Spatial distribution of net surface mass balance on Greenland

The spatial distribution of surface mass balance on the Greenland ice sheet is mapped on a 50 km grid using a combination of methods depending on a zonal characterization of the diagenetic snow facies. In the zones of dry snow and upper percolation facies, the accumulation rate is calculated from microwave emissivities derived from satellite measurements using a model that is calibrated with field accumulation data. In the lower percolation zone, accumulation rates are obtained from visual interpolation of previously compiled field data, with some modification so the balance is zero at the equilibrium line. Tn the ablation zone, ablation rates are calculated as a function of ice-surface elevation and latitude. Average values of the surface balance are 263 kg m(-2)a(-1) in the m(-2)a(-1) in the ablation zone and 128 kg m(-2)a(-1) overall. accumulation zone, (-) 1259 kg m(-2)a(-1) in the Compared to the findings of a previous study using practically the same approach but different models, our bulk estimate of balance (216 Gt a(-1)) is 57% smaller, but the differences iri the estimates of net accumulation and net ablation are, respectively, 30%; and 172% larger. In this and other comparisons, there is evidence that the differences in estimates are primarily due to differences in the delineation of the equilibrium line and the estimate of ablation, and secondarily to the estimate of accumulation and interpolation of field data. The differences noted with sis other estimates reported in the last two decades are all of a size close to the composite variation of the difference (+/-50 Gt a(-1)). Our surface balance is smaller than three estimates, larger than one and in agreement with two. If substituted in the latest mass-budget estimate that indicates equilibrium, our surface balance estimate would suggest a negative budget of 55 Gt a(-1) and thus a positive contribution to sea-level change of 0.15 mm a(-1).