Impact of frontal ablation on the ice thickness estimation of marine-terminating glaciers in Alaska

Abstract. Frontal ablation is a major component of the mass budget of tidewater glaciers, strongly affecting their dynamics. Most global scale ice volume estimates to date still suffer from considerable uncertainties related to i) the implemented frontal ablation parameterisation or ii) for not accounting for frontal ablation at all in the glacier model. To improve estimates of the ice thickness distribution of glaciers it is thus important to identify and test low-cost and robust parameterisations of this fundamental process. By implementing such parameterisation into the ice-thickness estimation module of the Open Global Glacier Model (OGGM v1.0.1), we conduct a first assessment of the impact of accounting for frontal ablation on the estimate of ice stored in glaciers in Alaska. We find that inversion methods based on mass conservation systematically underestimate the mass turnover (and therefore the thickness) of tidewater glaciers when neglecting frontal ablation. This underestimation can rise up to 17 % on a regional scale and up to 47 % for individual glaciers. The effect strongly depends on the size of the glacier. Additionally, we perform different sensitivity experiments to study the influence of i) a constant of proportionality ( k ) used in the frontal ablation parameterisation, ii) Glen's temperature-dependent creep parameter ( A ) and iii) a sliding velocity parameter ( f s ) on the regional dynamics of Alaska marine-terminating glaciers. OGGM is able to reproduce previous regional frontal ablation estimates applying a number of combinations of values for k , Glen's A and f s . These different model configurations show that volume estimates after accounting for frontal ablation are 15 to 17 % higher than volume estimates ignoring frontal ablation. Our sensitivity studies also show that differences in thickness between accounting for and not accounting for frontal ablation occur mainly at the lower parts of the glacier, both above and below sea level. This indicates that not accounting for frontal ablation will have an impact on the estimate of the glaciers' potential contribution to sea-level rise. Introducing frontal ablation increases the volume estimate of Alaska marine-terminating glaciers from 9.01 p 0.35 to 10.43 p 0.44 mm. SLE, of which 1.41 p 0.18 mm. SLE (0.58 p 0.05 mm. SLE when ignoring frontal ablation) are found to be below sea level.