This paper provides a useful method to automatically extract short-term velocity variations over large areas using Landsat 8 GoLIVE velocity fields. Previous studies have done this for large areas for single velocity snapshots, or over small areas for multiple periods, but few have done it at the regional scale over multi-year periods for a large variety of glacier types. The authors provide a rigorous description of their methodology, and demonstrate its applicability for monitoring glacier variations in Alaska/Yukon, including the identification of several glacier surges.
This paper has previously been reviewed, and the authors have done a good job of addressing the previous reviewer’s comments. However, there are still many minor language and technical corrections to be made, as I’ve listed below. Most of these aren’t substantive, but a little annoying to have to point out when some of the co-authors are native English speakers.
The only substantive issue that I would like the authors to address is the effect of snow cover on the ability to monitor glacier velocities. For example, how effectively are you able to determine velocities in permanently snow-covered terrain in the accumulation area? This has been a major limitation in the previous use of optical satellite imagery to determine glacier motion. Similarly, are there poorer velocity matches in the winter than the summer, due to greater snow cover and more shadowing in the winter? Does this limit the ability to detect seasonal velocity variability? For example, Fig. 12a appears to show dramatic changes in velocity during a surge between summer and winter, so I would like to understand whether these changes are real or whether they’re an artifact of poorer velocity matching during the winter.
P1, L13: Wrangell mountains, not Wrangell’s
P1, L16 (and paper title): you write as if the paper only considers Alaskan glaciers, but this isn’t correct as you also include substantial glacier area in the Yukon (indeed, most of the glaciers that you highlight in Fig. 13 are in the Yukon). So I would suggest changing ‘Alaskan glaciers’ to something like ‘Alaskan/Yukon glaciers’ to better describe your study area. The paper title should also be updated to reflect this.
P1, L16: The Gardner et al. (2013) paper is a bit outdated now, as it only considers data up to 2009. A paper such as Harig and Simons (2016) is better as it brings the s.l. contribution record up to 2014:
Harig, C., Simons, F. J. 2016. Ice mass loss in Greenland, the Gulf of Alaska, and the Canadian Archipelago: Seasonal cycles and decadal trends. Geophysical Research Letters, 43, 3150-3159.
P1, L19: I don’t understand what you mean by ‘topographic glacier change’
P2, L6: you could also reference this paper for regional glacier velocities in the St. Elias:
Van Wychen, W., Copland, L., Jiskoot, H., Gray, L., Sharp, M. and Burgess, D. 2018. Surface velocities of glaciers in western Canada from speckle-tracking of ALOS PALSAR and RADARSAT-2 data. Canadian Journal of Remote Sensing, 44(1), 57-66
P2, L9: change ‘with over sub-seasonal’ to ‘over sub-seasonal’
P2, L15: change to ‘knowledge of glacier flow’
P2, L16: clarify which ‘mission objectives’ you’re referring to. I assume of Landsat 8, but the current wording is a bit ambiguous.
P2, L31: change to ‘In this study we describe the construction…’
P3, L3: wording is a little unclear with the double use of present in this sentence, so I would suggest changing the start to: ‘Our method retains…’
P3, L13: although it’s inferred, you don’t actually state that you use the GoLIVE velocity fields and don’t spell out the GoLIVE acronym. It would also be useful to provide the URL for the dataset. So I suggest changing the wording of this sentence to:
‘The Global Land Ice Velocity Extraction from Landsat 8 (GoLIVE) velocity fields used in this study are based upon repeat optical remote sensing imagery and are distributed through the National Snow and Ice Data Center (NSIDC; https://nsidc.org/data/golive) (Scambos et al., 2016).’
P3, L24: the meaning of ‘pairs across tracks are neglected’ is a bit unclear, so reword. I assume that you mean that only images with the same path and row number are matched with each other?
Fig. 1 caption: change from ‘studied region’ to ‘study region’
P4, L6-L8: it would be useful to provide a reference, and perhaps some specific numbers, to back up your statements here. This paper could help:
Fleming M, Chapin F, Cramer W, Hufford GL and Serreze MC (2000) Geographic patterns and dynamics of Alaskan climate interpolated from a sparse station record. Global Change Biol., 6, 49-58
P4, L12: change to ‘composed of products’
P4, L14: change to ‘time-series’
Fig. 2: the word ‘acquisition’ is misspelled as ‘acquition’ in this figure
P5, L4: I wouldn’t call southern Alaska particularly high latitude; this sentence is also missing a comma and has ‘the’ too many times. So it should be rewritten as: ‘At the latitude of Southern Alaska, scenes…’
P6, L3: change ‘acquiring dates’ to ‘acquisition dates’
P6, L18 (& elsewhere): perhaps ‘aliased’ is a better term to use than ‘smeared’?
Fig. 3: I don’t understand why the two d1,2 displacements (purple and blue arrows) are different from each other. Please explain.
P7, L17: change ‘through out’ to ‘throughout’ and ‘such shape’ to ‘such shapes’
P8, L7: the lines are described as being blue, purple and yellow, but they look to be blue, purple and orange to me
P8, L16: change ‘dependents’ to ‘depends’
P9, L5: you mention undersampling as a result of cloud cover, but presumably undersampling could also occur due to snow cover (particularly from new snowfall between image acquisitions)? This follows on from my major comment above.
Fig. 5: unclear what ‘needs to extent’ means in the figure caption. Perhaps this is meant to be written as ‘needs to extend’?
P11, L1: add a sentence break: ‘…shown in Figure 6. In this time…’
P11, L3/4: ‘both’ is repeated twice at the end of L3 and start of L4
P11, L8: change to ‘completeness increases’
P11, L19: it would be useful to provide the distance of the ‘velocity bulge retreat’ to help the reader understand how significant it is
P12, L4: correct spelling of ‘therefor’ to ‘therefore’
P12, L20: correct spelling of ‘therefor’ to ‘therefore’
P14, L10: correct spelling of ‘two-dimenstional’ to ‘two-dimensional’
P14, L13: This sentence doesn’t make sense on its own: ‘When the x-component of these displacements are compared against the RapidEye displacements.’ I think that it needs to be merged with the next one?
Fig. 9b: add light grey lines to indicate 0 x- and y-displacement (to match Fig. 9a)
P15, L8: should be written as ‘…imagery (Fahnestock et al., 2016), as geo-location…’
Fig. 10: correct spelling of ‘emperical’ to ‘empirical’. Make inset larger as the mask is currently too small to see clearly
P16, L21: this is a new para, so you need to make it clear as to which glacier you’re referring to here (Klutlan?)
Fig. 12a: can you make a comment in the paper as to whether the horizontal ‘stripes’ in the velocity pattern are real? E.g., are these due to seasonal variability in motion during the surge? Or lack of good velocity matches during the winter?
Fig. 12 caption: correct spelling of ‘morraine’ to ‘moraine’
P18, L6: changes ‘estimates’ to ‘estimated’
P19, L23: and snow cover can also hamper velocity estimation?
P19, L28: change to ‘surges of different glaciers…’
P20, L18: missing figure number (??). Also reword this sentence, as I have a hard time trying to figure out its meaning: ‘In the example of Figure ?? the graph is annotated with dates, in order to better
understand the other graphs corresponding to other path and rows which are given afterwards.’
P20, L23: it’s a bit difficult to tell which path number goes with which row number in the table, so list the path number for every column. Also need to add a caption for this table and specify that the time interval is in days
Fig. A1 caption: specify what the grey vertical bars and associated numbers refer to
P21, L2: missing figure number (??)
Fig. B1 caption: Fig. 12 doesn’t show all the reference marks; I think that you mean to refer to Fig. 11a here