the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Nov 2023
The Radiative and Geometric Properties of Melting First-Year Sea Ice
Nathan J. M. Laxague
Christopher J. Zappa
Andrew Richard Mahoney
John Goodwin
Cyrus Harris
Robert E. Schaeffer
Roswell Schaeffer Sr.
Sarah Betcher
Donna D. W. Hauser
Carson R. Witte
Jessica M. Lindsay
Ajit Subramaniam
Kate Elyse Turner
Alex Whiting
Abstract. In polar regions, sea ice is a crucial mediator of the interaction between earth's atmosphere and oceans. Its formation and breakup is intimately connected with local weather patterns and larger-scale climatic processes. During the spring melt and breakup period, snow-covered ice transitions to open water in a matter of weeks. This has a profound impact on the use of sea ice in coastal Arctic regions by Indigenous People, where activities such as hunting and fishing are central to community livelihood. In order to investigate the physical phenomena at the heart of this process, a set of targeted, intensive observations were made over Spring sea ice melt and breakup in Kotzebue Sound, Alaska. This program is part of the Ikaaġvik Sikukun project, a collaborative effort in which an Indigenous Elder advisory council from Kotzebue and scientists participated in co-production of hypotheses and observational research, including a stronger understanding of the physical properties of sea ice during spring melt. Data were collected using high-endurance, fixed-wing uncrewed aerial vehicles (UAVs) containing custom-built scientific payloads. Here we present the results of these measurements. Repeated flights over the measurement period captured the early stages of the transition from a white, snow-covered state to a broken up, bare/blue-green state. We found that the reflectance of sea ice features depend strongly on their size. Snow patches get darker as they get smaller, an effect owed to the geometric relationship between bright interior and the darker, melting feature edges. Conversely, bare patches get darker as they get larger. For the largest ice features observed, bare blue-green ice patches were found to be ~20 % less reflective than average, while large snowy/white ice patches were found to be ~20 % more reflective than average.
Nathan J. M. Laxague et al.
Status: open (until 14 Dec 2023)
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RC1: 'Comment on egusphere-2023-2541', Anonymous Referee #1, 27 Nov 2023
reply
This manuscript presented a study on the radiative and geometric properties of sea ice observed by a fixed-wing UAV, which is interesting to Arctic sea ice society. The geometric properties part (Figures 9, 10) was something common study, which didn’t present novel information. The second part, the dependence of reflectance on ice size is indeed novel and could be useful as thinking about the impact of incorporating ice size into new sea ice models. However, I am confused by the motivation of this work. The problem that this manuscript seeks to solve needs to be clarified.
Major concerns:
Figures 11, 12, 13: It is reasonable that ice reflectance increases with the floe size, because of the lateral loss of light out the ice boundary. Furthermore, when the size is big enough (e.g. > 15 m), the reflectance is nearly identical. This result agrees with other simulated results. However, I don’t understand why the reflectance of ice with size was 20 and > 20 became smaller.
It seems that the manuscript didn’t present the observing time of UAV flights. The figure shows ice reflectance but not albedo. So, there is a question, whether the effects of solar zenith angle on ice bidirectional reflectance been considered?
I also didn’t see the view angle of the radiance sensor. At a flight altitude of 1000 m, its field of view may be over 40 m (assume the view angle is 5°). So, it is doubly that if the observed reflectance is from bare ice or ice and seawater. If it is the latter one, the word ‘size’ used here is misleading. The changing radiance is mostly due to the changing ice concentration but not size. Then, the novelty of this work disappeared.
Other concerns:
The abstract need re-write, and there are lots of introduction but less results.
There are too many abbreviations in the manuscript that weren’t defined in the main text. So, it is difficult to understand the figures.
L36-39: The Arctic amplification can’t be regarded as a total result of the changing ice situation.
Figure 3: It seems this figure didn’t show any useful information.
Section 3.1 (Figure 8): I don’t understand what this figure seeks to present. The difference in ΔEs was mostly controlled by the reflected radiance. It is a matter of course that mean or spectral reflectance decreases with increasing ΔEs.
Citation: https://doi.org/10.5194/egusphere-2023-2541-RC1 -
RC2: 'Comment on egusphere-2023-2541', Anonymous Referee #2, 02 Dec 2023
reply
Review of “The Radiative and Geometric Properties of Melting First-Year Sea Ice” This manuscript presents observational data from a field campaign deploying UAV with payloads designed to characterize the radiative properties of the sea ice surface. Data were collected over landfast Arctic sea ice during the melt season as the ice transitioned from snow covered to bare and melting. During this time the floe size distribution was rapidly evolving from larger floes to smaller floes.
The text is written very clearly and concisely. The figures are clear and informative. There is a lot of information here and the results are of interest to the sea ice radiative transfer modeling community.
I find the conclusions intriguing, but would be informative to have more explanation of the effects that were documented. For example, I can think of a variety of explanations for the results showing sensitivity of reflectivity to feature size:
- Smaller features are more influenced by their perimeter, perimeters are subject to some strong effects that can alter their reflectivity, such as lateral melting, wave wash.
- Light leakage: do photons propagate laterally out of floe boundary before they can be backscattered? Likewise, do photons enter the ice from the side and increase the albedo?
It would be helpful to the reader to consider these mechanisms (and there may be more?) and either substantiate them or refute them. I think it might not be that the larger floes are darker, but rather that the smaller floes get light leaked in from the sides?
I do agree that the reflectivity is reduced when light propagation out the sides becomes significant, so, yes less light backscattered to atmosphere, but does that mean that these snowy features are absorbing more solar radiation? The surface is absorbing more radiation, but I wouldn’t say that the ice is.
Am I interpreting correctly that 18 m is the size scale that roughly divides whether a floe is on the “large” or “small” size? From a radiative transport perspective, that “boundary” sounds very large to me.
I am a bit surprised that dust and sediment were mentioned on a couple of occasions, but not explicitly considered in the explanation. Further, not much was said about the green appearance of the ice cover. Is this a result of biologic activity? Or dust/sediment?
There are a lot of interesting results and intriguing discussions in this manuscript. I feel that the overall story of how the pieces fit together could be tightened up.
Minor comments:
Title: Add words “Arctic” and “landfast”
Line 3: matter of weeks? Sure, some areas undergo the transition from snow covered sea ice to open water in a matter of weeks, but that process takes a lot longer in other regions, depends on location and ice type.
14-15: “than average” than average of what? All features?
40: “Arctic system” less resilient to change? Of should this say “Arctic ice cover” is less resilient?
Fig 1: this figure shows the color of the ice, but why choose the same color for the mask?
160: estimation, retrieval of feature length scale hasn’t been sufficiently motivated, why should it be measured? Section 2.2 is ‘how’, but there is no ‘why’
Fig 11: “thickness of each trace corresponds to the mean effective feature diameter…” not clear how “thickness” is being used here. all look same thickness (width?) to me. Or does ‘thickness’ refer to “D” and hence the gray/yellow shade?
329-331: “…to strengthen positive feedbacks associated with radiation uptake. In short: the tendency of large blue-green features to absorb radiation increases with their size, while large snowy features absorb more solar radiation as they are subdivided and split by melt and degradation.” I don’t understand the mechanism whereby an increase in absorption with increasing size is a positive albedo feedback. That would suggest that as the floe size decreased (due to melt, increased absorption), the absorption would also decrease (which sounds like a negative feedback)
Citation: https://doi.org/10.5194/egusphere-2023-2541-RC2
Nathan J. M. Laxague et al.
Data sets
Codes and Data for "The Radiative and Geometric Properties of Melting First-Year Sea Ice", pre-submission Nathan J. M. Laxague and Christopher J. Zappa https://doi.org/10.7916/rrbv-k026
Nathan J. M. Laxague et al.
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