The review article by Royer et al. provides some insight into the measurement principles, accuracy, and advantages/disadvantages of four instruments that employ electromagnetic waves to measure in situ snow water equivalent (SWE). Instruments reviewed are the Cosmic Ray Neutron Probe (CRNP) (deployed in two configurations: above and below the snowpack), Gamma Ray Monitoring (GMON) sensor, Frequency Modulated Continuous-Wave Radar (FMCW-Radar), and Global Navigation Satellite System (GNSS) receivers. The paper is a combination of reporting results from previous evaluation and intercomparison studies and the reporting of results from intercomparisons at two Canadian research sites: Foret Montmorency and the Site Interdisciplinaire de Recherche en ENvironnement Extérieur (SIRENE). The purpose is to inform the reader of the relative advantages and disadvantages of each of these systems in the context of employing them in operational monitoring networks.

I thought that the paper offered a complementing balance of previous assessment results and associated measurement issues with fresh results to build on the experience. SWE sensor evaluation is complex because advantages and disadvantages are only realized in differing measurement (snowpack) conditions which are impossible to assess at one, or even several, concurrent intercomparison sites. In this way, the paper is both interesting and contributes to the body of knowledge related to automated in situ SWE measurements. Having said that, there are several deficiencies in the paper that require revision before the paper can be published in TC. These are incorporated into the attached annotated manuscript, but the major issues are as follow:

1)There are general wording and grammar issues and some terminology inconsistencies that need to be fixed. These are noted in the annotated manuscript.

2)The abstract is quite vague. It would benefit from some additional detail about each of the sensors assessed and perhaps a short overview of the offered recommendations.

3)There are some redundancies and therefore readability to be gained in Sections 3/4/5. For example, by the time we get to Section 4 (in particular, lines 700-702), we shouldn’t be reading more about instrument description, such as the field of view of the CRNP, as this should be outlined already in Section 3. I think that Section 4 can be organized a bit better to improve readability, perhaps by converting the bulleted list starting at line 637 to paragraphs using heading names organized by instrument.

4)Similar to point #2, the Conclusions section contains redundant details, such as the bulleted list starting on line 740, that should only be briefly summarized at this point. Since your pros and cons are a bulleted list in Section 4, summarize these in a short paragraph in Section 5, followed by some brief recommendations to potential users.  

 General comments

This paper presents an assessment of methods to measure autonomously and continuously the water equivalent of snow cover. The presented data (one winter each on two sites) are augmented by a wealth of already published data, including own ones. This allows for a thorough analysis of pros and cons of these instruments that are increasingly used worldwide. Furthermore, an emerging, truly low-cost method based on FMCW radar is presented. These two last points form the major strength of the paper while I cannot really call it a ‘review’.

On the other hand, it is sometimes difficult to find out whether the authors speak about their own data or data taken from the literature. This ambiguity is one of the major drawback of this contribution, that also reflects itself in a poor structure of Section 3 (see comments below). Another major problem is the ‘classification’ of the Cosmic Ray Neutron Probe as an ‘electromagnetic wave-based field sensor’. With that respect the authors are required to make necessary corrections throughout the text. Furthermore, the author should eliminate quite a number of weaknesses I address below and in the manuscript. This to make the paper less ambiguous and easier to read, thereby increasing its value for the community.

In summary I recommend accepting the paper after the authors addressed the issues below and consider the revisions as suggested in the annotated manuscript.

Specific comments

• Title:
  
  In view of my general comments and because CRNP is not based on EM waves, I would suggest to change the title to, “Performance of autonomous field sensors for continuously monitoring the water equivalent of snow cover (SWE)”
• Line 25: ‘relatively low cost’
  
  In Table 3, cost is named as a drawback for at least three instruments! I would also argue it is a matter of appreciation whether one considers USD 10 000.- to be ‘low-cost’. I would thus propose to drop this argument, at least in the abstract, the introduction, and the conclusions.
• Line 40: ‘kg m^-2, or in mm’
  
  You nicely define water equivalent of snow cover as ‘its mass per unit area’. Why then use mm as a unit? I would prefer by much seeing only kg m^-2 throughout the paper.
• Line 50: ‘reliable and automatic instrument alternatives exist’
  
  This is questionable. At least I could not find corresponding evidences in the references provided.
• Section 4:
  
  Consider shortening the last paragraph (lines 367-376) or moving it to the introduction as it is not further considered in the paper.
• Lines 533-534: ‘winter period, uncertainty can be introduced by well-known local SWE spatial variability that can occur at fine scales around the sensors.’
  
  I would argue this variability is less present in bulk snow density. Thus a comparison of SWE recomputed with regard to one reference snow depth may have been beneficial.
• Line 653:
  
  clearly, in your own words, CRNP is NOT EM wave!

Comments on structure

• Section 3 is quite difficult to follow and not free of ambiguities. I’d suggest the following to improve its readability:
  
  Rename Section 3 as ‘Results’ and start with a description of both sites (Sub-section 3.1). Adding a figure showing the location of the instruments at each site as well as a summarizing table would extremely helpful.
  
  Then Section 3.2 would become ‘Validation of measurements’ with separate sub-sections on SIRENE and Neige-FM. 3.3 ‘Uncertainty of measurements’ would end Section 3.

Comments on terminology

• ‘snow water equivalent’:
  
  Consider switching to “water equivalent of snow cover” as defined by WMO (for example, see WMO 2018): Guide to instruments and methods of observation: Volume II - Measurement of Cryospheric Variables, 2018th ed., edited by: WMO, World Meteorological Organization, Geneva, Switzerland, 52 pp., 2018.)
• ‘accuracy’:
  
  Consider switching to “uncertainty”, see the VIM https://jcgm.bipm.org/vim/en/index.html or https://jcgm.bipm.org/vim/fr/index.html
• ‘CNRP’ (see line 23):
  
  Please correct to “CRNP” throughout the text.
• ‘GNSS’ (used 16 times) vs ‘GNSSR’ (used 34 times):
  
  The distinction between ‘GNSS’, that is the satellite system, and ‘GNSSR’, that is the instrument used, is sometimes misleading in the text. Also note that GNSSR can easily be misunderstood as GNSS reflectometry. Thus I’d suggest the following: on lines 24-25 and 67 write, “Global Navigation Satellite System receiver(s) (GNSSr)”. Using GNSS alone later should then be clear too.