Surface mass balance model intercomparison for the Greenland ice sheet

Abstract

Abstract. A number of high resolution reconstructions of the surface mass balance (SMB) of the Greenland ice sheet (GrIS) have been produced using global re-analyses data extending back to 1958. These reconstructions have been used in a variety of applications but little is known about their consistency with each other and the impact of the downscaling method on the result. Here, we compare four reconstructions for the period 1960-2008 to assess the consistency in regional, seasonal and integrated SMB components. Total SMB estimates for the GrIS are in agreement within 34 % of the four model average when a common ice sheet mask is used. When models' native land/ice/sea masks are used this spread increases to 57 %. Variation in the spread of components of SMB from their mean: runoff 42 % (29 % native masks), precipitation 20 % (24 % native masks), melt 38 % (74 % native masks), refreeze 83 % (142 % native masks) show, with the exception of refreeze, a similar level of agreement once a common mask is used. Previously noted differences in the models' estimates are partially explained by ice sheet mask differences. Regionally there is less agreement, suggesting spatially compensating errors improve the integrated estimates.Modelled SMB estimates are compared with in situ observations from the accumulation and ablation areas. Agreement is higher in the accumulation area than the ablation area suggesting relatively high uncertainty in the estimation of ablation processes. Since the mid-1990s each model estimates a decreasing annual SMB. A similar period of decreasing SMB is also estimated for the period 1960-1972. The earlier decrease is due to reduced precipitation with runoff remaining unchanged, however, the recent decrease is associated with increased precipitation, now more than compensated for by increased melt driven runoff. Additionally, in three of the four models the equilibrium line altitude has risen since the mid-1990s, reducing the accumulation area at a rate of approximately 60 000 km 2 per decade due to increased melting. Improving process representation requires further study but the use of a single accurate ice sheet mask is a logical way to reduce uncertainty among models.

Ice flux evolution in fast flowing areas of the Greenland ice sheet over the 20th and 21st centuries

“…At the regional scale, the simulated SMB agrees relatively well with RCM results (Supplementary Figure S6, Tedesco and Fettweis, 2012;Vernon and others, 2013), except in the northern and western regions, where the simulated melting is overestimated (Supplementary Figure S6). We simulated negative SMB values in the northern and northeastern basins, while Vernon and others (2013) and Tedesco and Fettweis (2012) simulated SMB values close to zero, but positive.…”

21st-century climate change around Kangerlussuaq, west Greenland: From the ice sheet to the shores of Davis Strait

“…In this case we compare the EC-Earth-driven scenarios relative to the corresponding EC-Earth-driven present-day run; although, when doing so, one implicitly ignores potential nonlinearities whereby the magnitude of climate change can depend on the reference state (e.g., Boberg and Christensen 2012;Fettweis et al 2013).…”

“…By fitting a second-degree polynomial to annual SMB values for the two time slices for the RCP8.5 scenario relative to the historical period, we estimate the cumulative change in SMB for the 2011-2100 period. Here, we interpret this cumulative change in SMB as a change in ice-sheet thickness (in a manner similar to what was done in the final sensitivity experiments in Fettweis et al 2013). If the resulting surface lowering exceeds the ice-sheet thickness (Morlighem et al 2014) for a specific grid cell, that grid cell is then shown as a nonglacial grid cell in Figure 9.…”

“…A comparison of the performance of early versions of these RCMs in Rae et al (2012) showed some differences in performance. However, analysis by Mottram et al (2017) and Vernon et al (2013) shows that these RCMs produce similar values for surface mass balance over the ice sheet as a whole, although with some significant differences in the components of mass balance and in the spatial distribution of these components (e.g., Langen et al 2015;Van As et al 2014), partly attributable to different model resolution as well as different ice masks and potentially different parameterization schemes. Here, we focus on validating the HIRHAM5 simulations against observations, excluding a comparison with previous high-resolution downscalings.…”

“…To resolve the finer details that are more useful for prescribed regions or for complex process studies, regional climate models (RCMs) are useful for dynamically down-scaling these climate projections. In Greenland, the RCMs RACMO (e.g., Ettema et al 2009;Noël et al 2016), MAR (e.g., Fettweis et al 2017Fettweis et al , 2013, and PolarMM5 (Burgess et al 2010) as well as HIRHAM5 ) have previously been used for climate research. A comparison of the performance of early versions of these RCMs in Rae et al (2012) showed some differences in performance.…”