effects of firn on determining bed topography of polar ice sheets using radar
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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Effects of firn on determining bed topography of polar ice sheets using
radar
Kenny Matsuoka1,Stefan Ligtenberg2, Michiel Van den Broeke2
1. Norwegian Polar Institute2. IMAU, Utrecht University
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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)
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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
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Effects of firn on determining bed topography of polar ice sheets using
radar
Kenny Matsuoka1,Stefan Ligtenberg2, Michiel Van den Broeke2
1. Norwegian Polar Institute2. IMAU, Utrecht University
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Radio-wave propagation speed
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Air in the Antarctic ice
Van den Broeke (2008, Antarctic Science)
40 m
30 m
20 m
10 m
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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 group paper
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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-wide knowledge base.
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Ice thickness estimate using radar12
H v T H : Ice thickness
<v>: Depth-averaged propagation speed
T : Two-way travel time
v : Local propagation speed
c : Propagation speed in vacuum
n : Refraction index
<n>: Depth-averaged n
cvn
cvn
0
Hn z dz
nH
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Estimating depth-averaged <v>
1.Pick a reasonable relationship between density and propagation speed.
2.Assume approximate depth profiles of density3.Using 1 & 2, estimate depth-averaged
propagation speed
Pure-ice propagation speed vi = 168.5 m/ms (ni = 1.78)
- Range of vi = 168 – 169.5 m/ms- Function of ice temperature, fabrics, and chemisty (e.g. Fujita et al., 2000)
Fujita et al. (2000, Physics of ice core reocrds)
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Frequently-used relationships
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Depth profiles of density
2f
Surf i i f2f
i f
z zz z
H z
z z
Equation 9.81 in Greve and Blatter (2009, Dynamics of ice sheets and glaciers)
surf: 400, 450, 500, 550 kg/m3.
hf : 60, 80, 100 m
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Depth-averaged speed <v>CRIM Looyenga Kovacs Frolov
Red: firn thickness hf = 100 m; Green: hf = 80 m; Blue: hf = 60 m
Regardless of the refraction index models,<v> is largest when (surf, hf) = (400 kg/m3, 100 m)
and smallest when (600 kg/m3, 60 m).
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•Variations between models: ± 0.64 m/msIndependent of ice thickness and choice of densification parameters•Variations in pure ice: ± 0.75 m/msDependent on icetemperature andfabrics(Fujita et al., 2000)
Source of refraction-index uncertainty
Red: (400 kg/m3, 100 m)Blue: (600 kg/m3, 60 m)
Fujita et al. (2000, Physics of ice core reocrds)
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Which n relationship is best?• Estimated propagation speeds depend
minimal on the choice of the density/refraction-index relationship.
• So, use the simplest, linear equation, CLIM.
a in n A n I
Now <v> can be derived from air and ice thicknesses.We don’t need depth variations of the density.
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Depth-averaged <v>
vi = 168.5 m/ms
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Firn correction DH
• The first guess of the ice thickness H0 can be derived using pure-ice value of the depth-averaged propagation speed vi
• The best estimate of the ice thickness can be H0+DH, using firn correction DH:
0 i12
H v T
ii 0
i
12
v vH v v T H
v
D
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DH is usually assumed 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 group paper
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Firn correction DH variations
DH is virtually independent of ice
thickness.
vi = 168.5 m/ms
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DH for ice shelves
DH is virtually independent of ice
thickness.
vi = 168.5 m/ms
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DH for the Antarctic Ice Sheet
Input data: Fretwell et al. (2013, TC) and Ligtenberg et al., (2011, TC)
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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
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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.