A partial perspective of satellitederived ice sheet topography Jonathan Bamber & Jennifer Griggs Bristol Glaciology Centre

Very briefly-altimetry: • Radar altimetry (pulse limited) ☺coverage (interior) Only method that measures absolute elevation ☺vertical accuracy – margins + break in slope (GL) – footprint size (~4 km over rough terrain)

• Laser altimetry (beam limited) ☺footprint size (~70 m, c.f. ~4 km) ☺no slope induced error – currently sampling poor for topo

Beam limited geometry:

ERS/ICESat coverage:

Amery ice shelf: ERS vs ICESat

Greenland, circa 2000:

How’s the accuracy?

InSAR ( TanDEM-X…launch this year?) ☺resolution
vertical accuracy dependent on GCPs – coverage (largely limited to margins) – availability ???

Photogrammetry

over to you! GCPs….?

A few conclusions/thoughts: • Obvious potential to combine stereo and SRA DEMS to complement ± but non trivial • CryoSat II could address current limitation (~3 years time). 92 deg inclination, 369 day repeat, 100 m azimuth resoln. • ICESat 2…?

Ice shelf thickness for all Antarctic ice shelves Jennifer Griggs and Jonathan Bamber Bristol Glaciology Centre

Motivation • Reducing errors in mass budget estimates • Need accurate estimate for modelling studies of shelf and sub-shelf cavity • Created gridded product for all Antarctic ice shelves using radar altimetry

Method • Infer thickness from surface elevation assuming hydrostatic equilibrium

(e − δ ) ρ w Z −δ = ρ w − ρi • where Z = thickness, e = elevation wrt sea level, δ = firn density correction, ρw = density of water and ρi = density of ice

Data

• Use ERS-1 geodetic phase • Can’t just use DEM • Don’t include GLAS in data rich regions due to dH/dt considerations

Data availability - Larsen C • Black dots are data coverage • White contours are elevation • Coloured background is thickness

Elevation validation Error = -2.3 ± 4.9 m Error = -1.7 ± 4.5 m if accounting for dH/dt between GLAS and ERS1 Error = -0.5 ±1.1m in central area

Ice thickness

Thickness validation Ross ice 0% shelf

20% -100m

+100m

Saunders Coast

Mean diff = 27.35 m RMS = 33.50 m

Thickness validation

Mean diff = 0.41 m RMS = 53.66 m

Larsen B, C and D ice shelves 0%

20%

Conclusions for ice shelf thickness • Geodetic phase data from ERS-1, merged with IceSaT GLAS data, can be used to create a fully validated dataset of ice shelf thickness at 1 km spacing for all ice shelves in Antarctica. • The dataset relies on careful interpretation of the input data • The firn correction allows the effect of the spatially variable nature of the firn to be included in the derived ice thicknesses. • Comparison with airborne estimates of ice thicknesses shows that the differences are in general small • Differences rise to over 20% close to the grounding line where hydrostatic equilibrium breaks down • The spatial patterns of differences are as might be expected with high differences coming from convergent flow and possible marine ice presence.