the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Aug 2024
Variations in the microstructure of saline ice during its growth and decay: Evidences from an experimental study
Abstract. Ice physics are highly sensitive to the ice temperature and are primarily determined by the distribution of inclusions such as gas bubbles and brine pockets within ice. However, a detailed understanding of their distributions and evolution patterns during ice freezing and melting is lacking. To address this issue, in situ experiments were conducted to collect detailed information on the variations in the microstructure of ice using continuous sampling and a high-resolution imaging system. The results revealed a 5- to 10-fold increase in the volume fraction and a 2-fold increase in the size of gas bubbles during the melting phase of ice. Moreover, the size of brine pockets in the ice surface, middle, and bottom layers clearly increased for different reasons. The nearly 30 % increase in gas bubbles observed in the middle layer was thermally driven, while the increase in the surface layer was influenced by the net shortwave radiation. Additionally, the variation in the inclusion size distribution was attributed to the merging process, most of which occurred among smaller inclusions rather than among larger inclusions. The changing ice temperature is a significant factor in the merging process of the middle layer but not for the surface or bottom layers. This study could enhance the understanding of the effect of the transfer of energy between the atmosphere and water beneath ice on the ice microstructure.
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RC1: 'Comment on egusphere-2024-2155', Anonymous Referee #1, 23 Sep 2024
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This manuscript describes a study that was carried out to assess the variability of brine and gas inclusions within saline ice.
The methods and results of the study are described thoroughly.
The study sampled an ample number of brine, gas inclusions to get satisfying statistics on the population of individual inclusions. There was not however ample sampling carried out to achieve significant statistics on the spatial variability of the microstructure itself, specifically this study provides no information on the cm – m - decimeter scale variability in an ice cover.
The text is not very concise, but rather verbose, with low information content. Particularly in Section 4.
There really are no substantial conclusions. There may be a few new insights, but I am not seeing any well composed conclusions or key points that describe new insights into microstructural changes occurring during ice growth and/or decay.
Minor:
21: “ice” is frozen water, it becomes a multiphase material consisting of ice crystals, gas bubbles, and impurities when discussed in the context of (in this case) the frozen surface of a water body
27: “subglacial”, use of this word implies the authors are addressing ice as found in a glacier?
30-31: “The content of newly formed brine pockets in ice can be quantified by an effective segregation coefficient and the salinity of the underlying water.” As I read this statement, it implies there is a relationship between the content of newly formed brine pockets (i.e., the brine salinity) and the salinity of the ocean. In fact, the relationship cited by Cox and Weeks (1988) defines a relationship between the salinity of newly formed sea ice and the salinity of the ocean. Those are not the same things.
35-36: “For example, voids form due to the partial evaporation of ice or brine during internal melting.” Are the authors describing the formation of Tyndall figures in this statement? It’s not clear what this is referring to. I believe the bubbles referred to as active bubbles are gas bubbles that form within brine inclusions when the ice temperature is increased.
38: It would be more accurate to say “freezing equilibrium relationships in the water-salt system” than “ice phase diagrams”. An ice phase diagram would refer to the different phases of ice as a function of temperature and pressure.
41: Define Vb
57-58: “This inevitability introduces inaccuracies into ice models in response to varying circumstances.” This overstates the problem, as I am aware of no models that even attempt to treat the details of sea ice microstructure at a level where seasonal variations in microstructure would even be considered.
64: If this study was carried out for a water body with salinity 5 – 7 ppt, it would be important to describe how this differs from sea ice grown from water with salinity generally 30 – 32 ppt. This is especially in light of the critical threshold of 24.7 ppt, below which the temperature of maximum density exceeds the freezing temperature. The formation of sea ice in the open ocean generally occurs under conditions where the temperature of maximum density is equivalent to the freezing temperature.
364: “However, estimating the variation in Va remains challenging due to a lack of information on its influencing factors.” I am not clear what this means, or why it is written.
Figure 10 (and associated text): Is the point of this figure to show that inclusion (presumably brine inclusion?) sizes did not change in response to changes in temperature? If so, then I assert that the inclusions that were measured where not isolated and in freezing equilibrium, or it is possible that only the long dimension of inclusions was measured, and that dimension showed little sensitivity. The total volume of isolated (not connected, not drained, surrounded by ice) brine inclusions should show obvious size change as a result of change in ambient temperature (between -10, -15, 20 C), even in the lab. This fact appears to not be consistent with the statement made in lines 445-446.
473: A power law could also describe the opposite, with numbers decreasing with decreasing size.
473-474: It’s not clear what is meant by “…the minimum number of bubbles was small”
476: infinite? No, there can be (lower and upper) limits on the size.
482: This ice is not warmer than studies from melting ice.
486-487: “Therefore, it could be concluded that the methods used for classifying small inclusions in different works do not considerably affect the overall statistical results.” It could also simply be that the cm / m / decimeter scale has significantly larger variability than what was observed here.
495: “…significant increase in Va of the surface layer during the melting phase.” Is this a reference to the well documented process of brine drainage? See Niedrauer and Martin 1979 https://doi.org/10.1029/JC084iC03p01176
506-507: “These bubbles partly accumulate in the middle layer as a result of their buoyancy, while others escape from the ice bottom.” Please supply a reference for these processes.
559 – 560: “…newly formed bubbles in the middle layer were partly thermally driven.” I am not clear what this means. Please rewrite for clarity.
561 – 562: I think this is describing a well-established brine drainage process.
564-565: “The changes in the brine pocket size during the freezing phase were not as notable as those during the
melting phase.” I suspect this is due to merging and draining processes, and those processes are well documented.
570: what “other mechanisms”?
Citation: https://doi.org/10.5194/egusphere-2024-2155-RC1
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