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Deducing thickness changes of an ice sheet: comments on the paper by J. F. Nye

Published online by Cambridge University Press:  30 January 2017

Bruno Federer and
H. von Sury
Bruno Federer
Affiliation:
Atmosphärenphysik ETH, Hönggerberg, 8049 Zürich, Switzerland
H. von Sury
Affiliation:
Atmosphärenphysik ETH, Hönggerberg, 8049 Zürich, Switzerland
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Information

Type
Correspondence
Information
Journal of Glaciology , Volume 17 , Issue 77 , 1976, pp. 531
Copyright
Copyright © International Glaciological Society 1976

The Editor,

Journal of Glaciology

Sir,

We have read with great interest the article of Reference NyeNye (1975) in which he corrects an error in our earlier paper (Reference Federer, Federer, Sury von, Philberth and de QuervainFederer and others, 1970) and proposes a better model for calculating the surface lowering of the Greenland ice sheet. A surprising fact is that the movement of the reference points in the shaft of 40 m depth at Jarl Joset Station does not agree with Nye's prediction. If his model were applicable,. the value of ∂h/∂t should be constant (or a plot of δh/δt versus depth should at least approach a steady value at the lower layers). According to Nye's equation (3)

the second term on the right should not be constant. From our measurements, however, we find that V(z)p(z) is constant for a dozen reference points from 5 m to 40 m depth (Reference Sury von and HaefeiiSury and Haefeli, 1975).

Nevertheless the measured movement of the lowest reference point can be used to calculate the surface lowering according to Nye's model and to compare this value with the final result of the survey, now reported by. Reference SeckelSeckel (unpublished) in his thesis. From this it is seen that the measured surface lowering is 1.0 ±0.2 m from 1960 to 1968. The difference between this and the earlier value (1.74 m) is due, among other things, to improved values of the horizontal surface velocity v.r 17 ma and an increased surface slope tan α - 0.35%. Thus the values in our earlier paper (Reference Federer, Federer, Sury von, Philberth and de QuervainFederer and others, 1970) must be corrected as follows: v x tan α = 0.06 m a-1; V,r = Ur-Ux tan α = 0.45 — 0.06 = 0.39 m a-1; and V,* = 0.27 m a-1 (not 0.29 m a-1).

The yearly mass deficit (1959-68) now becomes a*— Vr* = 0.193 -0.27 - —0.077 ma -1. The surface lowering according to Nye's model

Thus the theoretical value of the surface lowering between 1960 and 1968 of 0.89 in is in good agreement with Seckel's measurement of 1.0±0,2 m. This suggests that Nye's model can eventually be used for layers which are not too close to the surface.

23 December 1975

References

Federer, B. 1970 Outflow and accumulation of ice in Jarl–Joset station , Greenland, by Federer, B.,Sury von, H. V.,Philberth, K,de Quervain, M. R. Journal of Geophisical Research, Vol. 75, No. 24, p. 4567–69 Google Scholar
Nye, J.F. 1975 Deducing thickness changes of an ice sheet from radio–echo and other measurements. Journal of Glaciology, Vol. 14, No. 70, p. 49–56. Google Scholar
Seckel, H.J. Unpublished. Zur geodatischen Erfassu ng von Inlandeisbewegungen. [Dr. rer. nat. thesis, Univers itat Fridericiana Karlsruhe, 1974) Google Scholar
Sury von, H.V. Haefeii, R. 1975 Rheologisch–glaziologischc Untcrsuchungen im Firngebiet des gronlandischen Inlandeises.Eidg Institut fur Sehnee– und Lawirrenforsclwng , Weissjluhjoch–Davos. Interner Berieht No. 540. Google Scholar