the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Oct 2025
Version 3.0 of the Crocus snowpack model
Abstract. This article presents a comprehensive description of the 3.0 stable release of the Crocus snowpack model in the SURFEX modelling platform. It synthesizes and harmonizes a number of equations disseminated in various previous publications, introduces a number of unpublished parameterizations and includes new developments implemented since 2012. Among the novelties, an explicit representation of the evolution of impurity mass in snow (e.g. black carbon, mineral dust) allows representing their impact on solar radiation absorption in the snowpack at different wavelengths and their feedback on all snowpack properties. The model also allows the formation of surface ice layers due to freezing rain. In addition, Crocus is coupled to the MEB "big-leaf" vegetation scheme and can therefore be applied in forested areas. A module for snow management can also be optionally activated to simulate the snowpack on ski slopes in ski resorts. The model can be coupled with various blowing snow schemes. The MEPRA expert system which analyses the mechanical stability of the simulated snowpack has been implemented directly within SURFEX. A multiphysics version of the model (ESCROC) was also developed by implementing from 2 to 4 parameterizations from the literature for each physical process represented by empirical parameterizations. The different combinations allow the quantification of simulations uncertainty for various applications. Finally, a technical solution was proposed for externalized applications allowing the use of the scheme in other Land Surface Models. The paper also reviews the available scientific evaluations and applications of the model. It describes its numerical efficiency and the main scientific and technical challenges providing guidance for the future of snow modelling.
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Status: open (until 25 Dec 2025)
- RC1: 'Comment on egusphere-2025-4540', Richard L.H. Essery, 06 Dec 2025 reply
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RC2: 'Comment on egusphere-2025-4540', Anonymous Referee #2, 08 Dec 2025
reply
This paper attempts to summarize the recent model advancements to Crocus since its previous documentation effort in 2012. The study addresses inaccurate and insufficient documentation as shortcomings of scientific model advancement, which is prevalent throughout the industry. It focuses on synthesizing the most recent Crocus model advancements including black carbon, ice layer development, ski resort impacts on snow modeling, blowing snow schemes, to name a few. This research effort will be important as Crocus continues to advance, be incorporated into other LSMs, and used in other scientific applications by modelers, snow hydrologists and practitioners, alike; therefore, I recommend that it should be published with minor changes.
I agree with referee #1 that the format of the paper is a little odd in that the only evaluation of the model is showing the computational efficiency of the different modules using “default physical options” (Section 4.2). I think this information is valuable but for it to be the only results shown was a little puzzling. If this is purely a paper synthesizing recent advancements of the physical formulas and parameters used in Crocus and referencing the specific studies they come from, I think a schematic diagram for what Crocus does (e.g. Vionnet et al., 2012 Fig. 1) and which processes are relevant to this current study would be helpful. It could even be something like Figure 1. I would have liked this to be referenced earlier to provide some context about what was traditionally incorporated in the Crocus model and what’s been added. Some other general comments include; grammatical errors, and unclear wording, and many of the provided equations are never actually referenced in the text (e.g. 135 and 136).
Specific comments:
Lines 10-12: confusing as written.
Lines 43: “to evolve”
Lines 49: It wasn’t immediately clear to me what was meant by diagnoses
Lines 51-53: Confusing as written.
Line 69 :”are set to zero”
Line 82: “in an appendix.”
Line 85: might be helpful to describe LAP in more context. I think its actually defined later in the manuscript.
Line 132-133: confusing as written.
Line 136: “relatively” -- “relative”
Line 160: “When a layer i is split”
Line 246-249: Confusing as written. Either .. either.
Line 252: “They depend on conditions of…”
Line 258-260: Confusing as written.
Line 348: “In addition, a threshold is applied on the mobility indice in case melt forms:”
Line 394-395: “The last modification of absorption related parameters” ??
Line 513: “and G0 is the flux obtained by applying Eq. 95 with T1.”
Line 530: “This allows us to represent” ??
Line 549: “…And to solve the linear system… modified according to the new values.
Line 558: “solving MEB”
Figure 1: Reference earlier.
Line 578-579: Confusing as written.
Line 598-599: “Either… or” instead of “Either … either”
Line 623-625: Confusing as written.
Line 640: “removing” instead of “deleting”
Line 644-645: Reads as somewhat conflicting. Crocus does not modify or update snow microstructure, yet you cannot track surface hoar buried underneath the snow?
Line 655-57: Does this mean model output variables that can be used for model evaluation and diagnostics? I think a little more of a description here and possibly in the introduction would be helpful.
Line 695: “For understanding stability indices of the snowpack, shear resistance Rsi is computed for each layer i.”
Line 707-708: Confusing as written
Line 709: typo -- “values”
Line 790: typo “then” -- “that”
Line 817: typo “spatialized” -- “spatial”
Line 848: typo “score” -- “accuracy”
Line 866-870: confusing as written
Line 903-905: could be written clearer.
Citation: https://doi.org/10.5194/egusphere-2025-4540-RC2
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- 1
This paper provides exhaustive (and quite exhausting!) documentation of Crocus, historically and still currently one of the most important models of snow on the ground. I have never before reviewed a manuscript with so few figures or so many nested braces! The authors do not follow the GMD encouragement that “Examples of model output should be provided, with evaluation against standard benchmarks, observations, and/or other model output”, but they do review other published evaluations and applications. The documentation collates developments in several previous publications and corrects errors in some of them. As such, this is an essential reference for users of Crocus. I recommend that it should be published with minor changes. A table of all the options and their dependencies would be a useful addition.
Specific comments:
70
Parametrizations which do not make this assumption that the diffuse fraction depends on solar zenith angle only are available. How much difference does using this parametrization make in simulations when data are available?
95
At atmospheric pressures, should T0 not be the melting point of water rather than the triple point?
264
Why are values 3 and 5 not included here in “occurrence of depth hoar at any time since the layer creation”?
469
For vertical heat transport between layers, the harmonic weighted mean to add conductivities in series seems more natural.
486
What is the justification for now using the harmonic mean between the bottom snow layer and the ground?
737
SOF and HAR are not explained.
1026
The Clausius-Clapeyron equation gives de_{sat}/dT. Equation (F6) is an approximate integral of the formula.
1040
Is there any justification for adjusting the Richardson number for slope? Atmospheric stability adjustment has bigger issues on slopes than the component of gravity.
1049
Letter gamma is used on this page for both slope angle and psychrometric constant without distinction.
Minor corrections:
48
delete “make”
160
“is split”
307
“dependence on snow microstructure”
372
Start a new line before “where”.
373
“volumetric mass” is more commonly called density.
394
“The last modification is”
411
“at Col de Porte”
430
“a homogeneous repartition is applied”
432
“the highest integer such that”
434
“not inconrporated in”
441
Strictly, q_sat is also a function of pressure.
531
“This allows representation of”
558
“cannot”
567
missing )
578
“either by the heating energy or by …”
598
“either by the layer colling after diffusion or by …”
640
“accounted for by adding”
695
“a shear resistance R_{s_i} is diagnosed”
709
“for slope angle \gamma”
714 and throughout
“indice” should be “index”
786
“it can vary significantly from one domain to another”
812
“an independent package called snowtools with full user documentation”
851
“discrepancies”
883
“adequation” is a very uncommon word in English. I think that the simplest fix would be “to reproduce the properties of …, or complex remote sensing signals”
904
“make the application of a number of data assimilation algorithms to this model challenging”
921
“discrepancies”
946
“are repeated here”
967
“cannot”
968
“between dendritic and non-dendritic cases”
1036
“The derivative”
1092
“preventing the vectorization”
1096
“prevents vectorization”