the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Oct 2025
Implementation of predicted rime mass in the bin microphysics scheme DESCAM 3D: Heavy Snowfall event during ICE-POP 2018
Abstract. Due to their wide variety of properties, the representation of ice particles in cold and mixed-phase clouds are challenging to represent for microphysical schemes. To improve their representation, this study evaluates the implementation of predicted rime mass distribution in the bin microphysics scheme DESCAM. Based on the ‘fill-in’ concept, the model allows a smooth transition in ice particle properties between unrimed and graupel particles. Consequently, the terminal velocity and collision kernels of ice particles were updated as a function of rime fraction. These implementations are tested for a heavy snowfall event observed from March 7–9 during the ICE-POP 2018 field campaign in the mountainous Pyeongchang region of the Korean Peninsula. This event consists of a deep cloud triggered by a low-pressure system, followed by a shallower cloud system formed by orographic lifting of marine air. We found that the rime mass fraction at ground simulated by DESCAM evolves similarly to the rime index measured by the MASC instrument. Furthermore, during the shallow cloud phase, the predicted rime implementation leads to an increase in ice particle number concentration and a decrease in mean particle size (from 1.5 to 1.0 mm). The new version of DESCAM leads to significant changes in the spatial distribution of precipitation, with strong local variations exceeding 10 mm, resulting in an increase of 6.5 % in total precipitation amount. Accounting for predicted rime mass gives a better agreement between the model and the ground based observations of ICE-POP 2018.
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Status: open (until 14 Dec 2025)
- RC1: 'Comment on egusphere-2025-3202', Anonymous Referee #1, 21 Nov 2025 reply
Data sets
Ground based observations of ICE-POP 2018 campaign used for DESCAM-3D Pierre Grzegorczyk, Wolfram Wobrock, Antoine Canzi, Frédéric Tridon, Gyuwon Lee, Kwonil Kim, Kyo-Sun Sunny Lim, and Céline Planche https://doi.org/10.5281/zenodo.17278661
Model code and software
DESCAM-3D setup for ICE-POP 2018 7-8 March case Pierre Grzegorczyk, Wolfram Wobrock, Antoine Canzi, Frédéric Tridon, and Céline Planche https://sdrive.cnrs.fr/s/TEeLBMdzFPwpoM3
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SUMMARY
This manuscript describes the modifications to the DESCAM bin microphysics scheme whereby the predicted rime fraction for ice is added. The authors then conducted high-resolution real-case mesoscale model simulations, using the original and modified scheme, for a heavy snowfall event that was well observed during the ICE-POP 2018 field campaign in South Korea. The authors conclude that the modified scheme improves the simulated precipitation and other fields compared to observations. This is a potentially valuable paper in a few ways. First, although the introduction of prognostic rime mass is not novel in the microphysics modeling community, it is new for DESCAM and it represents an important development to that model. Second, the use of microphysical data from field campaigns such as ICE-POP 2018 is very useful and interesting for examining detailed microphysics schemes. In that and other regards, the manuscript is nice in lots of way, however, it has some major shortcomings (described below) that must be addressed in order for this to be considered for publication.
Given that I am recommending that a DESCAM be further developed (with explicit melting) in order to properly study the effects of predicted rime mass and that idealized tests/demonstrations be added, then to be followed by a modifications to the real-case simulation examination section, one possible path forward would be to re-cast this as a two-part paper: 1) description of new developments + idealized tests; 2) Real-case simulation of the ICE-POP case. It may simply be too long as a single paper and I believe that the additions I am recommending are important. I will leave that to the authors to decide, but the comments below must be addressed. With that, I will recommend major revision.
SPECIFIC COMMENTS
1. The implementation of predicted rime mass in the DESCAM scheme is a major development. The authors go from describing the new method to attempting to illustrate the impacts through a full real case 3D simulation. This is a big leap. Understanding and evaluating the changes to a microphysics scheme is complicated enough; the authors have gone directly to the most challenging approach. Microphysical pathways in a 3D model are very complicated and evaluation based on comparison to observations is inherently challenging. For a major development of the type presented in this study, the authors should really start by illustrating the behaviour of the modified scheme in a very simple context, such as a 0D or 1D model framework, in order to provide the reader a basic understanding of how the new scheme works and what it does, as well as to illustrate that the changes do indeed do what they are supposed to do. I strongly recommend adding a section on idealized tests and demonstrations, even if it means reducing the amount that is presented for the 3D case.
2. The setup of sensitivity experiment is backwards. Normally in a control experiment one takes a baseline configuration, which is the control, and then sets out to conduct one or more sensitivity tests to examine the impact of one or more changes. In this case, the most logical setup would be to prescribe the control (or control simulation), which in this case would be the simulation with the unmodified DESCAM scheme, the control configuration, and define the experiment simulation to be the one with the modified code. Following from point 1, the one could define the CTR (baseline DESCAM) and MOD (DESCAM with rime fraction) configurations and apply them to the idealized simulations/demos and then to the ICEPOP case simulations.
3. When one uses a real-case simulation and comparison to observations to illustrate the benefits (or any kind of impact) of a particular set of changes to a model, one first needs to demonstrate that the control (unmodified) simulation is sufficiently realistic that one may proceed to use the modeling framework to meaningly examine the impacts of sensitivity tests using the modified model. This is why it is necessary to start off with the baseline control. When jumping straight to comparisons between observations and simulations with the modified model, as the authors have done, one cannot tell if discrepancies between the observations and the model are due to limitations of the model set up (which could include a number of things, starting with the initial conditions) or negative impacts resulting from the changes. Figures 5, 6, 7, 8, 10 compare observations and the simulation with the modified code, but no comparison to what should be called the control run (with no rime). Every comparison here should include simulations from both the original and modified microphysics scheme.
4. In the illustrations of model precipitation (Fig. 11), the observations are conspicuous in their absence. There was a dense network of surface precipitation observations in that region for ICEPOP. This needs to be added.
5. The explanation of particle density needs to be expanded upon in section 2.2, not just summarized by a reference to Heymsfield et al. 2018 (line 107). What is the density of ice with a rime fraction of 1? Is there no distinction between graupel and hail? I gather from line (“… it is currently not the case in DESCAM [variable density]”).
6. Line 349, “… in DESCAM [it is assumed that] ice particles melt instantaneously at the 0C isotherm”. This is a huge weakness in the DESCAM microphysics scheme in terms of modeling ice – it is not just a minor simplification. That might be fine for tiny crystal, but certainly not for ice that would be considered to be “snow” (large crystals or aggregates) or graupel. Presumably it would make more sense to address this deficiency in the scheme before adding predicted rime fraction. I definitely think the calculation of explicit melting should be added (it should be added anyway) in order to examine the impacts of rime fraction in the context of a case like ICE-POP. The rime fraction will affect the ice fall speed, which will affect the horizontal distance ice is transported before completely melting, which will therefore affect the spatial distribution of precipitation, particularly in the mountainous regions. So in order to understand the impact of predicted rime fraction, melting has to be treated more rigorously.
MINOR POINTS
1. The title could be improved.“Heavy snowfall event during ICE-POP 2018” by itself does not mean much; it is just a noun.
2. Line 310, “The overproduction of these small ice particles likely originating from a numerical artifact.” This sounds like a guess and is not very satisfactory, particularly given the negative impacts on the simulation.
3. The discussion on the impact the fixed rime density could be expanded upon. If variable density were to be added in DESCAM, this would exploit the predictive aspect of aerosols since the liquid droplet size is important for the rime density.
Given the magnitude of the major comments, I will stop with the minor points and address them in detail if/when a revised manuscript is submitted.