General comments

This manuscript describes a pan-Canadian data set on snowpack water equivalent (SWE), along with snow depth and snow density for observations for which snow depth has been reported in addition to SWE. The data set is an updated version of the Canadian Historical Snow Survey Data (CHSSD) archive, which has been used in a number of research studies since its publication in 2019. The current version corrects a number of issues in the earlier version, incorporates additional data sets, and applies a consistent quality control protocol. The steps involved in the updating are all clearly described and logical.

Based on the usage of the earlier version by the international community, I anticipate that this updated version will be an important resource for a range of studies related to atmospheric and climate science, cryospheric science, hydrology and ecology. I have a few suggestions for some additional information and technical corrections, as outlined below.

Specific comments

The introduction seems to me a bit long, and I wonder if all of the information is necessary in the context of introducing the data set. I would suggest that the authors consider ways to shorten it. For example, perhaps some of the information on measurement approaches in the first two paragraphs could be summarized in a table. That table could also be referred to later in the manuscript in relation to the metadata.

To help set the motivation for producing the current data set, it may be useful to add a couple of sentences to the introduction about the use of the earlier CHSSD by the international community. For example, a Web of Science search on the article by Brown et al. (2019, Atmos. Ocean) showed that it has already been cited eight times.

Figures 4 to 7 provide a good overview of the spatial and temporal coverage that will be useful for potential users. The only suggestion I would have for additional figures would be one showing the elevational distribution of observations in relation to hypsometry, perhaps at a provincial or regional scale (e.g., based on the national level ecoregions; see https://open.canada.ca/data/en/dataset/ade80d26-61f5-439e-8966-73b352811fe6). As a researcher who focuses on the mountainous regions of western Canada, I believe that it is important for users of SWE data to appreciate that most of our observations represent mid-elevation locations below treeline.

It would be useful for potential users to have information about the different types of snow samplers used in the different regions of Canada, If such information is available. For example, Goodison et al (1987, https://doi.org/10.4296/cwrj1202027) reported that many samplers used in Canada overmeasure by varying amounts.

Another methodological point that may be useful to mention, if information is available, relates to the siting of snow courses and snow pillows. For example, some snow pillows I have seen in British Columbia are located in small forest openings, such that I suspect that they tend to accumulate snow more like an open site and melt at rates more like forested sites.

When comparing model output to SWE observations, it is important for users to understand the accuracy of the locational coordinates in order to extract simulated SWE from a model unit that is representative of the monitoring location. If possible, I suggest that the authors add some information about the typical horizontal and vertical accuracies of the coordinates.

Technical corrections

line 104. I suggest reorganizing the sentence to avoid beginning with “91.”

line 192. Should “Northern America” be “North America” – i.e., Mexico, USA and Canada?

line 192. Change “expect” to “except.”

line 207. Change “was” to “were” to be consistent with earlier usage of “data” as plural.

line 283. Insert “of” between “majority” and “the.”

This paper presents an update of the 2019 Canadian Historical Snow Survey dataset (CHSSD). Errors in metadata as well as a large amount of duplicate data led to this overhaul. Furthermore, the dataset was renamed ‘Canadian historical SWE dataset (CanSWE)’ to reflect the inclusion of automated data and the weight put on water equivalent of snow cover (SWE) data. The paper is well written and well structured, describing clearly the processing steps as well as the cleaning procedures used. Great work!

The paper is timely as it emphasizes the need for standardized formats and procedures, “Python routines specific to each agency and corresponding data format were written to process the data and metadata and arrange them in a consistent NetCDF format”. While I very much welcome this effort towards standardization I may have wished an even larger convergence towards WMO standards, for example regarding terminology (see comments below). However, as the authors conclude that “Regular updates are required to make such datasets useful for the community.”, I would strongly suggest following WMO standards even more closely in future. For example, I anticipate a standardized WMO NetCDF format will emerge soon to which the CanSWE format may comply too.

In summary I very much appreciate the effort put in homogenising the Canadian snow survey data sets and therefore recommend accepting the paper after the authors addressed the issues below and do some minor revisions as suggested in the annotated manuscript.

Specific comments

• General introduction about snow courses and snow surveys, lines 23-60:
  
  I miss a clear statement whether you only consider snow courses as relevant measurements of water equivalent of snow cover. While on lines 39-40 you state that, “SWE observation networks using different measurements methods have been deployed at a national scale in various countries to provide valuable in situ information.”, the following lines only include countries performing snow surveys, which are most typical in North America indeed, but not everywhere. I would suggest including a short paragraph on other methods, referring as you do to the ‘European Snow Booklet’ but also to WMO-No.8 (WMO, 2018) and also to López‐Moreno et al. (2020).
• Section 3, Quality control of the final dataset :
  
  The homogenization of data quality flags shows how important standardized report practices are.
  
  Do I understand correctly that a H or W flag automatically results in a D flag? In other words, a quality flag set at one step influences the final number of flagged values?
• Section 4, Data availability and Table 6, lines 328-336:
  
  From my perspective, except snw, snd, den, most of the entries in this table are observational metadata in the sense defined by WMO (WMO-No. 49, Volume I and WMO-No. 1160). It may be worth considering whether the WMO standard could be implemented in a future version of CanSWE, both regarding the name of metadata and data as well as within the NetCDF file. A note on this in the text would be appreciated.

Comments on terminology

• ‘snow water equivalent’ vs “water equivalent of snow cover”, lines 1 (title), 10, 23, 336 (table 6), 338:
  
  Consider switching to new terminology introduced in WIGOS Metadata Standard (WMO-No. 1160) and WMO-No. 8, keeping SWE as abbreviation.
• ‘the depth of water that would be produced if all the snow melted’, line 10:
  
  Both the WIGOS Metadata Standard & WMO-No. 8, Vol 2, p. 13 state: “Water equivalent of snow cover (SWE) is the vertical depth of water that would be obtained if the snow cover melted completely, which equates to the snow-cover mass per unit area.”
• ‘[derived] bulk snow density’:
  
  I very much welcome you using ‘bulk snow density’!
  
  - Consider defining it on line 211 as ratio of SWE to SD – and also in the abstract, line 14 – Consider adding ‘derived’ only in tables (4, 5, and 6) but not in the text (line 339)
  
  - Use it throughout the text (see lines 245, 246, 250, 278, and 344) and not on line 255 as it is referring to a range.

References

López‐Moreno, J. I., Leppänen, L., Luks, B., Holko, L., Picard, G., Sanmiguel‐Vallelado, A., Alonso‐González, E., Finger, D. C., Arslan, A. N., Gillemot, K., Sensoy, A., Sorman, A., Ertaş, M. C., Fassnacht, S. R., Fierz, C., and Marty, C.: Intercomparison of measurements of bulk snow density and water equivalent of snow cover with snow core samplers: Instrumental bias and variability induced by observers, 34, 3120–3133, https://doi.org/10.1002/hyp.13785, 2020.

WMO: 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.

https://library.wmo.int/doc_num.php?explnum_id=9870