review of paper egusphere-2024-651, “Retrieval of airborne Ku-Band SAR Using Forward Radiative Transfer Modeling to Estimate Snow Water Equivalent: The Trail Valley Creek 2018/19 Snow Experiment”

Montpetit et al.

General

The study presents an experiment simulating radar backscattering intensity of snow-covered ground in a tundra environment using a coupled ground-snow radiative transfer model. Model simulations, driven by a combination of pre-retrieved and measured parameters, are compared to backscattering at Ku-band measured by an airborne radar instrument. Ground properties defining surface backscattering are retrieved from satellite-based measurements at lower frequencies, using these to guide the Ku-band retrievals. The forward simulation setup is used to retrieve optimized values for snow polydispersity in a two-layer simulation setting.

The text is generally well written and clear, and the subject is of high interest to the community since it provides new insight into the recently proposed concept of microwave grain size and how this is related to radar backscatter observed in remote sensing. However, I feel the section describing the forward model setup is currently insufficient to fully comprehend the experiment, with some details explained only in the discussion. The origin and use of structural polydispersity, a key aspect of the paper, is not fully explained, nor how this is used together with measured optical grain size to calculate the scattering coefficient in radiative transfer. Furthermore, some measurements such as those on soil permittivity, are meticulously presented but finally not used anywhere in the paper. Likewise, it is unclear how snow pit measurements were finally used in the simulations. These aspects should be improved before considering the paper ready for publication. Please see detailed comments below.

Figures are clear and present the results in a useful way, although figure captions are on occasion too concise to fully describe the figure contents. This could be improved.

Major comments

1. Section 3.2 is the main problem of the paper, and the only reason why I suggest a major revision. Many key details are missing, and the text in the section does not fully describe the model setting. E.g., which of the multitude of SMRT scattering models was used? How many layers were used in the simulation? How were other parameters than the retrieved ones defined for these layers (e.g. snow depth and density)? One has to read between the lines or reach out all the way to the end of the discussion to get some answers. For example, line 208 refers to “the multi-layer analysis”, but it is not clear how many layers there are (two, three, or more?). This only becomes clear (perhaps) only later in the manuscript, that two layers were used. As a further example, the last sentence states that “The different effective parameters were thus constrained by values found in the literature”, without giving the values. Please modify the entire section and explain fully 1) the model setup, including which scattering model was used in SMRT 2) state carefully all model parameters and from what source these were derived from (measured, retrieved, literature etc). Maybe a Table could help? E.g. at present it is not fully traceable how all the measurements described in Section 2.3 were finally used in simulations, validation or both.
2. The section could also benefit from a (brief) introduction of the SMRT and the GO models. Just a sentence or two placing the models in context for a potential reader who has no idea what these models actually do, could be sufficient. e.g. “SMRT (Picard et al., 2018) simulates propagation of microwaves in snow, generating estimates of microwave emission and backscatter from a stacked system of snow layers, with each layer described by…” and so on.
3. A similar short intro should be added on snow microstructure, in particular the relatively new concept of microwave grain size introduced by Picard et al., how this is obtained from field measurements, and where Polydispersity comes in. Maybe this warrants a separate subsection in Methods?

Minor & editorial comments

1. The title is a bit awkward to me. “Retrieval of airborne Ku-band SAR…” does not really say anything (retrieval of what? radar backscatter?). Please consider changing the title to e.g. “Retrieval of snow and ground properties from airborne Ku-band SAR…”, or even “Retrieval of snow polydispersity and ground effective permittivity from Ku-band SAR…” since those are the parameters you finally retrieve.
2. Abstract line 4. Not sure about “quality snow information”. I would change this to something less ambiguous, e.g. simply “snow information”
3. Abstract line 6. “It becomes possible to properly characterize” is also quite a strong statement. What is properly? I’d suggest something like “(SMP and SMRT)… are promising tools for characterizing the snow cover … etc”
4. Abstract line 15. Polydispersity is a very new concept in snow microwave modeling and you should give some short context for the term. e.g. “The polydispersity of the snow microstructure, which together with snow autocorrelation length has been proposed as a basis for scattering calculations of microwaves propagating in snow,…” (please use some wording better fitting your text). See also later comment on adding a paragraph, or even a separate section, on the concept of microwave grain size.
5. Abstract, last sentence. Since you have not demonstrated actual SWE retrieval in this study, I suggest to modify this a bit: “… using a priori knowledge of the snow conditions to simulate backscattering, facilitating also the possible retrieval of SWE from these measurements” (or similar).
6. Introduction line 38-39: “decomposing…” I’d rather say the overall problem in SWE retrieval is separating the influence of both variable microstructure and ground backscatter on total backscatter (as opposed to separating these from one another, maybe just change “from” to “and”?). Also change “ground permittivity” to “ground backscatter”. I also suggest to change the wording so that you acknowledge these vary also spatially, not only temporally.
7. Intro line 68. “Is most sensitive to SWE when a priori snow microstructure is known” does not really make sense. The sensitivity to SWE remains the same (weak or strong) whether or not we know what the microstructure is. I guess you mean “the sensitivity to SWE can be predicted when a priori snow microstructure is known”, or similar. Please reword.
8. Section 2.1 line 117 “prevents good forward modeling” is a bit inaccurate, since you could simulate also wet snow effects with e.g. SMRT, which would be just as “good”. Rather, the problem is that meaningful retrievals of SWE, microstructure or ground properties become difficult due to the overwhelming influence of wet snow on the radar signal? Please reword.
9. section 2.2 lines 128-131. The sentence is a bit awkward to me. Maybe split into two sentences after “were considered”?
10. section 2.3 lines 142: you state that “it was possible to determine the freeze-thaw state of the soil and provide modelling inputs to estimate the background scattering”. However, to what I see, these measurements were finally not used, but all ground values were retrieved ones? Did you try comparing retrievals to measured permittivities? If not, it is not necessary to report these measurements.
11. Section 4.1 line 240. The values stated in the text seem to contradict Figure 4 and Table 3, please verify.
12. Line 259 typo, remove (Calonne2000)
13. Section 4.4 line 307. Permittivity parameter? Do you mean soil permittivity?
14. lines 311-312. Here you introduce a previous scaling factor, but no explanation is given why this should be comparable to polydispersity. One has to dig into the paper by Picard et al. for clues, but it should be explained it here. See previous comment on adding short introduction/explanation of the microwave grain size concept.
15. Figure captions in general, here Figure 6 as an example: The captions should enable the reader to understand the figure without referring to the text. Instead of a generic “Results of calibrated SMP snow density measurements”, explain the two panels in the figure (scatterplot and histogram) fully in the caption, including information on what was used in the calibration (i.e. snow pit profiles). Please check that captions of all figures and Tables provide sufficient information.
16. line 281 typo: Grandell
17. Discussion: did you compare retrieved soil permittivity values at all to measurements? What does literature and models say about the frequency dependence of permittivity of frozen ground (largely controlled by permittivity of ice)? At least the latter should be discussed, even though the retrievals represent “effective” values. If a comparison to the measured ground permittivities did not yield anything conclusive, this could also be at least mentioned.
18. Discussion, lines 400-403. One has to read all the way here to understand the simulation setup. Please move this explanation to Methods, adding other necessary details (see previous comments)
19. Conclusions, lines 418-419. Increased and decreased compared to what? I am assuming the ranges introduced by Picard et al., but this should be made clear. Also, note that Picard et al presented the first estimates of these parameters, so finding values outside of these ranges is not surprising.
20. line 430 “background properties should be similar…” I do not follow. Why should these be similar?
21. line 442: “…skillful forward modelling of the radar signal in the tundra region”.