Intercomparison of Surface Albedo Retrievals from MISR, MODIS, CGLS Using Tower and Upscaled Tower Measurements
Round 1
Reviewer 1 Report
The authors present a very interesting procedure to compare ground albedo mesaurements to satellite-derived albedo, taking into account direct and diffuse albedo contributions separately.
I have some concerns about the methodology that should be explained:
1.- On line 194-195, the authors use the mean of the ground albedo calculated over 30 days. This maybe too large a time in some cases, especially in areas that can be either snow-covered or bare soil, since the properties of the cover may change dramatically, especially close to the snow melting season of at the beginning of the snow season. Why have they chosen such a long time lapse?
2.- I do not understand why the authors increase the value of the uncertainty. What is the purpose of it? Could you explain better?
3.- On lines 244-245, the authors state that they establish a non liner fit between endmembers abundance and MODIS albedo-to-nadir-reflectance in order to obtain the weighting factors. Are they referring to Equation (9)? If so, there seems to be a linear relation between the endmembers abundance and MODIS albedo-to-nadir-reflectance. I do not understand the claim on lines 244-245.
4.- The explanation from lines 247 to 250 (“This is based on the assumption……in an area) is not clear. What do the authors mean, can you explain?
5.- According to lines 253-254, the authors seem to apply the same calibration factor to scale ground measurements up to high resolution albedo and from high resolution albedo to low resolution albedo. Please provide a reason for that.
6.- Regarding Equation 8. I think this equation is used to calculate broadband albedo directly from spectral reflectance. Is not Eq(8) the broadband albedo already? This point should be made clear, since the aurthors identify Eq(8) as the broadband BRF, not the broadband albedo.
7.- Regarding Equation 12. It seems to me that L refers to shortwave reflectance (integrated using Equatino 8) and B to broadband albedo. If so, I do not understand why they refer to Landsat 8 bands.
Author Response
Please find the answers to the first reviewer's comments in attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments to the manuscript ID: remotesensing-449987 of Rui Song, Jan-Peter Muller and Said Kharbouche, entitled " Intercomparison of surface albedo retrievals from MISR, MODIS, CGLS using tower and upscaled tower measurements”
This work compares two corresponding sets of the albedo values of homogeneous and heterogeneous land surfaces covered with different vegetation cover, snow and ice. One set was obtained using corrected satellite data from MISR, MODIS, VEGETATION and Proba-V radiometers, and other from measurement collected between the years 2012-2016 by pyranometers from almost twenty towers belonging to the FLUXNET, SURFRAD and BSRN networks located on five continents. The authors of this manuscript also present a newly developed method to derive DHR and BHR directly from tower and diffuse shortwave radiation. This work is very important because the data obtained from these towers allow to quantify the correctness of the albedo of land surfaces using data from the above mentioned satellite radiometers on a global scale. The authors indicate what determines the correctness of data obtained through these radiometers. The results presented show that not all compared albedo sets of the tested surfaces give satisfactory correctness. The minimum albedo values of the lands on a given day of the year were compared because they related to satellite observation around noon of solar local time. They do not refer to the average diurnal albedo values of the analysed surfaces, which would best describe the energy transfer between them and the atmosphere in the models of environmental biophysical processes on a regional and global scale in periods longer than a few days, in a season or a year. The results of this work indicate the need to further improve the methods of obtaining the albedo values of land surfaces via satellite data.
In my opinion, the authors of this manuscript unnecessarily announce at the end of the "1. Introduction" section what will be in the next sections of it, but the purpose of their work has not been clearly defined. Even in the abstract this goal is more clearly expressed.
MINOR commentS
· The use of several abbreviations or acronyms in the text of the manuscript requires in my opinion an earlier explanation of what they mean. I think about: FLUXNET, SURFRAD, BSRN (line 20), IGPB (line 101), RSLSR (line 274), UTM (line 289), HJ (line 237).
· What does "T" mean in equation (10)?
· The symbols "βB" and "σ" in Figure 2 should be explained before viewing Fig. 2 or in the caption under this figure.
· The reader can guess what "HR" means in "HR-BO" (Figure 6), but it's better to be sure, explaining it earlier in text or caption in this figure. For example, in the first line "1. Introduction" section (line 40) is an opportunity to do this.
· Describing individual graphs by the letters (a), (b), (c),... (k), (l) in figures 7-12 is probably unnecessary.
· The reference [16] (line 132) appears in the text before the reference [15] (line 141).
· Reference [21] is not marked in the text of the manuscript.
I recommend this manuscript for publication in the Remote Sensing after improving it, taking into account the above comments.
Author Response
Please find the answers to the second reviewer's comments in attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
I would like to thank the authors for improving the paper according to my comments.
However, there is still one point that is not clear yet. I also have to apologize since I made a mistake in my previous revision with the numbering of Equations: in points 6 and 7 of my previous review, when I refered to Equation (8), I meant Equation (11).
So the problem with Equations 11 and 12 remians unsolved. My questions are:
1.- Regarding Equation 11, I think this equation is used to calculate broadband albedo directly from spectral reflectance, as explained in Ref. 18 in the new version (Liang, S. Narrowband to broadband conversions of land surface albedo: I Algorithms. Remote Sensing of Environment 2000, 76, 213–238, doi:10.1016/S0034-4257(00)00205-4.). Are you sure Equation 11 is not the broadband albedo already?
2.- Regarding Equation 12. If Equation 11 is the braodband albedo alreday, Equation 12 makes no sense at all, and the procedure to calculate Landsat 8 bradband albedo makes no sense either.
Author Response
I would like to thank the authors for improving the paper according to my comments.
However, there is still one point that is not clear yet. I also have to apologize since I made a mistake in my previous revision with the numbering of Equations: in points 6 and 7 of my previous review, when I refered to Equation (8), I meant Equation (11).
So the problem with Equations 11 and 12 remains unsolved. My questions are:
1.- Regarding Equation 11, I think this equation is used to calculate broadband albedo directly from spectral reflectance, as explained in Ref. 18 in the new version (Liang, S. Narrowband to broadband conversions of land surface albedo: I Algorithms. Remote Sensing of Environment 2000, 76, 213–238, doi:10.1016/S0034-4257(00)00205-4.). Are you sure Equation 11 is not the broadband albedo already?
2.- Regarding Equation 12. If Equation 11 is the broadband albedo already Equation 12 makes no sense at all, and the procedure to calculate Landsat 8 bradband albedo makes no sense either.
Response:
I would like to thank the reviewer for reviewing this paper. My response to the reviewers’ further comments is as following.
Comments 1 and 2:
Yes, in equation (11) a narrow-to-broadband conversion is used. However, equation (11) is used to convert the reflectance value (please see line 555), not the albedo value, from narrowband to broadband. The narrow to broadband conversion coefficients provided by Liang (2000) can be applied to either surface reflectance or surface albedo values. Because albedo is the directional integration of reflectance over all sun-view geometries. This means equation (11) is working on the conversion from narrowband surface reflectance to broadband surface reflectance Equation (12) is establishing the linear model between broadband reflectance and broadband albedo.
