Probabilistic parameterisation of the surface mass balance–elevation feedback in regional climate model simulations of the Greenland ice sheet

摘要

We present a new parameterisation that relates surface mass balance (SMB: thesum of surface accumulation and surface ablation) to changes in surfaceelevation of the Greenland ice sheet (GrIS) for the MAR (Modèle AtmosphériqueRégional: Fettweis, 2007) regional climatemodel. The motivation is to dynamically adjust SMB as the GrIS evolves,allowing us to force ice sheet models with SMB simulated by MAR whileincorporating the SMB–elevation feedback, without the substantial technicalchallenges of coupling ice sheet and climate models. This also allows us toassess the effect of elevation feedback uncertainty on the GrIS contributionto sea level, using multiple global climate and ice sheet models, without theneed for additional, expensive MAR simulations.We estimate this relationship separately below and above the equilibrium linealtitude (ELA, separating negative and positive SMB) and for regions northand south of 77° N, from a set of MAR simulations in which we alterthe ice sheet surface elevation. These give four "SMB lapse rates",gradients that relate SMB changes to elevation changes. We assessuncertainties within a Bayesian framework, estimating probabilitydistributions for each gradient from which we present best estimates andcredibility intervals (CI) that bound 95% of the probability. Below theELA our gradient estimates are mostly positive, because SMB usually increaseswith elevation: 0.56 (95% CI: ?0.22 to 1.33) kg m?3 a?1 forthe north, and 1.91 (1.03 to 2.61) kg m?3 a?1 for the south.Above the ELA, the gradients are much smaller in magnitude: 0.09 (?0.03 to0.23) kg m?3 a?1 in the north, and 0.07 (?0.07 to0.59) kg m?3 a?1 in the south, because SMB can either increaseor decrease in response to increased elevation.Our statistically founded approach allows us to make probabilisticassessments for the effect of elevation feedback uncertainty on sea levelprojections (Edwards et al., 2014).