Classification of Sea Ice Types in the Arctic by Radar Echoes from SARAL/AltiKa
Round 1
Reviewer 1 Report
Overall, an excellent paper presenting a comparison of Ka-band vs Ku-band derived radar backscatter and their ability to classifiy ice type. While the overall manuscript, methods, and presentation are great, my only recommendation to these authors (that would greatly strengthen the story) would be to include some direct in situ ground-based Ku and Ka-band scatterometer data in your comparisons.
Author Response
We are very grateful to the reviewer for reading and reviewing our paper and very thankful for the positive feedback!
We agree that including ground-based scatterometer data – the KuKa data from MOSAIC in particular – will provide a lot of information on the differences between Ku and Ka waveforms. Alas, we think this would be better written up as a study separate from the one at hand. Our main goal here is to see how the methodology existing for Ku band satellite sensors work when applied on a Ka-band sensor on a hemispheric scale. KuKa data would allow approaching the problem from a small scale and theoretical direction, in contrast to the rather experimental approach in this paper. Even if this is outside the scope of this paper, it is something we plan to do in another paper well before the launch of CRISTAL.
Reviewer 2 Report
This paper introduces Altika as a tool to classify sea ice types (MYI and FYI) benefiting from Ka-band microwave characteristics. The authors investigated four classifiers and presented their accuracy in sea ice classification.
Overall, I found this paper with clear objective, approaches, results and discussion, forming a valuable research paper. I have only quick revisions/suggestions for the authors:
1. In method, when the authors described the sources of difference between MYI and FYI signatures on altimetric data, their focus is only on the surface characteristic, i.e., surface roughness, while in radar literature, other factors are taken into account as well, including attenuation factor. This factor is controlled by some other sea ice characteristics, such as salinity, and may affect some waveform parameters used in this paper for sea ice classification, e.g., PP and backscatter. Although in Ka-band case, the surface scattering dominates, and brine particles in ice may have very little impacts on reflections, saline snow and saline gray ice may affect instead. More details may be found in a recent paper by Stroeve et al (2020) in https://doi.org/10.5194/tc-14-4405-2020. Authors are suggested to include this type of impacts as well.
2. In discussion (section 5.2), Authors mentioned that the volume scattering at Ka-band is only from a very thin subsurface snow layer, and this explains differences between Ku- and Ka-band. Although this statement is valid, the difference in “radar penetration factor” for Ku- and Ka-band over MYI and FYI can be suggested as a more general explanation. As suggested by Armitage and Ridout (2015) in https://doi.org/10.1002/2015GL064823, Ka-band shows more uncertainties in case of radar penetration factor over MYI and FYI, while the radar penetration factor of Ku-band works better in distinguishing MYI and FYI. This factor my play a key role in sea ice classification.
3. The link provided in line 134 doesn't work, at least for me!
4. The paper appears in a clear and accurate English writing; however, a minor writing checking is required. For the authors' convenience, I've listed some of them:
Line 24, “the sea ice cover” is redundant - Line 94, the word “furthermore” is better to replace with another similar word because it’s already used in this paragraph - Line 115, a comma is needed before the word “we” - Line 156, comma is not required - Line 198, “identify as” is redundant - Line 208, a comma is required after “echoes”, and the coma after “PP” should be removed - Line 229, as it “makes” - Line 562, Ka-band “are” encouraged; and the comma after “encouraged” is not required.
Author Response
We thank the reviewer for the positive feedback, for the suggestions and for taking the time to review our paper. Please find our response below.
- We appreciate the reviewer's comments on this, and will gladly include these aspects. We have added the following in the methodology section:
“While surface roughness is one of the factors that change the shape of the waveform, other factors also affect the radar signals behaviour in the snowpack. This in turn may be seen in the resulting waveform. A recent study by Stroeve et al., 2020 based on a ground-based radar system, KuKa deployed during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAIC), showed that factors such as snow grain size and neighbouring grains, brine particles, the salinity of snow and the general scattering within the snowpack should be taken into account when interpreting waveforms. However, for satellite-based systems, an assumption of a non-coherent sum of all scatterers is still usually accepted. The shape of the waveform is not dependent on surface roughness only, and scattering in snowpacks also plays a role. We note that this is still an ongoing research topic, and it is still unclear exactly how these scatterers within the local snowpack propagate through the resulting radar signal from a larger surface (larger footprint) recorded by the satellite compared to a local surface observed by a ground-based system. ”
References:
Stroeve, J., Nandan, V., Willatt, R., Tonboe, R., Hendricks, S., Ricker, R., Mead, J., Mallett, R., Huntemann, M., Itkin, P., Schneebeli, M., Krampe, D., Spreen, G., Wilkinson, J., Matero, I., Hoppmann, M., and Tsamados, M.: Surface-based Ku- and Ka-band polarimetric radar for sea ice studies, The Cryosphere, 14, 4405–4426, https://doi.org/10.5194/tc-14-4405-2020, 2020.
- Indeed, this is an important factor. We have added the following paragraph to include this aspect as well.
“Another explanation from this difference could be the difference in radar penetration factor for Ku- and Ka-band over MYI and FYI, as suggested by Armitage and Ridout (2015). The radar penetration factor describes the radar dominant scattering horizon in relation to the snow and ice surfaces, such that a value of zero indicates the air-snow interface is the dominant scattering horizon, and a value of one will indicate that the ice-snow interface is more dominant. Based on their results, the Ku-band shows a stronger dominance of the ice-snow interface in their results, but also a higher variance in radar penetration factor depending on MYI and FYI. In comparison, Ku-band shows a radar penetration factor around 0.5 for both sea ice types, suggesting a somewhat equal measure of dominance from both interfaces, but almost no difference in radar penetration factor depending on the type (FYI vs MYI). This factor may also play a role in the classification of sea ice types since the penetration factor seems to be more ice-type-dependent in Ku-band than Ka-band.“
Armitage, T. W. K., and Ridout, A. L. (2015), Arctic sea ice freeboard from AltiKa and comparison with CryoSat-2 and Operation IceBridge, Geophys. Res. Lett., 42, 6724– 6731, doi:10.1002/2015GL064823.
3. Thank you! This is indeed true because OSI SAF recently changed their website and made their former link invalid. It was valid when we accessed it back in 2019! We have changed the link to “www.osi-saf.org”, which will redirect the user to the new website. This is also a website listed to be used for the citation of the sea ice type product: (http://osisaf.met.no/p/ice/edge_type_long_description.html).
4. We thank the reviewer for the comments, and the suggestions – as English is not the first language of any of the authors. We shall include all of the above, and shall do an additional copy-editing before submitting the manuscript with the minor revisions.
