Quantifying mass balance processes on the Southern Patagonia Icefield

Schaefer, M.; Machguth, H.; Falvey, M.; Casassa, G.; Rignot, E.

doi:https://doi.org/10.5194/tc-9-25-2015

Articles | Volume 9, issue 1

https://doi.org/10.5194/tc-9-25-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/tc-9-25-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 9, issue 1

Research article

|

06 Jan 2015

Research article |

| 06 Jan 2015

Quantifying mass balance processes on the Southern Patagonia Icefield

M. Schaefer, H. Machguth, M. Falvey, G. Casassa, and E. Rignot

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Final revised paper (published on 06 Jan 2015)
Preprint (discussion started on 11 Jun 2014)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

SC C1000: 'Quantifying mass balance processes on the Southern Patagonia Icefield', Mauri Pelto, 26 Jun 2014
- SC C1051: 'Comments on evaluations proposed by M. Pelto', Marius Schaefer, 01 Jul 2014
- AC C1702: 'Detailed answers', Marius Schaefer, 03 Sep 2014
RC C1200: 'Comment on "Quantifying mass balance processes on the Southern Patagonia Iceﬁeld" by M. Schaefer et al.', Helmut Rott, 09 Jul 2014
- SC C1224: 'SMB of non-calving glaciers', Marius Schaefer, 10 Jul 2014
- AC C1703: 'Detailed answers', Marius Schaefer, 03 Sep 2014
RC C1273: 'Review of Schaefer et al.', Anonymous Referee #2, 14 Jul 2014
- AC C1704: 'Detailed answers', Marius Schaefer, 03 Sep 2014
EC C1797: 'Editors comments', Jonathan L. Bamber, 14 Sep 2014

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Marius Schaefer on behalf of the Authors (04 Oct 2014) Author's response Manuscript

ED: Referee Nomination & Report Request started (16 Oct 2014) by Jonathan Bamber

RR by Helmut Rott (02 Nov 2014)

Suggestions for revision or reasons for rejection

I wish to thank the authors for their detailed response to the reviews. This clarifies many of the critical points addressed in the reviews. However, the issues which the authors are communicating in the response to the reviews have not been fully taken into account when revising the manuscript.
A main critical issue that is not well communicated is the comparatively high uncertainty in modelling of precipitation and the resulting values of net accumulation:
- In line 193 ff the authors state: “In Fig. 3 we compare the results of our simulations to these direct point measurements of the surface mass balance. Satisfactory agreement can be observed between the modelled and the measured data.” This is true for the ablation area, but by no means valid for the accumulation area, as obvious from the scatter of the 3 points of Fig. 3 (Nr. 3, 5, 6) in the accumulation area (which the authors correctly recognize as an inadequate sample).
- In line 206 ff the authors state: “The process of wind drift …… These local effects are important when comparing point measurements with modelled surface mass balance, but should not play an important role when estimating the surface mass balance of larger areas as glacier basins or even the entire SPI.” This is rather speculative, not supported by analysis or references. Elevation gradients of precipitation depend on several factors, not just wind drift. Also, there should be major differences in wind effects between the luv and lee sides of the ice field.
- On page 6 of the response document the authors state: “We can get an idea about the uncertainties of the individual mass balance components at every glacier by comparing columns 1 and 2 to columns 3 in Table1. We think that this is much more informative then inventing some arbitrarily high a priori uncertainty to the modelled accumulation.” These statements imply that a clearly defined and traceable error assessment is missing and that the uncertainty of modelled net accumulation is not known, and thus could possibly be quite high. This should be communicated not only in the author’s response, but also in the manuscript.
- “ comparing columns 1 and 2 to columns 3 in Table1” in the statement above probably refers to Table 1 of manuscript Version 1 (Qc inferred and Qc from velocities) . “Comparing” these columns does not enable any clear quantitative error assessment. On one hand, two multi-year periods are compared with a short term data set. On the other hand, column 4 (Qc from 2004 velocities) has high error bars; these translate into high error bars for the modelled surface mass balance if the relation of Equation 1 is used.
- Table 1: overall uncertainties should be provided for Qc inferred rather than uncertainties considering only one of the error sources. The rather low error bars in the present version of Table 1 may lead to misinterpretation and are telling little about the uncertainty of the modelled mass balance.
- Table 1, columns 1 and 2 of revised manuscript: ELA data of the same years should be compared (2002 and 2004). It is not meaningful comparing the mean value of a long time series with data of 2 specific years.
- Conclusions: uncertainty estimates should be provided together with the mass balance numbers. The basis for estimating uncertainties needs to be explained in the manuscript (or in Supplementary Material).

Further issues:
- In the previous review there was a comment on the 1975 to 2000 (resp. 1968 to 2000 for Argentinean glaciers) volume change that has not been taken into account (or communicated) in the revised manuscript: Rignot et al.(2003) explain that the 1975 DEM did not cover areas at elevations above 1200 m. Obviously, this results in is a mismatch with the area covered by SMB simulations that extend over the whole ice field.
- Line 335 ff: “A literature reference on the 2004 SPI velocity field is missing.
- In line 335 ff it is explained: “The model ELAs are considerably lower than the average SLA at the end of the summers 2002 and 2004 for the glaciers HPS12, HPS13, HPS29 and HPS34. This discrepancy can be explained by the possibility of snowfalls before the acquisition of the examined satellite images.” Snowfall effects should be rather the opposite. Summer snowfall in satellite images should lead to underestimation of inferred ELA.

Referee Report: PDF

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RR by Anonymous Referee #2 (04 Nov 2014)

ED: Publish subject to minor revisions (Editor review) (07 Nov 2014) by Jonathan Bamber

AR by Marius Schaefer on behalf of the Authors (15 Nov 2014) Author's response Manuscript

ED: Publish subject to technical corrections (20 Nov 2014) by Jonathan Bamber

Short summary

We use a meteorological-glaciological multi-model approach to quantify, for the first time, melt and accumulation of snow on the Southern Patagonia Icefield (SPI). We were able to reproduce the high measured accumulation of snow of up to 15.4 m water equivalent per year as well as the high measured ablation of up to 11 m water equivalent per year. Mass losses of the SPI due to calving of icebergs strongly increased from 1975-2000 to 2000-2011 and were higher than losses due to surface melt.