An Improved Retrieval of Snow and Ice Properties Using Spaceborne OLCI/S-3 Spectral Reflectance Measurements: Updated Atmospheric Correction and Snow Impurity Load Estimation

Round 1

Reviewer 1 Report

In this manuscript, the authors present an update of the Snow and Ice (SICE) property retrieval algorithm and explain in detail the main improvements introduced, among which are the following: the introduction of a new atmospheric correction, retrieval of snow impurity load and properties, retrievals for partially snow-covered ground and also accounting for various thresholds to be used to assess the retrieval quality. The manuscript in general is very well written and the topic is very interesting considering the challenge of working with remote sensing data in polar areas especially in Antarctica where it is very complex to obtain in situ measurements. Therefore I consider that the manuscript could be accepted after minor changes.

 

General comments:

Although in several lines of the manuscript reference is made to the use of the broadband albedo derived from MODIS observations e.g. line 526, I think it should be added which product and which MODIS version was used for comparison with the results obtained with the SICE-3 algorithm. 

 

Additional Comments:

1. What is the main question addressed by the research? The authors present the update of the snow and ice properties retrieval (SICE) algorithm initially proposed by Kokhanovsky et al. (2019). 2. Do you consider the topic original or relevant in the field? Does it address a specific gap in the field? The topic is original and relevant considering that this algorithm allows studying some aspects of snow that for work in remote areas such as the Greenland ice sheet or Antarctica could be very useful. 3. What does it add to the subject area compared with other published material?  The main contribution is to evaluate the performance of the proposed algorithm not only with in situ data but also with data from other sensors. 4. What specific improvements should the authors consider regarding the methodology? What further controls should be considered? Although in several lines of the manuscript reference is made to the use of the broadband albedo derived from MODIS observations e.g. line 526, I think it should be added which product and which MODIS version was used for comparison with the results obtained with the SICE-3 algorithm. 5. Are the conclusions consistent with the evidence and arguments presented and do they address the main question posed? The conclusions are consistent with the evidence and arguments presented and address the main question posed. 6. Are the references appropriate? The references are appropriate.

Author Response

 We have used MODIS MOD10A1 broadband albedo product (collection 6). The paper has been modified.

Reviewer 2 Report

The paper titled “An improved retrieval of snow and ice properties using space-borne OLCI/S-3 spectral reflectance measurements: updated atmospheric correction and snow impurity load estimation” complete an update of the Snow and Ice (SICE) property retrieval algorithm which is based on the spectral measurements of Ocean and Land Color Instrument (OLCI) on board Sentinel-3 satellites, combined with the asymptotic radiative transfer theory valid for weakly absorbing turbid media. The main improvements of the algorithm include a new atmospheric correction, retrieval of snow impurity load and properties retrievals for partially snow-covered ground. To do this, the authors combine SICE with the asymptotic radiative transfer theory valid for weakly absorbing turbid media. The algorithm targets retrieval of snow and ice optical properties such as snow spectral and broadband albedo and has applicability in climatic studies or those related to the snow microstructure and composition.

The authors list the following updates:  the retrievals of snow fraction and sub-pixel snow albedo; the improvement of the atmospheric correction technique; characterization of snow impurities in terms of their type (dust, soot) and load; retrieval of total ozone column; introduction of retrieval quality metrics based on the root-square-mean-difference(RSMD) between measured and retrieved OLCI spectra; updated value of the calibration coefficient for the relation between the effective absorption length and the effective snow grain diameter. The authors describe an updated version of previous SICE algorithms and validate the retrieval the spectral and broadband albedo results with ground truth in situ observations over Greenland and Antarctica.

 

This work is very suitable for the journal, but being a review of previous papers, it is note that the authors have incorporated part of them. The work needs a final review that polishes errors and standardizes formats

 

Revisions:

1.      There are errors in the numbering of the sections that need revision:

Line 108: 2.2. Assumptions, should be 2.3

Line 126:  2.2. Pre-processing, should be 2.4

Line 156: 2.2. Post processing and retrieval quality checks, should be 2.5

Line 169-170: 2.2. Snow property retrieval, should be 2.6: also 2.2.1. in Line 170, should be 2.6.1

2.      I think it would be nice to standardize the units in which the wavelengths are expressed.  Most of the time wavelengths are in nm, but also appear micrometers. Moreover, they are expressed with different symbology.

In Line 267: “0.3-2.4 um wavelength range” um are micrometers?

In Line 290: ??

3.      There are problems with font type and size

In Line 457: (45°2′ 4″ N, 6°24′ 18″ E)…font type

Table 4: font  size

Table 5:  font type

4.      Problems with table numbering

There are two tables numbered as 5……Table 5 in Line 508 and Table 5 in Line 590

Line 638:  Table 6 should be Table 7, also font type and size

In Line 670: Table 8…. is correct

 

5.      Appendices : Standardize formats and numbering

Line 730:  Title appear in blue

Problems detected in References:

6.      Not all citations appear with doi. Review the missing doi.

For example: Bond, T. C., Bergstrom, R. W., 2006: Light absorption by carbonaceous particles: an investigative review, Aerosol 740 Science and Technology, 40: 1, 27- 67. The missing doi is : doi:10.1080/02786820500421521

7.      Not all doi have the same format. Choose a uniform format compatible with the journal

For example: https://doi.org/10.5194/acp-17-7175-2017 or doi: 10.1002/2016JD024783 or https://doi.org/10.3389/fenvs.2021.757575

8.      Sometimes the abbreviation of the name of the journal is used and sometimes not. For example Line 811_812

9.      Formatting issues:

 Line 822_824 font and paragraph

Line 832-833 font and paragraph

Line 854-855 font and paragraph

Line 863-864 font size

Line 872-873 font and paragraph

Author Response

We have accounted for your comments  in the revised version of the paper.

 

Reviewer 3 Report

In this paper authors integrate the asymptotic theory developed by one of the authors in the retreival algorithm and use it for retrieving of snow properties.

The manuscript is well written and contains a very detailed description of the algorithm. It is very important that the manuscript is supplied with the link to the open source code.

I recommend

1. to add some comments about the performance enhancement due to using teh asymptotic model w.r.t. full radiative transfer computations.

2. a more general question. Clearly, the use of simplier models can be regarded as a sort of regularization. Should we expect some differences in retrieval results obtained with the proposed algorithm as compared to the conventional retrieval algorithms which do not rely on approximations of Dr. Kokhanovsky?

Author Response

The approximate solutions have been compared with exact radiative transfer calculations (Kokhanovsky et al., 2019) and the differences found are small and can be ignored as far as practical problems of snow remote sensing using satellite observations are of concern. We have  added a comment on this subject in Conclusions.