Evaluation of the updated regional climate model RACMO2.3: summer snowfall impact on the Greenland Ice Sheet

Abstract

Abstract. We discuss Greenland Ice Sheet (GrIS) surface mass balance (SMB) differences between the updated polar version of the RACMO climate model (RACMO2.3) and the previous version (RACMO2.1). Among other revisions, the updated model includes an adjusted rainfall-to-snowfall conversion that produces exclusively snowfall under freezing conditions; this especially favours snowfall in summer. Summer snowfall in the ablation zone of the GrIS has a pronounced effect on melt rates, affecting modelled GrIS SMB in two ways. By covering relatively dark ice with highly reflective fresh snow, these summer snowfalls have the potential to locally reduce melt rates in the ablation zone of the GrIS through the snow-albedo-melt feedback. At larger scales, SMB changes are driven by differences in orographic precipitation following a shift in large-scale circulation, in combination with enhanced moisture to precipitation conversion for warm to moderately cold conditions. A detailed comparison of model output with observations from automatic weather stations, ice cores and ablation stakes shows that the model update generally improves the simulated SMBelevation gradient as well as the representation of the surface energy balance, although significant biases remain.

Impact of MODIS sensor calibration updates on Greenland Ice Sheet surface reflectance and albedo trends

“…Continuing around the periphery of the northern GrIS, the remainder of the ice sheet margin from approximately Scoresbysund to Humboldt Glacier, we see positive trends in NIR albedo and no significant trends in short wavelength visible albedo. In this region of low annual accumulation and long surface exposure times, this signature appears to indicate increasing snowfall, which is confirmed by our separate preliminary analysis of MAR data in northeastern Greenland showing an increase in accumulation patterns starting in 2013 as well as GrIS surface mass balance climate model results (e.g., Noël et al, 2015). A very small addition of snowfall would cause more rapid surface burial and result in a lower age of surface grains and higher NIR albedo.…”

On the recent contribution of the Greenland ice sheet to sea level change

“…These are combined into an uncertainty for ice-sheet-wide SMB assuming accumulation and ablation to be independent. We note that this assumption is debatable, as ablation and accumulation tend to be connected via surface albedo, especially in summer (Fyke et al, 2014;Noël et al, 2015). The scarcity of accumulation and ablation measurements does not allow for a further regionalization of the uncertainty, but obviously uncertainties can be significantly larger for smaller areas and sub-climatological time periods.…”

“…For a more detailed description of recent changes in RACMO2.3 model physics and how they impact the modelled SMB of the GrIS, the reader is referred to Noël et al (2015) and references therein.…”

“…Other RACMO2.3 modelled SMB components and atmospheric parameters have been extensively evaluated against in situ observations both in the accumulation and ablation zone of the GrIS Noël et al, 2015). Cumulative surface mass balance (SMB Greenland ) for the full island (indicated by subscript "Greenland", amber line), i.e.…”

Reassessment of 20th century global mean sea level rise

“…19. The Antarctic ice sheet is modeled with the RACMO2.3 model (40), as in ref. 20, assuming no mass changes before 1979, long-term balance between the surface mass balance from RACMO2.3 and ice discharge between 1979 and 1993, and small acceleration in ice discharge after 1993 to match GRACE estimates (20).…”