the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Lake Ice Break-Up in Greenland: Timing and Spatio-Temporal Variability
Christoph Posch
Jakob Abermann
Tiago Manuel Ferreira da Silva
Abstract. Synthetic aperture radar (SAR) data from the Sentinel-1 (S1) mission with its high temporal and spatial resolution allows for an automated detection of lake ice break-up timings from surface backscatter differences across South (S), Southwest (SW) and Northwest (NW) Greenland (< 71° N latitude) during the period 2017 to 2021. Median break-up dates of the 563 studied lakes range between 8 June and 10 July, being earliest in 2019 and latest in 2018. There is a strong correlation between break-up date and elevation, while no relationship with latitude and lake area could be observed. Lake-specific median break-up timings for 2017–2021 increase (i.e., are later) by 3 days per 100 m elevation gain. When assuming an earlier break- up timing of 8 days which corresponds to the observed median variability of ± 8 days, the introduced excess energy due to a changing surface albedo from ice to water translates to melting 0.5 m thick ice at the melting point or heating up a water depth down to 35 m by 1 K across the entire surface area of each respective lake. Upscaling the results to 100486 lakes across the regions S, SW and NW which correspond to 64.5 % of all lakes or 62.1 % of the overall lake area in Greenland yields an estimate of 1.8 * 106 TJ additional energy input. This translates to melting 5.8 Gt ice at the melting point or warming 432.3 Gt water by 1 K.
- Preprint
(4200 KB) - Metadata XML
- BibTeX
- EndNote
Christoph Posch et al.
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2023-1762', Anonymous Referee #1, 02 Sep 2023
This study uses Sentinel-1 (S1) combined with surface backscatter differences to obtain disintegration dates for 563 lake ice in Greenland between 2017-2021, obtaining a relationship between disintegration date and altitude. Estimates of the additional radiative input due to the advancement of the disintegration dates are presented, and the possible consequences of the excess energy are discussed. The results reveal the uniqueness of Greenland's geographic location and topographic features in response to climatic conditions, and enhance understanding of the extent to which altitude, latitude and lake size correlate with disintegration dates.
However, more detailed descriptions should be added in some areas, e.g. the introduction clarifies the importance of lake ice phenology studies but lacks a description of climate studies or ice condition studies in Greenland. In addition, why did the authors only analyse the date of disintegration and ignore the date of freezing, were they limited by the detection method? It would be more convincing to quantify the additional heat input in terms of spatial and temporal variations in the length of the complete freeze-up. For the lack of credibility of the authors' assumption of a pre-melt lake ice albedo αi = 0.9, which is perhaps only achievable with fresh snow; the effect of cloudiness on incident solar radiation should also be considered.
The research work is interesting and covers a large number of lakes in Greenland, and the authors are advised to improve the manuscript in two ways. Ⅰ, emphasise the climatic characteristics of Greenland and the progress of ice condition research, suddenly the significance of this study; Ⅱ, determine empirical values of regional lake ice albedo and cloudiness to ensure that the estimation of additional heat input is accurate. In addition, it would have been better to include freezing dates in the analyses, or to explain the reasons for not considering freezing date identification. Below are some specific comments aimed to provide guidance on the revision.
L9-10: “There is a strong correlation between break-up date and elevation, while no relationship with latitude and lake area could be observed” 'no relationship' is too absolute, suggest 'weakly relationship'.
L18: “Introduction” the authors describe the relationship between lake ice and climate and the lack of break-up time studies in Greenland, I would also like to know more about Greenland's climate and ice conditions, e.g. what is the rate of warming? Is the lake ice surface dominated by bare ice or snow cover? What are the special features compared to other regions?
L21: “The duration of lake ice controls......” 'Duration of lake ice' might make more sense than 'break-up time'.
L34-35: “The seasonal changes in solar radiation, however, are the main influence for the overall energy availability to form and decay lake ice cover” It is for this reason that solar radiation may have a greater effect on lake ice melt than air temperature. Does it make sense to study the use of "Calculating Cumulative Positive Degree Days"?
L43: “Wang et al., 2018” missing information in References
L63-65: “While still being dependent on field measurements for validation, lake ice studies from remote sensing are no longer dependent solely on a small number of ground observations but can produce results and extrapolate measurements across landscapes and regions” I didn't understand the sentence.
L91-92: “…below 2 days for most of Greenland” This is a great idea, has anyone else also achieved 2 days accuracy with the help of this approach?
L117-118: “…we acquired incoming shortwave radiation and air temperature at 2 m data as climatological daily mean values for the period 1991-2020 from RACMO2” The study used "daily mean values". The credibility of Cumulative Positive Degree Days can be enhanced if there are measured daily mean values between 2017-2022.
L132-133: “…when most of the lake surface is…” "Most" is not a quantitative description; 80% or 90% would be a better standard.
L150: “Figure 1 (a)” could the sudden rise in November 𝜎0 also determine the freezing of the lake?
L150: “Figure 1 (c)” What does the pentagram represent?
L161-162: “The progressing melt on the lake surface leading to a rougher, wetter surface explains the 𝜎0 recovery before the major backscatter decline in summer indicating lake ice break-up” rougher and wetter ice is unlikely to result in αi = 0.9.
L164-165: “This is due to the nature of break-up processes being more complex due to melting on top and bottom or varying acquisition conditions” could it be a secondary freeze due to lower air temperatures?
L179: “Calculating Cumulative Positive Degree Days” whether climate averages for the period 1991-2020 are representative of today's ice-season environmental conditions?
L214: “…of lake ice αi (0.9) …” the value of 0.9 is too high, please revise or give a basis.
L223-224: Does the authors take into account the effect of cloud cover, which attenuates incident shortwave radiation?
L280: “Figure 4 (a)” I want to know r = 0.76, then p-value = ?
L284: “…we there is no relationship between break-up timing and lake size” it is necessary to control for equal elevation and latitude, and to analyse only the size of the lake in order to consider that there is no relationship between disintegration and lake size
L290-291: “Figure 5b shows that lakes with similar cumulative PDDs experience a later lake ice break-up at higher elevation” are the air temperature data corresponding to lakes at different elevations at the same latitude the same? Because altitude must bring about differences in air temperature, defaulting to the same value will lead to bias.
L332: “…down to 35 m by 1 K across…” 35 m is the average depth of regional lakes?
L349: “(between 40°N and 82.5°N)” perhaps because Greenland's latitudinal span is not large enough to become only weakly correlated.
Citation: https://doi.org/10.5194/egusphere-2023-1762-RC1 -
RC2: 'Comment on egusphere-2023-1762', Anonymous Referee #2, 25 Sep 2023
General comment:
I enjoyed reading the paper. The text is clear. The quality of the plots are very good. I think it is a nice paper and contributes to the field.I have one general (major) comment:
I think the discussion is somewhat lacking in putting the work in the broader context. It is not well motivated why this study matters from a global perspective and what it adds to the general understanding of the climate change impacts. More specifically:- US is a wide continent covering various Köppen-Geiger climatic zone but Greenland is specifically located in one climatic zone and is not spanning across multiple latitudes with different climatic features. Therefore, I am not too surprised that for US there was a strong spatial correlation while such correlation could not be observed in Greenland. Please consider adding more to this discussion and make the comparison a bit stronger.
- Put the whole study in the broader context: Maybe mention implication for hydropower. You can use snow-dominated locations for the sake of comparison. I tried to find some examples and the discussion section in paper https://www.sciencedirect.com/science/article/pii/S0022169423007497 and https://hess.copernicus.org/articles/24/3815/2020/ might work. Please try to find other papers to add to this point.
- Compare the finding with other boreal countries: Note https://tc.copernicus.org/articles/16/2493/2022/tc-16-2493-2022-discussion.html where they studied ice break-up patterns in Sweden. Please try to find other papers to add to this point.
- Mention what is expected to happen to Greenland under climate change. You can use latest IPCC report for this.
Specific comments:
- L280: please remove we.
- Please report the significance of the correlations whenever you report the correlation strength.
- The paper lacks climatic description of Greenland.
Citation: https://doi.org/10.5194/egusphere-2023-1762-RC2
Christoph Posch et al.
Christoph Posch et al.
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 184 | 69 | 10 | 263 | 4 | 4 |
- HTML: 184
- PDF: 69
- XML: 10
- Total: 263
- BibTeX: 4
- EndNote: 4
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1