Altimeter as a sounding radar on the Amery Ice-shelf

1. Altimeter as a sounding radar on the Amery Ice-shelf P. Lacroix¹, B. Legrésy¹, R. Coleman², M. Dechambre³, F. Remy¹ ¹ LEGOS, Toulouse, France ² University of Tasmania, Hobbart, Australia ³ CETP, Velizy, France

2. ENVISAT • 2 frequencies: classical Ku (13.6 GHz) and S band (3.2 GHz) • Useful to correct for ionospheric delay • Above penetrable media , differences of penetration properties: =>leads to altitude bias. altitude difference hKu-hS (m)

3. ENVISAT (2) Difference of diffusion processes perception leads to waveforms differences: =>difference of waveform parameters reflects surface and subsurface properties Idea: use the dual-frequency to extract sub-surface characteristics Backscatter difference Ku-S (dB)

4. Envisat over Amery Ice-shelf Altitude Ku (black) and S (red)

5. Strong second echo in S band and not in Ku => Ku wave is not penetrating (high T, coarse grains) => sub-surface echo attributed to snowbridge bottom interface Waveform analysis Waveforms over cracks Waveforms outside cracks

6. Backscatter evolution • Strong echo in Ku (Hyperbolic shape) above crack, situated at the surface => Surface or near-surface of the crack is a strong reflector for Ku and not for S band. • Lower backscatter over cracks than outside mostly in S band (-4dB) => Due to emptiness below snowbridge (well seen in the S waveforms) => Due also maybe to different characteristics of the surface or the snowbridge Waveforms and backscatter evolution over a crack

7. Contents • Crack geometric caracteristics • Diffusion processes over cracks

8. Crack model Cracks over ice-shelves can be as large as several hundred of meters

9. Crack signature in Ku band Hyperbola shape: Crevasse surface or near surface is a strong reflector for Ku band => Retrieve the crack position Axe transformation t => distance to nadir d elongated structure on the surface is seen as a line: Series of Ku normalised waveforms in time domain (left) and in distance to nadir domain (right)

10. Crack geometry • Angle between trace and the elongated reflector is in adequation with the crack on the MODIS image (~70°). • The position of the reflector on the trace is obtained at the intersection between the fitted line and the surface. • each branch leads to 1 reflector => Either 2 differents reflector or only 1 but at a different altitude Remark: 47cm in altitude=1200m in horizontal => Certainly only 1 reflector Linear regression of the crack echo

11. Application to glacier survey • We can follow one crack covered by snow over 2 years data. • We replace the echo on a direction parallel to the flow direction • Glacier velocity estimated: 1175m/year • Results in adequation with other observations (GPS, speckle tracking) Altitude records over one crack situated at the glacier terminal tongue

12. Diffusion processes the strong diffusion is not affecting S band => Not a surface diffusion process, but diffusion by snow grains (Diffusion by snow grains is assumed to have no effect below 5GHz ) Strong reflector situated at the surface or near surface Lower backscatter above cracks (and more sub-surface echo) => less surface echo

13. Conclusions • It is most of the time assumed that Surface signal is largely predominant in the total waveform. By frequencies comparison we showed here that sub-surface processes are important: Altimeter S band can see layers of high density contrast (snowbridge bottom) in the firn Altimeter Ku band over cracks is mostly sensitive to the first meter(s) directly under the surface constituted by coarse snow grains • Cracks are perfect distinguishable targets for the altimeter even if their size is smaller than footprint scale => Ability of a dual frequency radar to survey cracks web of a glacier, and then its healthy state. • Ability of a dual frequency radar to retrieve properties of the firn