Anita C. Brenner

Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002

Changes in ice mass are estimated from elevation changes derived from 10.5 years (Greenland) and 9 years (Antarctica) of satellite radar altimetry data from the European Remote-sensing Satellites ERS-1 and -2. For the first time, the dH/dt values are adjusted for changes in surface elevation resulting from temperature-driven variations in the rate of firn compaction. The Greenland ice sheet is thinning at the margins (-42 � 2G t a -1 below the equilibrium-line altitude (ELA)) and growing inland (+53 � 2G t a -1 above the ELA) with a small overall mass gain (+11 � 3G t a -1 ; -0.03 mm a -1 SLE (sea-level equivalent)). The ice sheet in West Antarctica (WA) is losing mass (-47 � 4G t a -1 ) and the ice sheet in East Antarctica (EA) shows a small mass gain (+16 � 11 Gt a -1 ) for a combined net change of -31 � 12 Gt a -1 (+0.08 mm a -1 SLE). The contribution of the three ice sheets to sea level is +0.05 � 0.03 mm a -1 .T he Antarctic ice shelves show corresponding mass changes of -95 � 11 Gt a -1 in WA and +142 � 10 Gt a -1 in EA. Thinning at the margins of the Greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate. The marked thinnings in the Pine Island and Thwaites Glacier basins of WA and the Totten Glacier basin in EA are probably ice- dynamic responses to long-term climate change and perhaps past removal of their adjacent ice shelves. The ice growth in the southern Antarctic Peninsula and parts of EA may be due to increasing precipitation during the last century.

Growth of Greenland ice sheet: measurement

Measurements of ice-sheet elevation change by satellite altimetry show that the Greenland surface elevation south of 72� north latitude is increasing. The vertical velocity of the surface is 0.20 � 0.06 meters per year from measured changes in surface elevations at 5906 intersections between Geosat paths in 1985 and Seasat in 1978, and 0.28 � 0.02 meters per year from 256,694 intersections of Geosat paths during a 548-day period of 1985 to 1986.

Analysis and retracking of continental ice sheet radar altimeter waveforms

The SEASAT-I radar altimeter data set acquired over both the Antarctic and Greenland continental ice sheets is analyzed to obtain corrected ranges to the ice surface. The radar altimeter functional response over the continental ice sheets is considerably more complex than over the oceans. Causal factors identified in this complicated response include sloping surfaces, undulating ice surfaces with characteristic wavelengths on the same spatial scale as the altimeter beam-limited footprint, off-track reflections, and dynamic lag of the altimeter tracking circuit. Retracking methods using the altimeter return pulse waveforms give range corrections that are typically several meters. The entire set of SEASAT-I altimetry over the continental ice sheets is being retracked by fitting a multi-parameter function to each waveform. Many waveforms have double ramps indicating near-normal reflections from two distinct portions of the ice surface within the altimeter beam. Two independent range measurements differing by less than 25 m are obtained from retracking the double-ramp waveforms.

Satellite Altimetry, Semivariograms, and Seasonal Elevation Changes in the Ablation Zone of West Greenland

Seasonal mean changes in the surface elevation of the ablation zone of West Greenland to 72 oN between spring 1985 and summer 1986 are measured using radar altimeter data from the 18-month Geosat Geodetic Mission. Semiva riograms are used to estimate the noise in the data as a function of pOSItIOn on the ice sheet. Mean elevation changes are computed by averaging the elevation differences measured at points where orbits ascending in latitude are later crossed by orbits descending in latitude (or the reverse) , with each cross-over difference weighted in proportion to the in verse square of the noise level in the neighborhood of the cross-over point. The mean surface ele vation of the ablation zone, relative to spring 1985, ran ged from 1.5 ± 0.6 m lower during summer 1985 to I. 7 ± 0.4 m higher during spring 1986.

Postprocessing of satellite altimetry return signals for improved sea surface topography accuracy

Large off-nadir attitude deviations and high surface wave heights cause an alteration in the ocean return signal from a satellite radar altimeter. This leads to an error in the on-board calculation of the height measurement. The error can be removed by reprocessing the full radar return signal on the ground. In the ground processing, the correct tracking point in the return signal is recomputed through a procedure called retracking. There has been a controversy over whether or not all altimeter data would be retracked. This study analyzes retracked southern ocean data from the first 34 repeat cycles of the Geosat Exact Repeat Mission (ERM), covering November 1986 through April 1988. The final data set consists of over 2.5 million smoothed one-per-second measurements of the ocean surface. The significant wave height (SWH) distribution as given on the NOAA geophysical data records (GDRs) for these measurements peaks at around 2.1 m (19% of the measurements) and drops down almost linearly to 2% of the measurements at 5.8 m. There are over 1100 observations with SWH greater than 15 m. The difference between the surface heights calculated from the retracked data and the original on-board estimates is less than 10 cm for SWH less than 10 m but increases to approximately 1.0 m at a SWH of 18 m. In general, the electromagnetic (EM) bias coefficient calculated using the retracked data is slightly less than that using the unretracked data and does not decrease as much with SWH as do the EM bias coefficients calculated from the unretracked data. A map of the sea surface height variability of the southern ocean created using the retracked data shows differences from variability maps created using the unretracked data in regions of high wave heights. Retracking can be done efficiently on modern UNIX work stations at 0.064 times real-time acquisition. This study shows that retracking will improve altimeter precision.

A global mean sea surface based upon GEOS 3 and Seasat altimeter data

A mean sea surface relative to the International Union of Geodesy 1980 Geodetic Reference System reference ellipsoid has been derived from Seasat and GEOS 3 altimeter measurements. This surface, called MSS-9012, has been computed on a grid of 1/8° resolution. Each elevation value was calculated by fitting all data within 111 km to a local biquadratic surface using Bayesian least squares. Individual data points were weighted inversely to the square of the distance to the grid location in the gridding process. The surface covers the global ocean between 70°N and 72°S. In the vicinity of sea ice the altimeter heights have been corrected for the on-board tracker error that occurs over non-Gaussian surfaces. Comparisons are made between MSS-9012 and ocean bathymetry. The eastern extent of the Chain Fracture Zone in the Gulf of Guinea is more apparent in the altimetry than in the bathymetry data, as are many other features. The combination of data from the two satellites has successfully retrieved more information about the sea surface than was previously possible using only Seasat data.

Motion of Major Ice Shelf Fronts in Antarctica from Slant Range Analysis of Radar Altimeter Data, 1978 - 1998

Abstract Slant-range analysis of radar altimeter data from the Seasat, Geosat and European Remote-sensing Satellite (ERS-1 and -2) databases is used to determine barrier location at particular times, and estimate barrier motion (kma–1) for major Antarctic ice shelves. The analysis covers various multi-year intervals from 1978 to 1998, supplemented by barrier location maps produced elsewhere for 1977 and 1986. Barrier motion is estimated as the ratio between mean annual ice-shelf area change for a particular interval, and the length of the discharge periphery. This value is positive if the barrier location progresses seaward, or negative if the barrier location regresses (break-back). Either positive or negative values are lower-limit estimates because the method does not detect relatively small area changes due to calving or surge events. The findings are discussed in the context of the three ice shelves that lie in large embayments (the Filchner–Ronne, Amery and Ross Ice Shelves), and marginal ice shelves characterized by relatively short distances between main segments of grounding line and barrier (those in the Dronning Maud Land sector between 010.1°W and 032.5°E, and the West and Shackleton Ice Shelves). The ice shelves included in the study account for approximately three-quarters of the total ice-shelf area of Antarctica, and discharge approximately two-thirds of the total grounded ice area.