Validation of high-resolution GRACE mascon estimates of glacier mass changes in the St Elias Mountains, Alaska, USA, using aircraft laser altimetry

We acquired center-line surface elevations from glaciers in the St Elias Mountains of Alaska/northwestern Canada using aircraft laser altimetry during 2000-05, and compared these with repeat measurements acquired in 2007. The resulting elevation changes were used to estimate the mass balance of 32 900 km(2) of glaciers in the St Elias Mountains during September 2003 to August 2007, yielding a value of -21.2 +/- 3.8 Gt a(-1), equivalent to an area-averaged mass balance of -0.64 +/- 0.12 m a(-1) water equivalent (w.e.). High-resolution (2 arc-degrees spatial and 10 day temporal) Gravity Recovery and Climate Experiment (GRACE) mass-balance estimates during this time period were scaled to glaciers of the St Elias Mountains, yielding a value of -20.6 +/- 3.0 Gt a(-1), or an area-averaged mass balance of -0.63 +/- 0.09 m a(-1) w.e. The difference in balance estimates (altimetry minus GRACE) was -0.6 +/- 4.8 Gt a(-1), well within the estimated errors. Differences likely resulted from uncertainties in subgrid sampling of the GRACE mass concentration (mascon) solutions, and from errors in assigning 4 an appropriate near-surface density in the altimetry estimates. The good correspondence between GRACE and aircraft altimetry data suggests that high-resolution GRACE mascon solutions can be used to accurately assess mass-balance trends of mountain glacier regions that are undergoing large changes.