Time Series of Greenland Ice-Sheet Elevations and Mass Changes from ICESat 2003-2009

Abstract:

We follow the repeat-track analysis (RTA) of ICESat surface-elevation data by a second stage that adjusts the measured elevations on repeat passes to the reference track taking into account the cross-track slope (α_c), in order to construct elevation time series. α_c are obtained from RTA simultaneous solutions for α_c, dh/dt, and h₀. The height measurements on repeat tracks are initially interpolated to uniform along-track reference points (every 172 m) and times (ti) giving the h(xi,ti) used in the RTA solutions. The xi are the cross-track spacings from the reference track and i is the laser campaign index. The adjusted elevation measurements at the along-track reference points are hr(ti) = h(xi,ti) – xi tan(α_c) - h₀. The hr(ti) time series are averaged over 50 km cells creating H(ti) series and further averaged (weighted by cell area) to H(t) time series over drainage systems (DS), elevation bands, regions, and the entire ice sheet. Temperature-driven changes in the rate of firn compaction, C_T(t), are calculated for 50 km cells with our firn-compaction model giving I(t) = H(t) - C_T(t) - B(t) where B(t) is the vertical motion of the bedrock. During 2003 to 2009, the average dC_T(t)/dt in the accumulation zone is -5 cm/yr, which amounts to a -75 km³/yr correction to ice volume change estimates. The I(t) are especially useful for studying the seasonal cycle of mass gains and losses and interannual variations. The H(t) for the ablation zone are fitted with a multi-variate function with a linear component describing the upward component of ice flow plus winter accumulation (fall through spring) and a portion of a sine function describing the superimposed summer melting. During fall to spring the H(t) indicate that the upward motion of the ice flow is at a rate of 1 m/yr, giving an annual mass gain of 180 Gt/yr in the ablation zone. The summer loss from surface melting in the high-melt summer of 2005 is 350 Gt/yr, giving a net surface loss of 170 Gt/yr from the ablation zone for 2005. During 2003-2008, the H(t) for the ablation zone show accelerations of the mass losses in the northwest DS8 and in the west-central DS7 (including Jacobshavn glacier) and offsetting decelerations of the mass losses in the east-central DS3 and southeast DS4, much of which occurred in 2008 possibly due to an eastward shift in the surface mass balance.