1 / 24

Effects of firn on determining bed topography of polar ice sheets using radar

Effects of firn on determining bed topography of polar ice sheets using radar . Kenny Matsuoka 1 , Stefan Ligtenberg 2 , Michiel Van den Broeke 2 Norwegian Polar Institute IMAU, Utrecht University. New GIS package for Antarctica. Based on free Quantum GIS software.

totie
Download Presentation

Effects of firn on determining bed topography of polar ice sheets using radar

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Effects of firn on determining bed topography of polar ice sheets using radar Kenny Matsuoka1, Stefan Ligtenberg2, Michiel Van den Broeke2 Norwegian Polar Institute IMAU, Utrecht University

  2. New GIS package for Antarctica • Based on free Quantum GIS software. • Tested for two years on ice and in office. • “Ready to go” package, i.e. • Range of useful data and satellite images • Thoughtful visualization • Standalone system (no Inteternet)

  3. Quantarctica = Quantum GIS + Antarctica • You can modify Quantarctica freely. • We solicit: • Contributions of continent-wide field data, model outputs, and remote sensing data. • Suggestions which data should be included. Download and Contribute! www.quantarctica.org

  4. http://www.youtube.com/watch?v=_HV_VbxKKDM skip ad

  5. Effects of firn on determining bed topography of polar ice sheets using radar Kenny Matsuoka1, Stefan Ligtenberg2, Michiel Van den Broeke2 Norwegian Polar Institute IMAU, Utrecht University

  6. Radio-wave propagation speed

  7. Air in the Antarctic ice 40 m 30 m 20 m 10 m Van den Broeke (2008, Antarctic Science)

  8. Firn correction “The majority of direct ice thickness measurements from radar and seismic techniques were calculated with the inclusion of a “firn correction”.” “ Routinely for radar measurements on thick ice, 10 m of additional ice thickness has been added by researchers to account for the low-density/high-velocity firn layers.” Fretwell et al. (2013, TC) BEDMAP2 grouppaper

  9. Is it a matter? • Accuracy of ice thickness and ice mass in polar regions • Data compilations • Errors in freeboard elevations of the ice shelves and eventually estimates of marine ice thickness • Errors in subglacial hydraulic potentials Individual researchers have made best estimates for specific studies,but there is no continent-wideknowledge base.

  10. Icethicknessestimateusing radar H : Icethickness <v>: Depth-averagedpropagation speed T : Two-way travel time v : Localpropagation speed c : Propagation speed in vacuum n : Refractionindex <n>: Depth-averagedn

  11. Estimatingdepth-averaged<v> Pick a reasonablerelationshipbetweendensity and propagation speed. Assumeapproximatedepthprofilesofdensity Using1 & 2, estimatedepth-averagedpropagation speed Pure-icepropagation speed vi= 168.5 m/ms (ni = 1.78) - Range ofvi = 168 – 169.5 m/ms - Functionoficetemperature, fabrics, andchemisty (e.g. Fujita et al., 2000) Fujita et al. (2000, Physicsoficecorereocrds)

  12. Frequently-usedrelationships

  13. Depthprofilesofdensity rsurf: 400, 450, 500, 550 kg/m3. hf: 60, 80, 100 m Equation 9.81 in Greve and Blatter (2009, Dynamics oficesheets and glaciers)

  14. Depth-averaged speed <v> Looyenga CRIM Kovacs Frolov Regardlessoftherefractionindexmodels, <v> is largestwhen(rsurf, hf) = (400 kg/m3, 100 m)and smallest when (600 kg/m3, 60 m). Red: firn thicknesshf = 100 m; Green: hf = 80 m; Blue: hf = 60 m

  15. Sourceofrefraction-indexuncertainty • Variationsbetweenmodels: ± 0.64 m/msIndependent oficethickness and choiceofdensification parameters • Variationsin pureice:± 0.75 m/msDependent onicetemperature andfabrics(Fujita et al., 2000) Red: (400 kg/m3, 100 m) Blue: (600 kg/m3, 60 m) Fujita et al. (2000, Physicsoficecorereocrds)

  16. Whichn -rrelationship is best? • Estimatedpropagation speeds depend minimal onthechoiceofthedensity/refraction-indexrelationship. • So, usethe simplest, linear equation, CLIM. Now <v> can be derived from air and ice thicknesses.We don’t need depth variations of the density.

  17. Depth-averaged<v> vi = 168.5 m/ms

  18. Firn correctionDH • The first guessoftheicethicknessH0can be derivedusingpure-icevalueofthedepth-averagedpropagation speed vi • The best estimateoftheicethicknesscan be H0+DH, using firn correctionDH:

  19. DH is usuallyassumed to be 10 m “ Routinely for radar measurements on thick ice, 10 m of additional ice thickness has been added by researchers to account for the low-density/high-velocity firn layers.” Fretwell et al. (2013, TC) BEDMAP2 grouppaper

  20. Firn correctionDHvariations DH is virtually independent of ice thickness. vi = 168.5 m/ms

  21. DH for iceshelves DH is virtually independent of ice thickness. vi = 168.5 m/ms

  22. DH for theAntarcticIceSheet Input data: Fretwell et al. (2013, TC) and Ligtenberg et al., (2011, TC)

  23. Properties in the modeled DH • Mean value: 9.2 m. • Inland Antarctica • ~15 m • Large (Ross, Ronne/Filchner) ice shelves • 8 -10 m • Small ice shelves in Dronning Maud Land • < 5 m

  24. Take-home messages • Firn correction values are virtually independent of ice thickness but gradually vary with air column thickness. • Firn correction values are < 5 m in the DML ice shelves and 15-20 m in the inland EAIS. • Please, show pure-ice propagation speed vi and firn correction DH in your paper. • Please, consider submitting two-way travel time “data” together with ice thickness “estimates” to a world data center.

More Related