# Presentation: 330525

## Getting into The Zone: Ice Shelf Margins from ICESat

**Getting into The Zone: Ice ****Shelf Margins from ICESat**

- Laurie Padman1 and Helen A. Fricker2
- 1 *Earth & Space Research, Corvallis OR*
- 2 *IGPP, Scripps, UCSD, La Jolla CA*

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002

**Notes:**
For those who may not know, ICESat is the Ice, Cloud and land Elevation Satellite that measures surface elevation using an active laser (the Geoscience Laser Altimeter System, or “GLAS”).  GLAS is the first altimeter to cover the polar regions which has sufficient resolution to resolve the flexure of the ice shelf due to ocean processes such as tides and the inverse barometer effect. 

## 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.

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002

## 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).

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002

**Notes:**
I’m not a glaciologist. My main goal in looking at ice shelf data is to determine ways to improve ocean models in regions which are important for exchange of heat and fresh water between the ocean and cryosphere. Some of the problems with current models are listed here (read through list).

## State of tide modeling: Model comparisons

**TPXO6.2******

**CSR4**
**         ****NAO.99b**

*King and Padman* (2005)

**GOT00.2: present**
**ICESat tide model**

**Same result with ocean tide loading cf. bedrock GPS (*****King et al*****., 2005)**

**“****Ocean” tide data set**

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002

**Notes:**
As an aside, we have looked at existing popular tide models to see which ones perform best. The first two models on this list are regional solutions; the rest are global or quasi-global models. We looked at data from all tide gauges that we could find, grouped into different categories. These are: Long records (>6 months); all records; open ocean (not ice shelves); ice shelves excluding the Rutford GZ; and Rutford only.

It is clear that TPXO6.2 from Oregon State is currently the best model. Two models perform poorly for ice shelves. These are CSR4 and NAO.99b.  The main problem with these models is poor geometry, mainly poor GL definition.

Even TPXO6.2 is not useful for GZ tide studies, since its grid spacing is 1⁄4 degree, i.e., 28 km in latitude. CATS and CADA are ~10 km grid spacing. Our best regional model, for the Peninsula, is at 2 km grid spacing, capable of coarsely resolving the broader GZs.

## ICESat near-global coverage

- Provided by Chris Shuman

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002

**Notes:**
ICESat covers most of the globe, with a maximum latitude of 86o. 

## ICESat:Antarctic (Laser 2a)

**Antarctic elevation (m) from ICESat Laser 2a operation period, Sept-Nov 2003.**

- Provided by Chris Shuman

**Convergence**
**of tracks: good for FRIS and RIS studies**

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002

**Notes:**
Here we zoom in on ICESat tracks for the Laser 2a operation period, for the Antarctic only. This is one of seven operation periods, each about 1 month long, that have been collected so far. Note how track spacing gets very close for the deep southern ice shelves, the Ross and the FRIS. 

## 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).

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002

## 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.

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002

**Notes:**
(read through list)

So, given all these concerns, it isn’t obvious yet whether ICESat repeat-track analyses will be generally useful.

## To cut a long story short ... it can be done!

**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.**

- Track across SW Ronne I.S.

**Dashed lines: Offshore tide prediction from CATS02.01.**

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002

**Notes:**
Here is a case, however, where the methodology is clearly successful. This plot shows 4 repeat tracks across the southwest GZ of the Ronne Ice Shelf, in a region of strong tides. We first filter the data at about 1 km resolution along each track, then plot the deviation of each track from the mean along-track profile. 

North of the offshore limit of flexure, the ice shelf elevations are reasonably consistent with one of the better ocean tide models for this region. Predictions for each track are shown by dashed lines. We also get some idea of the structure of flexure within the GZ. There is a hint that downward flexure for tides below mean sea level has a different characteristic than upward flexure (for Laser 2a, in red), but more study is required on this topic.

We see that the GZ is fairly well defined by comparison of ICESat repeat tracks crossing the GL, at least at this site.

## 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?)

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002

**Notes:**
Based on looking at many plots like this from around the FRIS, it looks like ICESat can be used to define the GL and the width of the GZ. 

## 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.

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002

## 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.

- Earth & Space Research, Presentation at Polar Research Board Meeting, April 2002