ESM; ∼ 100 km) (Van Angelen et al, 2013a; Fettweis et al, 2013; Mottram et al., 2017). ESMs from the fifth phase of the Climate Model Intercomparison Project (CMIP5) do not accurately represent the contemporary large-scale climate of the Greenland region (Rae et al, 2012; Fettweis et al, 2013). An alternative approach is to directly use outputs of ESMs to estimate GrIS SMB; most ESMs do not have (sophisticated) snow models that consider meltwater retention in firn, and their coarse spatial resolution does not accurately resolve the large SMB gradients at the GrIS margins (Lenaerts et al, 2019)
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