the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Dec 2023
Investigating the spatiotemporal features of glacier elevation changes over the southeastern Tibetan Plateau using multisource satellite data
Abstract. Glaciers in the southeastern Tibetan Plateau (SETP) feature the largest maritime glaciers on the Tibetan Plateau (TP) and have experienced accelerated melting in recent decades. Investigating the spatiotemporal features of glacier elevation changes in the SETP remains a challenging task since this region is highly heterogeneous and high spatiotemporal resolution observations for region-wide glacier change measurements are still insufficient. To better understand the spatiotemporal variations in glacier elevation changes in the SETP, multisource satellite observations, including ASTER DEM, ICESat, ICESat-2 and CryoSat-2, are integrated in this study. We derive the spatially resolved glacier change for each year based on the 0.5° × 0.5° geographical tiles, and the obtained glacier elevation change rate of the entire SETP is −0.710 ± 0.046 m/yr during 2000–2022. We divided the study period into a recent decade and the previous decade and found that glacier thinning accelerated at a rate of 31.2 % in the recent decade. We evaluated the correlation between the elevation measurements of different satellites and found that the elevation measurement of ICESat-2 had a slight negative bias relative to the measurements of the other satellites. The ICESat-2 elevation measurements of the strong beam and weak beam were also compared, and no significant difference was observed. We also compared the CryoSat-2 swath measurements with the Level-2 (L2) measurements, and we found that the CyroSat-2 swath data agreed significantly more with the other satellite data than with the L2 measurements. A comprehensive comparison is carried out for the glacier elevation changes obtained in existing studies. Our estimates are highly consistent with those of new published studies and have a finer temporal scale and less estimation uncertainty.
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RC1: 'Comment on egusphere-2023-2389', Anonymous Referee #1, 12 Feb 2024
Review
Overall assessment:
The paper deals with an important topic which is the estimation of mass balance of mountain glaciers by a combination of different satellite-based methods. It is focused on the SE of the Tibetan Plateau which is a region with fast glacier down-wasting rates. This region was part of several studies focusing on the whole of the High Asia, Tibetan Plateau, or Himalaya-Karakoram region using typically just a single remote sensing method.
This laborious study brings some interesting results in terms of the trends of glaciers in the studied region and the differences among the used methods. However, there are several serious shortcomings in the presented study.
The study does not provide necessary details on the application of the widely used methods which are applied differently by different authors. The authors also failed to show the reasons behind the differences amongst the methods. The study does not refer to any ground measurements that could support the superiority of one of the approaches and validate the overall results on specific glaciers. There is a paper by Yao et al. (2012) that includes ground measurements of glacier mass balance on the Tibetan Plateau from which two glaciers should fall into the extent of the presented study. Additionally, newer measurements could be available meanwhile.
The quality of the study would largely improve if the authors would focus on the reasons behind the differences, that the particular methods produce and on the glaciers with positive mass balance. An analysis of these anomalous glaciers in terms of accumulation area altitude, orientation of the glaciers, and feeding mechanisms would be highly beneficial. Such analysis would provide enough material to allowing the discussion to get more interesting. The section Conclusions in this form is rather weak.
Reference
Yao, T., Thompson, L., Yang, W., Yu, W., Gao, Y., Guo, X., ... & Joswiak, D. (2012). Different glacier status with atmospheric circulations in Tibetan Plateau and surroundings. Nat Clim Change 2: 663–667.
Detailed comments:
19 ‘recent decade’ to ‘last decade compared to the previous one.
36 ‘downstream humans’ to ‘downstream population’
58 remove ‘the most’
68 Add ‘Optical and SAR’ before ‘satellite images’
69 ‘along the climate change’: Do the satellite images really contain direct information about climate change? Rephrase the sentence.
80 ‘large scale’ to ‘large’
82 ‘using interferometric synthetic aperture radar’: better to write ‘using SAR interferometry’
83 ‘matter of debate’: Better write that the penetration can reach meters and that it depends mainly on the presence of liquid water in the superimposed snowpack and ice.
87 Optical satellite stereo processing is also limited in areas of fresh snow which lacks features for feature matching.
87b (ICESat missions): There was only one ICESat mission. The instrument was however switched on for several time periods (campaigns) due to the lack of energy.
91-93 Specify whether you mean lidar or radar altimetry here.
99 Remove ‘Above all’ as it does not match here.
111-115 This you can list as objectives
116-120 You do not need this. Clear chapter headings and sub-headings are enough to get quickly a good overview of the study.
129 This means that the mean glacier area is about 1km. Is this true?
134: Better to provide information on the mean annual temperature and its changes throughout the studied area. Some figures on total precipitation and basic facts about its regional variation would also be useful.
How about the accumulation type of the glaciers? See Maussion et al. 2014 for details on this.
140 Does the SE Monsoon really reach the great bend of Yarlung Tsangpo? Can you support this with a citation?
143 better to call this section ‘Datasets used’
148 ‘fourteen different bands’: better provide information on the bands that acquire the stereo pairs.
148b ‘You probably mean ‘almost simultaneously’ by the ‘quasireal-time’. You can remove it as it’s not needed here.
151 The limit of 60% of cloud cover is rather high. Would you get too few stereo pairs if you selected a lower threshold such as 30%? Please provide details on this.
158 The campaigns are more important for ICESat than the revisit time.
164 ‘collected through’ to ‘obtained from’
166 You should better introduce the dataset in terms of satellite, period of operation etc.
176 ‘lack of data available’: Not clear which data are not available.
184 ‘acquired’ to ‘used’
188 ‘was acquired’ to ‘was used’
189 ‘stable region’ to ‘stable off-glacier region’
212 It is not clear which landcover class is regarded as a ‘stable region’ out of the listed classes.
221 ‘altimetry data-based’ sounds strange. Write better ‘elevation differences based on altimetry data
221b what do you mean by ‘location’? The footprint, measurement point, area? Specify.
223 What bias do you want to prevent? Noise, clouds, fog?
226 ‘elevation range’ instead of ‘elevation bin’
238-240 Rephrase, not clear.
244 The formula is not clear. Is the line above the difference an average? If yes of what? The variables are named in an awkward way, simplify it.
248 the formula should be definitely on a separate line and numbered.
252 What reason is behind the selection of this ice density value? (Other values are also in use.)
255 Specify, what bias you mean.
285 Why not use the time periods 2000-2011 and 2011-2022 to make them the same length? If not, use the yearly difference which is comparable in any case.
297 ‘lower’ instead of ‘reduced’
300 Say at which elevation the mass balance becomes positive. A graph of this dependence would be highly interesting. Is that valid in the whole study area? Can you compare this with the equilibrium line elevation (ELA)?
313 This is an unfortunate style of referring to the figures. Why not writing: ‘Base on our results (Fig. 3 (a) and (c) it appeared ...’
Figure 3: The explanation for ‘(c)’ is missing in the caption. Also a legend is needed. ‘a’ and ‘b’ are not clear. The caption suggests that there are 3 similar sub-figures. Two are shown instead.
325 The meaning of this chapter is not clear. What the individual arbitrarily selected glacier show about the study region? I guess not much. More meaningful would be to take anomalous glaciers and to analyse those looking at their feeding mechanism, debris cover proportion, orientation, the elevation of the accumulation area or minimal elevation.
I suggest to cancel this chapter and deal with the glaciers with positive mass balance and with a sincere analysis of the differences between the various approaches (lidar altimetry, radar altimetry, DEM differencing.
Figure 4: The subfigure (a) should be a separate figure as it does not have much in common with the other sub-figures.
The sub-figures show how limiting is to confine the analysis only to the glacier outlines from a global glacier inventory RGI. In the case of a glacier surge, you would not see it at all.
361 In which way do the Gaussian curves provide information on correlation? I think such a comparison can reveal a systematic error in the methods provided you have a robust reference.
Figure 5: This figure is messy. No letters at the sub-figures. Week descriptions in the caption.
What is meant by ‘satellite measurement’, is it satellite altimetry by ICESat, Cryosat or something else?
Provide detailed descriptions of all sub-figures referring to them by a, b and c.
385 The fact, that there is no difference between the strong and weak beams was mentioned in some previous studies. You should cite them here.
Figure 6: You should explain what you mean by homologous measurements.
Figure 6b: What is the difference between the two means of the distributions? This should be discussed.
Figure 7: This is a highly important figure. The sub-figure a should be larger and the lines should be easier to distinguish, for instance by a small offset if all start on the same date. If the figure is large enough you might add the info on the instrument to (a) and remove (b). Why is actually the y-axis inverse? Is there any strong reason to have the negative values above?
Reference
Maussion, F., Scherer, D., Mölg, T., Collier, E., Curio, J., & Finkelnburg, R. (2014). Precipitation seasonality and variability over the Tibetan Plateau as resolved by the High Asia Reanalysis. Journal of Climate, 27(5), 1910-1927.
Citation: https://doi.org/10.5194/egusphere-2023-2389-RC1 -
AC2: 'Reply on RC1', Xin Luo, 10 Apr 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2389/egusphere-2023-2389-AC2-supplement.pdf
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AC2: 'Reply on RC1', Xin Luo, 10 Apr 2024
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RC2: 'Comment on egusphere-2023-2389', Anonymous Referee #2, 01 Mar 2024
The authors investigated the spatiotemporal features of glacier elevation changes over the southeastern Tibetan Plateau using multisource satellite data, it is an interesting work. While the current work is not sufficient. The authors introduced that glacier in the SETP experienced accelerated melting in recent decades in the Introduction. However, based on multisource satellite date investigation and comparison with previous studies, they got the same result, and then the research is over. There have no novel findings, even though they did a lot of work.
In addition, there have many confusions in the Introduction. For example, in the second paragraph, the authors introduced that glaciers in the HMA are greatly retreating, and then list several research and numbers, there have no summarize. And then the authors said that glacier elevation changes in the SETP have not been well quantified by existing studies due to inconsistent results and give some examples. I know the authors want to emphasize the importance of glacier elevation change in the SETP and it should have drawn more attention. However, more research and numbers were given to emphasize the inconsistence of previous studies in the end of this paragraph. It is a confusion to me.
In the third paragraph, the authors summarized the approaches for glacier observation through spaceborne remote sensing. The first approach is satellite images, including glacier outlines delineation from Landsat, Sentinel, and glacier elevation determination from satellite-based optical stereo images. The second approach is geodetic digital elevation model differencing, including InSAR and optical stereo images. That is a repeat.
About the data source, the specific time of DEM acquired from ASTER images is not clear, and Fig 2(a) showed the glacier elevation changes from ICESat, ICESat2, CryoSat and ASTER DEM, the ICESat2 measurements (the purple dots) covered from 2002 to 2020, how it could be?
Therefore, the potential of this work is great, but considerable work is required before making it publishable.
Citation: https://doi.org/10.5194/egusphere-2023-2389-RC2 -
AC1: 'Reply on RC2', Xin Luo, 10 Apr 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2389/egusphere-2023-2389-AC1-supplement.pdf
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AC1: 'Reply on RC2', Xin Luo, 10 Apr 2024
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