In the paper "A Flexible Snow Model (FSM 2.1.0) including a forest canopy", Essery et al. present the Flexible Snow Model FSM2, an extended version of the Fractional Snow Model (FSM) in which canopy snow processes are described. The manuscript includes a thorough model description of shortwave radiative transfer, longwave radiative fluxes and turbulent heat exchanges between the snow and the (single or double layered) canopy, as well as the canopy mass balance. The authors show simulation results with respect to the modeling options for radiative transfer, interception efficiency and canopy snow unloading. Modeling results are then compared with field observations in Scandinavia and Switzerland and with large scale remote sensing simulations over the Northern Hemisphere.

This is a high-level model description paper that I enjoyed reading. The model is clearly presented, the figures are clean and this paper is a relevant contribution to the snow forest modeling community. The manuscript is already very good and I have no major concerns. However, I do suggest a few changes that I think could improve the overall quality of this work. I recommend that the authors consider these before the publication of this paper in its final form.

General comments:

• Introduction. In my opinion, the introduction is very solid. It is well written and guides the reader to the section describing the model. As mentioned by the authors, FSM has been widely used in previous work by many people in the snow modeling community from different countries and has contributed to the development of scientists in academia (l. 19-20). However, I think the introduction would benefit from an additional paragraph mentioning and describing the context in which researchers have used FSM in previous studies. This would highlight some shortcomings in the description of canopy snow processes in the model and underline the importance of developing a robust and flexible modelling scheme for it.
• Discussion. As there are several options that have to be decided by the user, I think the authors should provide better guidance to the community on how to use their model. Based on the simulations performed at three different sites (two in Scandinavia, and one in Switzerland), based on the literature supporting the different parameterisations described in the paper, and based on their understanding of the physical processes represented in the model, I suggest that the authors provide insights regarding the options to use for specific climates or modeling purposes.
• Outlook. It would be interesting to have a few words about the ongoing and future developments of FSM2. I think the current limitations of the model should also be exposed.

Specific and technical comments.

• l. 1: I would explicitly name the model “FSM” instead of referring to a generic “model” in the first sentence of the manuscript.
• l. 11: I feel that a sentence could be added at the end of the abstract mentioning how this new model development can help the scientific community to improve their ability to model snow in forested environments.
• l. 66: Given that several models use the leaf area index (LAI) as a parameter for forest structure, and that the LAI is commonly (and rather easily) measured in the field, it is worth adding a few words on the difference between the effective vegetation index, as used in FSM, and the LAI.
• l. 94: Please add one or two sentences to briefly summarize the method from Erbs et al. (1982).
• Some default parameters are mentioned in the text (l. 129, l. 256, l. 262, l. 277, l. 349) without any explanation of the choice of the specific value for these parameters. Were these values determined from a sensitivity analysis? Are they based on previous modeling or experimental work or arbitrarily chosen? Please indicate how each of these parameters was defined.
• l. 171: What is the meaning of "canopy gaps"? Looking at equation 14, I understand that the authors refer to the space in a canopy layer without leaves or branches that allows the transmission of diffuse shortwave and longwave radiation. As "canopy gaps" generally refer to a sub-environment where energy and mass fluxes are altered compared to a "full canopy" environment in the snow-forest scientific literature, this could be misleading. Please consider replacing it with another term.
• l. 301. Please specify the parameter from which the iteration starts.
• l. 353. Given the questionable physical basis of this 2/3 exponent in equation 80, I am curious about the sensitivity of the model to this parameter. Have you tried running simulations with exponents other than 2/3?
• l. 365. This suggests that snow in the canopy evolves in the same way as snow on the ground. Please state this clearly in the manuscript.
• l. 397 to 405: Please consider moving this to a subsection of the method in which the study sites would be described.
• l. 441 to 443: Please specify which parameters have been adjusted and which ones have been let as default
• l. 446-447. I imagine that a subjective discrimination between 9 levels of canopy interception must be very difficult to do. Showing photos of these 9 interception levels would help the reader to see the nuance between each stage of canopy load. I suggest adding this in a supplementary material document.
• Table 8: Some plots are somehow a bit messy with all the black lines (especially 8b). Consider showing the ensemble median as a black line with a min-max envelope.
• Table 1: This is perhaps a personal preference, but as a reader I would find it easier to grasp differences in modelling results between each simulation with a figure rather than a table. This could be done by plotting two variables against each other on an x-y plot and a color map for the third variable. As there are only 16 points to plot, you could then write a short name for each simulation on the graph without oversaturating it.
• I find that Figure 9b is very interesting. I would like to see how this partitioning would change with the different simulation schemes (Table 1). I am curious about what drives the melt energy for sites with different canopy densities. Consider including this in the supplementary material.
• l. 521. Can you explain why this particular model behaves differently to the others?
• Figure 10. Please enlarge this figure.
• l. 554-555. Can you elaborate on how these modeling choices regarding canopy snow unloading and canopy snow-rainfall interactions may be more appropriate for some climates and less appropriate for others?
• l. 571-572: Please, revise this sentence. There seems to be a word missing.