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Getting into The Zone: Ice Shelf Margins from ICESat

Getting into The Zone: Ice Shelf Margins from ICESat. Laurie Padman 1 and Helen A. Fricker 2 1 Earth & Space Research, Corvallis OR 2 IGPP, Scripps, UCSD, La Jolla CA. Goals. Short Term

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Getting into The Zone: Ice Shelf Margins from ICESat

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  1. Getting into The Zone: Ice Shelf Margins from ICESat Laurie Padman1 and Helen A. Fricker2 1Earth & Space Research, Corvallis OR 2IGPP, Scripps, UCSD, La Jolla CA L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

  2. Goals Short Term Determine if repeat ICESat tracks across grounding zone (GZ) can precisely locate grounding line (GL) and define structure of GZ (relies on tides to give time-varying flexure of GZ). Long Term Build GZ-resolving ocean tide/IBE models with realistic flexure to improve accuracy of mass-balance calculations within GZ. L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

  3. State of tide modeling w.r.t. the GZ Problems GL poorly known in some locations: significant impact on tidal energy propagation around ice shelf perimeters; GZ is not resolved by present global and circum-Antarctic models. GZ is ~2-10 km wide; typical model grid spacing is 10-50 km; Flexure is not modeled, either empirically (to give time-dependent elevation within GZ) or dynamically (to give ice contribution to tidal energy loss). L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

  4. State of tide modeling: Model comparisons King and Padman (2005) TPXO6.2 CSR4 NAO.99b GOT00.2: present ICESat tide model “Ocean” tide data set Same result with ocean tide loading cf. bedrock GPS (King et al., 2005) L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

  5. ICESat near-global coverage Provided by Chris Shuman L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

  6. ICESat:Antarctic (Laser 2a) Antarctic elevation (m) from ICESat Laser 2a operation period, Sept-Nov 2003. Convergence of tracks: good for FRIS and RIS studies Provided by Chris Shuman L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

  7. ICESat for GZ studies: The good news 170-m along-track sampling: ~70 m footprint; easily resolves GZ (2-10 km wide); Orbit to 86oS; sees all Antarctic ice shelves, with closely-spaced tracks in southern RIS and FRIS; High intrinsic accuracy: ~3 cm over salar de Uyuni; ~11 cm RMS over RIS, with best available detiding (Padman and Fricker, 2005; (cf. ~50 cm for ERS radar altimeter). L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

  8. ICESat for GZ studies: The bad news Few repeats of each track (max 7 (as of 10/05), but usually fewer); Data loss due to clouds; Small tracking errors (tracks offset from each other) can cause significant height offsets, esp. near the GZ; Some tidal harmonics not resolved by time separation of repeat tracks; Regions of weak tides give low signal-to-noise. L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

  9. To cut a long story short … it can be done! Track across SW Ronne I.S. Dashed lines: Offshore tide prediction from CATS02.01. A and B are onshore and offshore limits of tidal flexure. Dashed lines show offshore tide model prediction of I.S. displacement. GZ width ~8 km. L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

  10. Therefore … (Based on looking at several repeat tracks from the FRIS …) ICESat can be used in regions of moderate-to-strong tides, to define location of GL and offshore limit of tidal flexure where they are not available from InSAR; ICESat, perhaps with InSAR, may define the curvature of the shelf ice in the GZ. (Is there an asymmetry between low and high tides?) L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

  11. Next steps … #1: Develop automated procedure to q-c analyses of ICESat repeat-track GZ data; #2: Empirically model attenuation of tide amplitude as fraction of distance through GZ: scale “free” tide by this function. #3: Develop new regional tide models at order 1 km grid spacing to take advantage of improved GL (from multiple methods): models can be high-res finite-difference or finite-element (as used in FES global models); #4: Incorporate shelf-ice mechanical properties into ocean tide model: feedback to ocean tide through surface pressure gradient and friction at ice/ocean interface. L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

  12. References King, M.A., N.T. Penna, P.J. Clarke, and E.C. King (2005), Validation of ocean tide models around Antarctica using onshore GPS and gravity data, J. Geophys. Res., 110. King, M.A., and L. Padman (2005), Accuracy assessment of ocean tide models around Antarctica, Geophys. Res. Lett., accepted. Padman, L., and H.A. Fricker (2005), Tides on the Ross Ice Shelf observed with ICESat, Geophys. Res. Lett., 32 (14). Knopoff, L., P.A. Rydelek, W. Zurn, and D.C. Agnew (1989), Observations of load tides at the South-Pole, Physics of the Earth and Planetary Interiors, 54 (1-2), 33-37, 1989. L. Padman and H. A. Fricker; Presentation at the West Antarctic Ice Sheet (WAIS) Meeting, Washington DC, 2005

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