Comparing elevation and backscatter retrievals from CryoSat-2 and ICESat-2 over Arctic summer sea ice
Abstract. The CryoSat-2 radar altimeter and ICESat-2 laser altimeter can provide complimentary measurements of the freeboard and thickness of Arctic sea ice. However, both sensors face significant challenges for accurately measuring the ice freeboard when the sea ice is melting in summer months. Here, we used crossover points between CryoSat-2 and ICESat-2 to compare elevation retrievals over summer sea ice between 2018–2021. We focused on the electromagnetic (EM) bias documented in CryoSat-2 measurements, associated with surface melt ponds over summer sea ice which cause the radar altimeter to underestimate elevation. The laser altimeter of ICESat-2 is not susceptible to this bias, but has other biases associated with melt ponds. So, we compared the elevation difference and reflectance statistics between the two satellites. We found that CryoSat-2 underestimated elevation compared to ICESat-2 by a median difference of 2.4 cm and by a median absolute deviation of 5.3 cm, while the differences between individual ICESat-2 beams and CryoSat-2 ranged between 1–3.5 cm. Spatial and temporal patterns of the bias were compared to surface roughness information derived from the ICESat-2 elevation data, the ICESat-2 photon rate (surface reflectivity), the CryoSat-2 backscatter and melt pond fraction derived from Seintnel-3 OLCI data. We found good agreement between theoretical predictions of the CryoSat-2 EM melt pond bias and our new observations; however, at typical roughness <0.1 m the experimentally measured bias was larger (5–10 cm) compared to biases resulting from the theoretical simulations (0–5 cm). This intercomparison will be valuable for interpreting and improving the summer sea ice freeboard retrievals from both altimeters.
Geoffrey Dawson and Jack Landy
Status: final response (author comments only)
RC1: 'Comment on egusphere-2023-31', Ellen Buckley, 24 Feb 2023
- AC1: 'Reply on RC1', Geoffrey Dawson, 14 May 2023
RC2: 'Comment on Dawson and Landy', Anonymous Referee #2, 27 Feb 2023
- AC2: 'Reply on RC2', Geoffrey Dawson, 14 May 2023
Geoffrey Dawson and Jack Landy
Geoffrey Dawson and Jack Landy
Viewed (geographical distribution)
This work presented in this manuscript addresses important concepts and questions in sea ice altimetry. The methodology is described clearly and with reproducible details. However, there are some points in the results and discussion that are confusing and seems to be drawing conclusions from very weak relationships. I also think there is too much confidence put into ICESat-2 as the “truth” when summer sea ice segments have not been validated. Generally, the use of the phrase “CryoSat-2 underestimates ICESat-2 heights” suggests that ICESat-2 is right and CryoSat-2 is wrong. Although some limitations of the study are mentioned, I think there are significant assumptions that need to be addressed and further discussed. There are specific comments in the next section. Overall though I think it is an important study and this line of work needs to be further investigated.
throughout: check in text citations- a lot of incorrect punctuation
50: is no snow always the case? in all regions? is there a reference for this? also what do you define as ‘mid-summer’ here? I see you address the snow assumption in line 124, perhaps move up.
56: is SAMOSA an acronym? if so, define. I see it is defined in line 141, move up to first instance.
62: ‘likely often’ is redundant and vague. consider rewording
78: newer studies say 11 m footprint. see:
Lori A. Magruder, Kelly Brunt, Thomas Neumann, et al. Passive ground-based optical techniques for monitoring the on-orbit ICESat-2 altimeter geolocation and footprint diameter. ESS Open Archive . October 27, 2020.
80-87ish: consider including these studies:
Buckley, E. M., Farrell, S. L., Herzfeld, U. C., Webster, M. A., Trantow, T., Baney, O. N., Duncan, K. A., Han, H., and Lawson, M.: Observing the Evolution of Summer Melt on Multiyear Sea Ice with ICESat-2 and Sentinel-2, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-189, 2023.
Herzfeld, Ute; Trantow, Thomas; Han, Huilin; Buckley, Ellen; Farrell, Sinead; Lawson, Matthew (2022): Automated Detection and Depth Determination of Melt Ponds on Sea Ice in ICESat-2 ATLAS Data — The Density-Dimension Algorithm for Bifurcating Sea-Ice Reflectors (DDA-bifurcate-seaice). TechRxiv. Preprint. https://doi.org/10.36227/techrxiv.21300153.v1
95: what about ICESat-2 Summer Sea Ice Campaign- cryo2ice underflights included in this campaign
100: regarding error propagation, consider discussing and referencing this:
Giles, K. A., Laxon, S. W., Wingham, D. J., Wallis, D. W., Krabill, W. B., Leuschen, C. J., ... & Raney, R. K. (2007). Combined airborne laser and radar altimeter measurements over the Fram Strait in May 2002. Remote Sensing of Environment, 111(2-3), 182-194.
135: “they”- who is they
151-160: this is confusing. the atl07 product does not sample 0.7 m along track. I think here you are mixing up atl07 and alt03 products. describe one at a time: ICESat-2 sampling, then what atl03 is, then how atl07 is created. where does 6 m come from? something you calculated? if not reference. seems very short. see reference: Kwok, R., Markus, T., Kurtz, N. T., Petty, A. A., Neumann, T. A., Farrell, S. L., et al. (2019). Surface height and sea ice freeboard of the Arctic Ocean from ICESat-2: Characteristics and early results. Journal of Geophysical Research: Oceans, 124, 6942– 6959. https://doi.org/10.1029/2019JC015486
“The length of the strong beam height segments (Ls) varies between ~10 and 200 m, while those from the weak beams are between ~40 and 800 m.”
153: same as line 78 comment (11 m)
165: no hyperlinks within text, move to references
163-170: would this be better in a table form?
175: does interpolating the MSS introduce new errors?
190: typo “known [to] classify significantly [fewer]”
191: do you think you need to define what snagging is or is that a well enough known term?
192: flow --> floe
206: no links within text- should just be in data accessibility section
215: awkward sentence, reword
216: “typically..” what do you mean, what percentage of the time, did you not use any CRYO2ICE tracks then?
223: again, where is this 6 m value from? reference, or mention that you calculated that.
249: be careful of the use of the word ‘underestimate’ (here and throughout this paragraph) I don’t think it’s an estimate of ICESat-2 you are producing and ICESat-2 isn’t the ‘truth’
264 Figure 3: note that this is height not thickness in fig caption or axis label.
279: figure 4, can you add total count of data points to the statistics displayed. also you need the y axis values for a)
283: less --> fewer
300: is there really more variability? the distance between the MAD lines seem almost steady. if there is in fact more variability, quantify.
305: 18th august or 16th august (as in Figure 5)
314: figure 5 caption- describe why 9th July and 16th august are marked
320: fig 6 caption. don’t the box plots also show the 50th percentile, add that: “2nd, 25th, 50th, …” Also what does the line show? best fit?
322: misspell CryoSat-2 also in line 329
323: what do you mean ‘overwhelming’ – use the word significant (if applicable)
340: I wouldn’t say “has an impact.” they are related, but for example it is not clear if the altimeter difference is affected by the photon rate or if the difference is affected by the surface type which also affects the photon rate.
342: I think you mean Figure 8, not 7.
343: “Here we observe a negative relationship between elevation difference and both photon rate and backscatter, inferring that ice floes with a higher coverage of surface water produce a lower height bias” – this is not what you see in figure 7 though- you see a positive relationship between backscatter and the difference.
353: misspell ICESat-2
361: regarding relationship between backscatter and photon rate and melt pond fraction. Why don’t you show it? could add another panel to figure 5 showing the melt pond fraction evolution in the central arctic.
366: typos in the Figure 8 caption, missing spaces, Sentinel-3
436: unit for the bias