the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Feb 2021
Effect of ephemeral snow cover on the active layer thermal regime and thickness on CALM-S JGM site, James Ross Island, eastern Antarctic Peninsula
Abstract. This study aims to assess the role of ephemeral snow cover on ground thermal regime and active layer thickness in two ground temperature measurement profiles on the Circumpolar Active Layer Monitoring Network – South (CALM-S) JGM site on James Ross Island, eastern Antarctic Peninsula during the high austral summer 2018. The snowstorm of 13–14 January created a snowpack of recorded depth of up to 38 cm. The snowpack remained on the study site for 12 days in total and covered 46 % of its area six days after the snowfall. It directly affected ground thermal regime in a study profile AWS-JGM while the AWS-CALM profile was snow-free. The thermal insulation effect of snow cover led to a decrease of mean summer ground temperatures on AWS-JGM by ca 0.5–0.7 °C. Summer thawing degree days at a depth of 5 cm decreased by ca 10 % and active layer was ca 5–10 cm thinner when compared to previous snow-free summer seasons. Surveying by ground penetrating radar revealed a general active layer thinning of up to 20 % in those parts of the CALM-S which were covered by snow of > 20 cm depth for at least six days.
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Interactive discussion
Status: closed
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RC1: 'Comment on tc-2021-5', Anonymous Referee #1, 03 Mar 2021
The manuscript “Effect of ephemeral snow cover on the active layer thermal regime and thickness on CALM-S JGM site, James Ross Island, eastern Antarctic Peninsula” reports on the effect of a large summer snowfall event on the active layer progression and thermal regime at a remote Antarctic location. The event is definitely interesting, but I am not convinced that there is any wider significance in documenting it in such detail. It seems to have a rather small impact on ground temperatures and active layer thickness, and it remains unclear under which conditions such events could play a role. I also have concerns on the evaluations and interpretation of some of the data that the authors should consider and address by providing additional data and analyses.
Major comments:
-Essentially, the impact on the ground thermal regime seems to be rather limited. To me, it looks like the main findings can largely be explained by the fact that ground surface temperatures are kept at 0°C for a prolonged period during the event. This is clearly visible in Fig. 4. So the effect is not really different from that of a long-lasting snow pack which reduces the thawing degree days and thus the active layer thickness. It is just that the timing is different. Since the snow cover lasts for only a small fraction of the thaw season, the changes in the end-of-the season thaw depth are relatively small and certainly smaller than the spatial variability due to ground conditions.
-I do not believe the statement “… the active layer refroze completely after a snowstorm” (l. 190). Fig. 6 shows that thaw depths go to zero almost as a straight line at the onset of the snow cover, but at the same time 5cm temperatures remain at 0°C for the entire time. This means that no energy related to the phase change water/ice can be lost through the surface, so the only possibility for the AL to refreeze would be by cooling from the colder permafrost below. However, this is not the case either, since the 50cm temperature also shows 0°C. The likely explanation is that the ground between 0 and 50cm is isothermal at 0°C, but no or only little new ground ice forms between 0 and 50cm. This also explains why the thaw depths increase sharply after the event and do not show a more gradual progression like in spring. Some limited refreezing down from the ground surface and up from the permafrost table is certainly possible, but the active layer must have remained largely unfrozen.
-In Fig. 7, only GPR evaluations are available for the areas below the snow cover. These suggest a large difference between snow-free and snow-covered areas. However, it is unclear how the GPR evaluation was performed for the snow-covered parts, and how a possible frozen surface layer and changes in soil moisture due to melting snow would affect the results. Was it possible to pick reflections from all these layers? Was the two-way travel time through the snow accounted for? For such application, a detailed uncertainty assessment of the GPR evaluations should be provided, also covering the effect of soil moisture and its spatial variability. The GPR velocity was calibrated with frost probe measurements, but these are only available for the areas outside the snow cover. Is it reasonable to assume that soil moisture is the same under the (possibly melting) snow cover?
Minor comments:
L. 20: It is estimated… These statements are too general, this will depend on the snow properties, e.g. the snow density and thermal conductivity.
L. 21: same here, this depends on the timing. During polar night conditions, any snow cover will insulate, while during the summer it can cool.
L. 27: the thaw depth is not necessarily affected by the snow cover, at least other factors are much more important, so that it is difficult to detect this relationship in the field. For a very thick snow cover that persists throughout most of the summer the active layer thickness is actually reduced compared to shallower snow covers.
L. 31: In many cases, this does not work as the frozen ground does not permit water flow.
L. 73: “The active layer thickness…” - please provide a few more details on this study, this is relevant in the light of the AL changes that are presented in this manuscript.
Fig. 1 has very poor resolution and overall quality.
L. 78: REF
L. 94: it is not really an AWS if it only measures ground temperature. Consider renaming to active layer temperature station or similar.
L. 130: does that mean that snow depth data are not available in 2016/17?
L. 132: how was the georeferencing done, did you use ground control points with known location?
L. 146: bracket too much.
L.147: Better use “Mean summer ground temperatures” for clarity.
L. 150: There is a strong offset between TDDa and TDD5. Are the reasons for this understood?
Fig. 4 has a very poor quality
L. 236: thawing
Fig. 6: Indicate that the first year is on the left side. Also, replace ALT by thaw depth, ALT in the most simplified definition corresponds to the annual maximum thaw depth.Citation: https://doi.org/10.5194/tc-2021-5-RC1 -
RC2: 'Comment on tc-2021-5', Anonymous Referee #2, 12 Mar 2021
The paper entitled Effect of ephemeral snow cover on the active layer thermal regime and thickness on CALM-S JGM site, James Ross Island, eastern Antarctic Peninsula presents and discusses ground temperature and weather records (snow depth only available for 1 year) over the summers 2016-2017 and 2017-2018, together with GPR measurements, mechanical probing and UAV survey on a permafrost monitoring site in the Antarctic Peninsula to infer summer snowfall effects for one of these two summers.
General comments
Overall, the paper is well written and easy to follow. However, I do not get the broad significance of the study. How does these results will impact other researches and current understanding of active layer and permafrost dynamics at a broader temporal and/or spatial scale ? Results point out very minor ground temperature and active layer thickness changes, and conclusions appear very speculative. One major concern here, is that the reported observations are so minors and questions about uncertainty in the measurements arise. Snow measurements are restricted to 1 summer only, and air temperature difference, despite presented as similar from year to another, are apparently not so similar (0.3°C difference reported for January in comparison to minor ALT change, this is meaningful, L 144). Furthermore, the paper lacks of integration of the international and recent literature. These limitations make the paper not suitable for publication in The Cryosphere.
Therefore, to lower the reviewing time, this review below is not complete. A few major concerns and suggestions for improvements are listed in order to guide the authors for manuscript improvement. I suggest to submit the study in another journal that rather targets researches with a regional impact. I also suggest to couple this study based on observations only with a modelling approach to see if the observed patterns are reproducible, and if yes, to extrapolate the results to give them a broader significance.
Specific comments
- Abstract :
The abstract is overall very poor and do not point out any peculiar finding that could help improving active layer thermal regime. The fact that summer snowfalls affect seasonal thawing dynamics is not new. Possibly, the study could have made some step forwards if these effects would have been more generally quantified (and not only for a few days in a specific summer) and that the significance of these results for long term permafrost dynamics in regards to climate change could be assessed.
The term « ephemeral » that is used all along the study must be defined.
The same is true for « AWS-JGM » and « AWS-CALM », it is not clear to what they precisely refer to.
« … a decrease of mean summer ground temperatures … ca 0.5-0.7 °C » : compared to what ? The day before ? the summer before ? but how consistent is it ?
- Introduction :
The introduction is based on very general sentences while the study aims at describing very specific patterns. More references should be considered. For example, to determined the threshold for snow insulation effect, the only considered study is Zhang, 2005, while, for example, and among many others (e.g. Luetschg et al., 2008) give other threshold. And of course, this threshold is depending on a variety of snow properties that are not describe here. This should be better explain all along the manuscript.
L 21-22 : the insulation effect of snow is not due to the albedo !!! Here again, a thorough understanding and accurate reference to the existing knowledge and literature is necessary.
Globally, references are only studies condcuted in Antarctica. Studies on the interactions of permafrost and snow in other regions worldwide must be considered to enlarge the impact of your study.
- Study area :
L73-74 : to which period does this ALT (50-120 cm) is valid for ? From which measurements it is known ? Given the variability described here (> 100%), the ALT changes reported in the abstract are ridiculously low !
Fig. 1 : Provide a map of the entire Antarctica. Reader in international journal such as TC do not all know where is the AP exactly.
L78 : reference is missing
Fig. 2 : add north . The AWS reported in the figure must introduced at least in the figure caption.
- Material :
L 93-94 : at which depth are the ground temperature sensors ?
L 98 : give correlation value
L 112-113 : this would be nice to report these measurement points on Fig. 2.
Overall, there is no consideration for uncertainty in your ALT and GPR measurements. One can wonder if the major results reported in the abstract, which are minor changes in ALT, are not in the range of uncertainty.
Fig. 3. This needs to be betetr explain. How are these values obtained ? Noyt every reader is familiar with GPR measurement and GPR data processing.
L 130-131 : when reading that the snow probe was not working before 2017-2018, I wonder how meaningful it is to present effect of snow fall by comparing temperature patterns to previous years while only 1 year of snow measurements are available.
- Results :
L 144 : is it mean montly temperature for january ?
L 168ff : explain how did you observe the snow storm and snow deposit evolution. Meteorological record ? Direct observation … ?
Fig. 6 : I do not understand from which data is built this figure. Probing was done 2 or 3 times during each summer. Is it interpolation within temperature sensor ?
Fig. 7 : it is for a specific day ? Average over the month ?
- Discussion
Section 5.1 is part of the results.
See general comments and comments on the introduction : this is broadly speculative and lack of references to the international literature.
Citation: https://doi.org/10.5194/tc-2021-5-RC2
Interactive discussion
Status: closed
-
RC1: 'Comment on tc-2021-5', Anonymous Referee #1, 03 Mar 2021
The manuscript “Effect of ephemeral snow cover on the active layer thermal regime and thickness on CALM-S JGM site, James Ross Island, eastern Antarctic Peninsula” reports on the effect of a large summer snowfall event on the active layer progression and thermal regime at a remote Antarctic location. The event is definitely interesting, but I am not convinced that there is any wider significance in documenting it in such detail. It seems to have a rather small impact on ground temperatures and active layer thickness, and it remains unclear under which conditions such events could play a role. I also have concerns on the evaluations and interpretation of some of the data that the authors should consider and address by providing additional data and analyses.
Major comments:
-Essentially, the impact on the ground thermal regime seems to be rather limited. To me, it looks like the main findings can largely be explained by the fact that ground surface temperatures are kept at 0°C for a prolonged period during the event. This is clearly visible in Fig. 4. So the effect is not really different from that of a long-lasting snow pack which reduces the thawing degree days and thus the active layer thickness. It is just that the timing is different. Since the snow cover lasts for only a small fraction of the thaw season, the changes in the end-of-the season thaw depth are relatively small and certainly smaller than the spatial variability due to ground conditions.
-I do not believe the statement “… the active layer refroze completely after a snowstorm” (l. 190). Fig. 6 shows that thaw depths go to zero almost as a straight line at the onset of the snow cover, but at the same time 5cm temperatures remain at 0°C for the entire time. This means that no energy related to the phase change water/ice can be lost through the surface, so the only possibility for the AL to refreeze would be by cooling from the colder permafrost below. However, this is not the case either, since the 50cm temperature also shows 0°C. The likely explanation is that the ground between 0 and 50cm is isothermal at 0°C, but no or only little new ground ice forms between 0 and 50cm. This also explains why the thaw depths increase sharply after the event and do not show a more gradual progression like in spring. Some limited refreezing down from the ground surface and up from the permafrost table is certainly possible, but the active layer must have remained largely unfrozen.
-In Fig. 7, only GPR evaluations are available for the areas below the snow cover. These suggest a large difference between snow-free and snow-covered areas. However, it is unclear how the GPR evaluation was performed for the snow-covered parts, and how a possible frozen surface layer and changes in soil moisture due to melting snow would affect the results. Was it possible to pick reflections from all these layers? Was the two-way travel time through the snow accounted for? For such application, a detailed uncertainty assessment of the GPR evaluations should be provided, also covering the effect of soil moisture and its spatial variability. The GPR velocity was calibrated with frost probe measurements, but these are only available for the areas outside the snow cover. Is it reasonable to assume that soil moisture is the same under the (possibly melting) snow cover?
Minor comments:
L. 20: It is estimated… These statements are too general, this will depend on the snow properties, e.g. the snow density and thermal conductivity.
L. 21: same here, this depends on the timing. During polar night conditions, any snow cover will insulate, while during the summer it can cool.
L. 27: the thaw depth is not necessarily affected by the snow cover, at least other factors are much more important, so that it is difficult to detect this relationship in the field. For a very thick snow cover that persists throughout most of the summer the active layer thickness is actually reduced compared to shallower snow covers.
L. 31: In many cases, this does not work as the frozen ground does not permit water flow.
L. 73: “The active layer thickness…” - please provide a few more details on this study, this is relevant in the light of the AL changes that are presented in this manuscript.
Fig. 1 has very poor resolution and overall quality.
L. 78: REF
L. 94: it is not really an AWS if it only measures ground temperature. Consider renaming to active layer temperature station or similar.
L. 130: does that mean that snow depth data are not available in 2016/17?
L. 132: how was the georeferencing done, did you use ground control points with known location?
L. 146: bracket too much.
L.147: Better use “Mean summer ground temperatures” for clarity.
L. 150: There is a strong offset between TDDa and TDD5. Are the reasons for this understood?
Fig. 4 has a very poor quality
L. 236: thawing
Fig. 6: Indicate that the first year is on the left side. Also, replace ALT by thaw depth, ALT in the most simplified definition corresponds to the annual maximum thaw depth.Citation: https://doi.org/10.5194/tc-2021-5-RC1 -
RC2: 'Comment on tc-2021-5', Anonymous Referee #2, 12 Mar 2021
The paper entitled Effect of ephemeral snow cover on the active layer thermal regime and thickness on CALM-S JGM site, James Ross Island, eastern Antarctic Peninsula presents and discusses ground temperature and weather records (snow depth only available for 1 year) over the summers 2016-2017 and 2017-2018, together with GPR measurements, mechanical probing and UAV survey on a permafrost monitoring site in the Antarctic Peninsula to infer summer snowfall effects for one of these two summers.
General comments
Overall, the paper is well written and easy to follow. However, I do not get the broad significance of the study. How does these results will impact other researches and current understanding of active layer and permafrost dynamics at a broader temporal and/or spatial scale ? Results point out very minor ground temperature and active layer thickness changes, and conclusions appear very speculative. One major concern here, is that the reported observations are so minors and questions about uncertainty in the measurements arise. Snow measurements are restricted to 1 summer only, and air temperature difference, despite presented as similar from year to another, are apparently not so similar (0.3°C difference reported for January in comparison to minor ALT change, this is meaningful, L 144). Furthermore, the paper lacks of integration of the international and recent literature. These limitations make the paper not suitable for publication in The Cryosphere.
Therefore, to lower the reviewing time, this review below is not complete. A few major concerns and suggestions for improvements are listed in order to guide the authors for manuscript improvement. I suggest to submit the study in another journal that rather targets researches with a regional impact. I also suggest to couple this study based on observations only with a modelling approach to see if the observed patterns are reproducible, and if yes, to extrapolate the results to give them a broader significance.
Specific comments
- Abstract :
The abstract is overall very poor and do not point out any peculiar finding that could help improving active layer thermal regime. The fact that summer snowfalls affect seasonal thawing dynamics is not new. Possibly, the study could have made some step forwards if these effects would have been more generally quantified (and not only for a few days in a specific summer) and that the significance of these results for long term permafrost dynamics in regards to climate change could be assessed.
The term « ephemeral » that is used all along the study must be defined.
The same is true for « AWS-JGM » and « AWS-CALM », it is not clear to what they precisely refer to.
« … a decrease of mean summer ground temperatures … ca 0.5-0.7 °C » : compared to what ? The day before ? the summer before ? but how consistent is it ?
- Introduction :
The introduction is based on very general sentences while the study aims at describing very specific patterns. More references should be considered. For example, to determined the threshold for snow insulation effect, the only considered study is Zhang, 2005, while, for example, and among many others (e.g. Luetschg et al., 2008) give other threshold. And of course, this threshold is depending on a variety of snow properties that are not describe here. This should be better explain all along the manuscript.
L 21-22 : the insulation effect of snow is not due to the albedo !!! Here again, a thorough understanding and accurate reference to the existing knowledge and literature is necessary.
Globally, references are only studies condcuted in Antarctica. Studies on the interactions of permafrost and snow in other regions worldwide must be considered to enlarge the impact of your study.
- Study area :
L73-74 : to which period does this ALT (50-120 cm) is valid for ? From which measurements it is known ? Given the variability described here (> 100%), the ALT changes reported in the abstract are ridiculously low !
Fig. 1 : Provide a map of the entire Antarctica. Reader in international journal such as TC do not all know where is the AP exactly.
L78 : reference is missing
Fig. 2 : add north . The AWS reported in the figure must introduced at least in the figure caption.
- Material :
L 93-94 : at which depth are the ground temperature sensors ?
L 98 : give correlation value
L 112-113 : this would be nice to report these measurement points on Fig. 2.
Overall, there is no consideration for uncertainty in your ALT and GPR measurements. One can wonder if the major results reported in the abstract, which are minor changes in ALT, are not in the range of uncertainty.
Fig. 3. This needs to be betetr explain. How are these values obtained ? Noyt every reader is familiar with GPR measurement and GPR data processing.
L 130-131 : when reading that the snow probe was not working before 2017-2018, I wonder how meaningful it is to present effect of snow fall by comparing temperature patterns to previous years while only 1 year of snow measurements are available.
- Results :
L 144 : is it mean montly temperature for january ?
L 168ff : explain how did you observe the snow storm and snow deposit evolution. Meteorological record ? Direct observation … ?
Fig. 6 : I do not understand from which data is built this figure. Probing was done 2 or 3 times during each summer. Is it interpolation within temperature sensor ?
Fig. 7 : it is for a specific day ? Average over the month ?
- Discussion
Section 5.1 is part of the results.
See general comments and comments on the introduction : this is broadly speculative and lack of references to the international literature.
Citation: https://doi.org/10.5194/tc-2021-5-RC2
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