the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Spatial characterization of near-surface structure and meltwater runoff conditions across Devon Ice Cap from dual-frequency radar reflectivity
- 1Institute for Geophysics, University of Texas at Austin, Austin, TX, 78758, USA
- 2Geological Survey of Denmark and Greenland, Copenhagen, Denmark
- 3Department of Electrical Engineering, Stanford University, Stanford, CA, 94305, USA
- 1Institute for Geophysics, University of Texas at Austin, Austin, TX, 78758, USA
- 2Geological Survey of Denmark and Greenland, Copenhagen, Denmark
- 3Department of Electrical Engineering, Stanford University, Stanford, CA, 94305, USA
Abstract. Melting and refreezing processes in the firn of Devon Ice Cap control meltwater infiltration and runoff across the ice cap, but their full spatial extent and effect on near-surface structure is difficult to measure with ground-based traverses or existing satellite remote sensing. Here, we derive the coherent component of the near-surface return from airborne ice-penetrating radar over Devon Ice Cap, Canadian Arctic, to characterize firn containing centimeter to meter-thick ice layers (i.e., ice slabs) formed from refrozen meltwater in firn. Comparison with reflectivities using a thin layer reflectivity model, informed by ground-based radar and firn core measurements, indicate that the coherent component is sensitive to the near-surface firn structure composed of quasi-specular ice and firn layers, limited by the bandwidth-constrained radar range resolution. By leveraging their differences in range resolution, we assess the use of dual-frequency airborne ice-penetrating radar to characterize the spatial and vertical near-surface structure of Devon Ice Cap. Our results suggest that average ice slab thickness throughout the Devon Ice Cap percolation zone ranges from 4.2 to 5.6 m. This implies conditions that can enable lateral meltwater runoff and potentially contribute to the total surface runoff routed through supraglacial rivers down glacier. Together with the incoherent component of the surface return previously studied, our dual-frequency approach provides an alternative method for characterizing bulk firn properties, particularly where ground-based and higher frequency radar data are not available.
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Kristian Chan et al.
Status: open (until 14 Nov 2022)
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RC1: 'Comment on tc-2022-181', Anonymous Referee #1, 25 Sep 2022
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Review of: Spatial characterization of near-surface structure and meltwater runoff conditions across Devon Ice Cap from dual-frequency radar reflectivity - by Chan et al.
General Comments
Chan et al. investigate the surface coherent return power of reflected radar waves by applying the Radar Statistical Reconnaissance method to multiple ice penetrating radar datasets with different center frequencies over Devon Ice Cap (Canada). The data is used to better characterize the composition of the firn pack (and if the firn pack contains ice layers) of the upper meters over the ice cap, which is important to better understand melting and refreezing processes as well as meltwater infiltration and runoff. The measured reflectivities are compared with modeled reflectivities using a reflectivity model informed by existing information on the firn pack (from ground-based ice penetrating radar data and firn cores). Their results suggest meter-thick ice slabs in certain parts of the ice cap, which permits surface water runoff away from the ice cap.Overall I find the study by Chan et al. to be informative and very well written. They applied a smart approach to characterize the firnpack with existing multiple airborne radar data sets and other auxiliary data sets (such as firn cores and land-based radar data). Although the methodology is not fundamentally new and many aspects have been already analyzed and built upon previous studies (such as in Rutishauser et al., 2016), I believe that this article deserves to be published in The Cryosphere.
The basis for my decision is that, in my opinion, this is a robust study that is well structured, clearly written, and represents a significant step forward in knowledge on which future studies can build on and which is very useful for the cryosphere community. What I particularly liked is that the authors use existing data sets and put the data into a new context with their method to find out more about the first meters of firn of the Devon Ice Cap. Below I have some comments and questions that I think might help to add clarity and make the article better readable and easier to follow.
Main RemarksIntroduction:
The introduction could benefit from a small introduction on the Devon ice cap and why it is a particularly good place to characterize the firn column. Either in a new paragraph (which I would prefer) or incorporated in one of the existing paragraphs.Figure 1:
(1) It would be a nice addition to have an overview map of the Canadian arctic or Canadian-Greenlandic arctic pointing out the location of the survey area. This would give the reader a much better impression of where the Devon ice cap is located.
(2) I also would suggest finding a better solution with the contour lines and their elevation labels. They appear very chaotic at the ice caps margins, which is rather confusing than helpful information. The same applies to all other figures (also in the supplement; S4) in which the contour lines are shown. Maybe only displaying contour lines only above 600 m would make the plot less overloaded.
(3) It would also be good to state what kind of satellite image you are using as a background image.Table 1:
Please explain the symbols in the table caption (e.g., that range resolution is z_0, etc.)
In addition, but very minor: a hline between the two systems would be nice to immediately see which z0 belongs to which system.Figure 2:
What about the following idea: To give the reader a better understanding of the different depth resolution of the radar systems and which parts of the firn column are affected, one idea would be to somehow draw or indicate the depths that HiCARS & MARFA and MCoRDS3 resolve in Figure 2b.Figure 3:
(1) I think the figure could be better arranged if, for example, (a) and (b) were in a row and (c) below. Then the subfigures would be bigger and the whole figure would take probably less space in the document at the same time. The same could be done with Figure 4.
(2) Shouldn't the label of the colorbar be "dB" instead of "db"?
(3) I would suggest a different color scale, preferably linear rather than divergent. This is because in the HiCARS display, for example, the transition from -10 to -15 dB is shown as a weak color change, while from -20 to -25 dB there is a strong color change (yellow to blue). Therefore, I would suggest a linear graded color scale to better interpret the changes in dB based on a color scale across the different data sets.Figure 4:
Caption: define again that interquartile ranges is IQR and P_c surface coherent power (as in Fig. 3).Discussion:
I have a question regarding the ice slab thicknesses in Zone II. In Line 336 you state that the HiCARS/MARFA system captures the entire thickness of the ice slabs. Maybe I have missed it, but why is that the case and how do you know that the ice slabs along these radar profiles are not thicker than the range resolution of the system?
My next question is very similar and refers to the average ice slab thicknesses. You calculated a mean ice slab thickness based on the range resolution of the two different radar (groups). Wouldn’t it be rather a minimum average ice slab thickness? Because since you are only analyzing the surface return within the limits of the range resolution of the radar system, you cannot estimate if the ice slab continues with depth and is thicker, right?
For me it seems that based on the surface GPR data it is assumed that the ice slabs in this region are not thicker as what is for example shown in Figure 2b. However, it might nevertheless be possible that thicker ice slabs might be present along the airborne radar profiles where no surface radar data exists. I think this should be clarified and also mentioned in the uncertainty section.Figure 5:
Here now appears a reference to the background satellite image, but the coordinates are missing. Again, I would prefer to get rid of the contour lines and labels below a certain depth.Supplement
Figure S4: Please mention once more in the caption that P_c is coherent specular and P_n incoherent/scattered. I'm sure many readers don't, but I often have the problem that I forget the abbreviations in the text while reading and then have to look for them again in the text when they appear in a figure.Line-item Comments
L 84-86: I think that Operation Ice Bridge should be mentioned here as well in addition to the University of Kansas. Moreover, I would suggest using the acronym MCoRDS3 instead of just MCoRDS throughout the document.
L 99-101: With respect to the factors affecting permittivity, I think that temperature and the anisotropy due to the orientation of the ice crystal fabric should also be mentioned (although COF may not be so important in the firn column). In that sense you could additionally cite for example Fujita et al. (2000):
“Fujita, S.,T. Matsuoka,T. Ishida,K. Matsuoka, and S. Mae (2000), A summary of the complex dielectric permittivity of ice in the megahertz range and its applications for radar sounding of polar ice sheets, in Physics of Ice Core Records, edited by T. Hondoh, pp. 185–212, Hokkaido Univ. Press, Hokkaido, Japan. ”L 128 (and L177-178): You mention that “[...] surface roughness is not the main contributor to surface scattering over DIC (Rutishauser et al., 2016).”. It would be interesting to mention in one sentence why this is not the case. Especially because this assumption is important for the interpretation of the results.
L 137-139: Here you state that: “Previous applications of the RSR method have empirically shown that an aircraft roll of 2 to 3° allows for a stable coherent radar return.” Is there a reference for this?
L 141: The airborne radar data in your study is also "ground-penetrating". From what I understood you refer to land-based or surface radar in this section. Therefore I would suggest making clear that all radar surveys are ground penetrating and some are airborne and this one is land-based/surface radar data.
L 248-252: I am not sure if I missed it, but is the difference between the old and refined Zones shown somewhere? If not, I think it should be (maybe in the Supplement). I guess the old Zones are those displayed in Rutishauser et al. (2016) in Figures 1a and 2?
L 252-254: Here you refer to the Discussion Section but I think it would be also good to refer to Figure 5.
Thank you for the exciting read.
Kristian Chan et al.
Kristian Chan et al.
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