Freeboard changes of Drygalski and Mertz ice tongues in east Antarctica using altimetry data

Transcription

1 Freeboard changes of Drygalski and Mertz ice tongues in east Antarctica using altimetry data XIANWEI WANG 1 and DAVID M. HOLLAND 1, 2 1. Center for Global Sea Level Change, New York University Abu Dhabi. Abu Dhabi, United Arab Emirates. 2. Courant Institute of Mathematical Sciences, New York University. New York 10012, United States of America. xw21@nyu.edu; wangxianwei0304@163.com

2 Outline Motivation Study area and data Mertz and Drygalski ice tongue ICESat/GLAS, ICE Bridge Riegl Laser Methodology of freeboard changes Influences on freeboard changing rate (G C, freeboard extraction method) Results and discussion Conclusion

3 Background 1. Both Mertz and Drygalski ice tongue are floating extensions of land based glaciers. 2. Mertz glacier stretches about 140 km to the sea from the grounding line, with ice front of about 34 km in width before Drygalski ice tongue stretched from David glacier is about 20 km wide and 70 km long. 4. Both ice tongues experienced ice calving in recent years and lost mass in the way of ice calving.

4 Drygalski Ice Tongue Terra Nova Bay Polynya 1. Long ice tongue??? / Iceberg collision Whitlock AWS Weather changed little 2. Freeboard??? Cape Denison AWS Mertz Ice Tongue Mertz Polynya Study area Drygalski and Mertz Ice Tongue (Background is MODIS Mosaic of Antarctica from NSIDC)

5 ICESat/GLAS Data ICESat is the first Earth orbiting Laser altimetry satellite with GLAS as its primary sensors. Laser: 1, 2, and 3 Life Time: 2003 to 2009 Footprint size: ~70 m Data: R 34 with G C correction ICESat/GLAS data coverage on the Drygalski ice tongue ICESat/GLAS data coverage on the Mertz ice tongue

6 Flow chart of freeboard changes ICESat/GLAS freeboard calculation data preprocessing G C correction no freeboard relocation velocity data yes track separation EGM08 height crossover sea level extraction <=30 days >=300 days freeboard freeboard accuracy changing rate

7 Method of freeboard change detection Samples of ICESat/GLAS on the Mertz Ice tongue Elevation profile along track local_max and local_min indicate the local maximum and local minimum of the profile. re_local_max and re_local_min indicate the revised local maximum and local minimum. sea_level indicates the footprint from the sea surface. (Wang et al. 2014)

8 Crossovers from ICESat with footprints relocation (Wang et al. 2014)

9 Crossovers without relocation crossovers on the Drygalski ice tongue crossovers on the Mertz ice tongue

10 Accuracy of sea level extraction If a crossover does not fall on sea ice or ice tongue, freeboard difference on crossover contains error of sea level extraction only. The accuracy of sea level extraction from ICESat/GLAS can be evaluated with crossovers from different tracks fallen on ocean water. Height referred to geoid, sea level selection by track and G C correction for GLA12 data.

11 Sea level selection Criteria: sea_level falls in mean value ±3 times of std slmean = slstd = Inside number= 121 Outside number= 5 slmean = slstd = Inside number= 88 Outside number= 1 Sea level height around Drygalski ice tongue (blue) and outliers (red) Sea level height around Mertz ice tongue (blue) and outliers (red)

12 Influence of G C correction For GLA06, 12, 13 and 15, difference of Gaussian Peak Waveform Centroid should be considered (Release 34) G C corrections for different campaigns using different lasers (1,2 and 3) Borsa et al. (2013)

13 Sea level comparison Mertz Minimum Maximum Mean Standard deviation count No G C EGM Our Method Drygalski Minimum Maximum Mean Standard deviation count No G C EGM Our Method

14 Accuracy of freeboard calculation Assuming the changes of freeboard of Mertz and Drygalski ice tongue in 30 days is small which can be neglected and constant ice flow. The accuracy of freeboard calculation from ICESat/GLAS can be evaluated with crossovers from different tracks on sea ice or ice tongue because of no in situ data. Height referred to geoid, sea level selection by track and G C correction for GLA12 data.

15 Freeboard accuracy Mertz Minimum Maximum Mean Standard deviation count No G C EGM Our Method Drygalski Minimum Maximum Mean Standard deviation count No G C EGM Our Method

16 Crossover selection Criteria 1. Crossovers fallen in heavily crevassed region should not be used. 2. For specific tracks, adjacent footprints should exist in both sides of crossovers < threshold Dis_left+Dis_right<= threshold

17 Freeboard changing rate of Mertz and Drygalski ice tongues from ICESat Region Minimum (m/a) Maximum (m/a) Mean (m/a) Standard deviation (m/a) count Mertz Drygalski Without crossover selection Region Minimum (m/a) Maximum (m/a) Mean (m/a) Standard deviation (m/a) count Mertz Drygalski

18 Freeboard changing rate of Drygalski ice tongue from ICESat and Riegl laser Mean: 0.61 Max: 0.90 Min: 3.64 Std: 0.99 Data and date ICESat/GLAS: ICE Bridge Riegl Laser: Nov, 2010 Steps Sea level height Freeboard calculation Freeboard relocation Freeboard comparison Results 49 crossovers

19 Influence on crossovers using footprints relocation or not The location of footprints observed on earlier campaigns appears downstream on later campaigns. To make sure the freeboard comparison on the same points, footprints relocation is what must be done. Spatial distribution of crossovers Freeboard difference in crossovers

20 Crossover distributions from ICESat and Rigel laser No with footprints relocation Drygalski ice tongue

21 Freeboard difference in crossovers from ICESat and Riegl laser altimeter onboard ICE Bridge Freeboard difference in crossovers with and without footprint relocation from ICESat and Rigel laser

22 Crevasse depth of Mertz and Drygalski ice tongue from ICESat/GLAS Freeboard changes of heavily rugged ice tongue is difficult 65% <10m 87% <10m

23 Conclusions 1. Accuracy of sea level extraction is about ±0.2 m and freeboard accuracy is about ±0.5 m from analysis of both Drygalski and Mertz ice tongue in east Antarctica. 2. Using cross validation, to detect freeboard changes, GLA 12 R33(34), adopting geoid height and simultaneous sea surface does not matter much. 3. Footprint relocation is required for freeboard comparison using ICESat/GLAS data from different campaigns especially when the ice flow is fast. 4. Freeboard changing rates of Drygalski ice tongue and Drygalski ice tongue are 0.20 ±1.33 m/a and 0.02 ±1.32 ( 0.61 ±0.99) m/a.

24 Future work Evaluation of footprints relocation Further validation of freeboard changing rates Further analysis of freeboard changes of Drygalski ice tongue ( 0.02 ±1.32 m/a 0.61 ±0.99 m/a) Reanalysis of freeboard changing rate of ice shelves in Antarctica using laser altimetry.

25 Thanks for your attention