the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Apr 2026
Meso-NH-ISO v1.0: a water stable isotopes scheme in the non-hydrostatic mesoscale atmospheric model Meso-NH. Application to a 2D West African squall line
Abstract. Better understanding how convective processes impact the isotopic composition of atmospheric water has implications for understanding present-day phenomena such as squall lines or tropical cyclones, and for reconstructions of past rainfall extreme events. With this motivation, we implemented water stable isotopes in the non-hydrostatic mesoscale atmospheric model Meso-NH. Water stable isotopes are implemented in the advection of water phases and in the microphysical scheme. The implementation is validated on a test case of a 2D simulation of a tropical squall line observed in June 1981 during the COPT81 field campaign in the Sahel region. The isotopic version of Meso-NH (Meso-NH-ISO) captures the expected evolution of the isotopic composition of both precipitation and water vapor along the squall line. This work opens the door to future isotopic studies using realistic cases of mesoscale convective systems (squall lines, tropical cyclones).
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-548', Anonymous Referee #1, 24 May 2026
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RC2: 'Comment on egusphere-2026-548', Anonymous Referee #2, 14 Jul 2026
Review of "Meso-NH-ISO v1.0: a water stable isotopes scheme in the non-hydrostatic mesoscale atmospheric model Meso-NH. Application to a 2D West African squall line" by Barthe, Vimeux, Risi and Francois, Manuscript egusphere-2026-548
Summary: This paper introduces Meso-NH-ISO, a water-isotope-enabled version of Meso-NH, a numerical model for the study of mesoscale meteorology. The implementation of water isotopes is described, and a 2D simulation of an observed squall line case is presented. The original squall line case didn't include observations of isotopes, so water isotope observations from other West African squall lines are used to inform (qualitatively) the performance of the water isotope implementation and simulation.
Assessment: The paper does its job well, introducing the model, its implementation and performance in a demonstration simulation, while highlighting the water isotopic signals in rain evaporation and how they relate to different regions/phenomena within squall lines. I have some questions about details of the isotopic implementation below, specifically the Bergeron process and the kinetic fractionation of deposition onto ice, and ask for clarification from the authors.
Recommendation: Minor Revisions. The comments below are minor, but I would be curious to read the responses to my queries about the details of the isotopic implementation.
Minor Comments (5/112 means page 5, line 112, REFERS TO TRACK CHANGES VERSION):
Reference: Looking at this analysis of rain evaporation made me think of the bin model implementation of H218O in Hiron and Flossman (2020, https://doi.org/10.1029/2019JD031753). They have a plot of isotopic composition as a function of droplet size in their figure 8. Not sure that paper needs to be cited here, but I find it interesting to think about how the broad range of radii in the rain category of a bulk scheme might have a variety of isotopic compositions in reality.
4/85: The arrow representing the Bergeron process in Figure 1, which connects cloud liquid water directly with cloud ice, causes some concern. My understanding of the Bergeron process is the transfer from cloud liquid water to cloud ice occurs through the vapor phase due to the differing saturation vapor pressures of liquid and ice. Cloud liquid evaporates (presumably at saturation with liquid and in isotopic equilibrium, since the scheme uses saturation adjustment), and deposition onto ice occurs in supersaturated conditions with fractionation as described in section 3.2.3. If this is the way that the Bergeron process is implemented, please adjust the arrow in Figure 1. If not, please explain in the reply how the Bergeron process is implemented and whether that representation of the process differs from what I've described here.
7/168: The exponents in the formula for dynamic viscosity should be "-5", not "5" as is shown in the pdf.
8/191: Add "equilibrium" when describing alpha_l/v
8/195: Couldn't c^{'iso} be described more simply as "c^{'iso} = c' (D/D_iso)^(1/3)"? This simpler definition might help the reader focus on the differences between the light and heavy water equations.
sec 2.3.2-2.3.3: I'm curious about how the ventilation factor affects the kinetic fractionation of water isotopes as a function of drop or particle size. If we defined alpha_k = D_iso*f_iso/(D*f) = (Diso/D)^n, the exponent n would tell us how strongly the different diffusivities impact the isotopic transfer between vapor and condensate across those particle sizes. Presumably, n~1 for small particles and decreases from there. Stewart (1975, https://doi.org/10.1029/JC080i009p01133) suggests such a formulation in the paragraph after their equation 1.
9/eqn 31: I compared this formulation of alpha_k with equation B26 in Blossey et al (2010), whose b and S_s seems equivalent to the present manuscript's F and (S_i+1), respectively. The two formulas don't quite match, making me wonder whether the (S_i - 1) terms in the numerator and denominator of equation 31 should actually be (1 + S_i), since S_i is defined as the supersaturation over ice just after equation 26. Please double check. If I've misunderstood something, please do clarify. I'm trusting that this is coded correctly in the code regardless.
10/246: Move the sentence about open boundary conditions to the the end of the paragraph on lines 239-240, since (in my mind) the boundary conditions should be included in the description of the model domain. Could the impact of inflow at the boundaries on water and its isotopes be described briefly? Something like: "Where inflow occurs at the domain boundary, the temperature, water vapor mixing ratio and its isotopic composition are taken from the values in the initial soundings at that height." I think an additional sentence would make more clear the impact of the open boundaries. Also, I didn't quite understand what was meant by "There is no lateral advection." Perhaps, clarifying that would also help the reader.
10/sec 4.3: While the water vapor mixing ratio and the isotopic composition of precipitation do reach a quasi-steady state from hours 6-8, the rain rate itself is changing with time (by about a factor of two) during this period. While the quasi-steady-state of some variables is interesting, the simulation isn't really in steady state, so perhaps a different name for section 4.3 would work better, maybe "Quasi-steady-state regime" or "Mature squall line behavior" or something like that?
12/290: "Fig. 4c and _4d_". In the figure itself, the fourth panel is labeled "e", which is consistent with the text but surprising to this reader.
13/305: Is this the first time (\delta^{18}O_p)_{eq} appears? I feel that this variable and (d_p)_{eq} appear so often in this paper that they deserve numbered equations defining them, perhaps back in section 3. I realize that equation 11 does this implicitly, but for the uninitiated, perhaps the definition could be made explicit.
14/316: The diffusivities of HD^16O and H_2^18O differ but only slightly, by about 3 per mille of each other (e.g., Merlivat and Jouzel, 1979, bottom of p. 5030). The larger fractionation factor of HDO relative to H218O makes it likely that the jump in isotopic composition (expressed as delta) between rain and vapor is larger (by roughly that factor of eight) for HDO, leading to fluxes that are roughly eight times larger, with a slight modification because D_HDO > D_H218O. Perhaps, this could be rephrased in terms of fluxes rather than as "eight times faster.
14/317-319: Re "rain evaporation leads rain to be more enriched ..." Graf et al (2019, https://doi.org/10.5194/acp-19-747-2019) nicely talks about this and could be a good reference here or wherever the d18O_p - (d18O_p)_eq quantity is introduced, because Graf spends a lot of time thinking about how that and the d_excess equivalent behave.
18/373-376: Lee and Fung (2008, https://doi.org/10.1002/hyp.6637) analyze how much the near-surface layer will affect the isotopic composition of precipitation to understand its dependence on drop size and rain rate. Perhaps, such arguments could apply here.
19/Fig 8: Please add thin, possibly dashed line showing the zero values in panels a-c, including a vertical one in panel c. This would make it easier for the reader to see the sign changes of disequilibrium and enriching/depleting rain-vapor exchange. If the authors don't like lines, at least add tick marks on the right-hand and possibly top axes.
20/Fig 9: Do the high evaporation ratios happen only in regions where the mixing ratio is very small?
21/431: "... despite the non-isotopic steady state of the near-surface water vapor ..." As noted above in my comment about section 4.3, I don't think the transient behavior of the isotopic composition of near-surface vapor is a problem. I would suggest rephrasing in a more neutral tone: "While the isotopic composition of near-surface vapor continues to evolve late in the simulation while the water vapor mixing ratio and the isotopic composition are quasi-steady, the isotopic results are actually ..."
21/435: "minor problem". As above, I don't think this is a problem.
SI/Fig S1: This is repeated in the pdf I have. Please check.
SI/Fig S2: Could additional panels be included showing the d18O and d excess of all non-precipitation condensed water? This could strengthen the argument about why the precipitation just below the melting layer is more depleted in the stratiform region.
SI/Fig S6: Same comments about zero lines as for Fig 8 above.
Typographical suggestions:
9/206: "interacts"
12/284: "snapshots"
21/432: "This is also _true_ earlier in the simulation ..."
22/457: "The _plans for_ this implementation ..." Is a simpler phrasing.
Citation: https://doi.org/10.5194/egusphere-2026-548-RC2
Model code and software
Meso-NH v5-5-0 Meso-NH team http://mesonh.aero.obs-mip.fr/mesonh55/
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The paper describes formulation and an application of a new extension to the Meso-NH mesoscale atmospheric cloud-resolving model. The extension enables the model to capture the dynamics of water isotopic composition across gaseous, liquid and solid phases of water. A single-moment mixed-phase bulk scheme is used. Three stable isotopologues are considered: light water, semiheavy water HDO, and heavy-oxygen water (Oxygen-18). The example application uses a two-dimensional setup from a prior Meso-NH study (1994) simulating a tropical squall line observed during a field project back in 1980-ties.
I consider the presented material matching well the journal scope, and useful for the community. In the following comments, there are numerous suggestion for how to improve the description, but overall I find the paper well written and balanced. My review focuses on the microphysics description and the isotope "basics", for I have less experience in interpretation of the isotopic features of large-scale weather systems or the hydrological implications.
Abstract:
Section 1:
Section 2:
Section 3:
Section 4:
Section 5:
Table 1:
Supplement:
Maths, symbols, nomenclature:
Code:
From software engineering standpoint, the repeated definition of constants across the codebase (15 times) and the lack of code reuse (i.e., copy-pasted lines instead of shared function definition) are both anti-patterns. This approach leads to bug-prone code, technical debt, high software maintenance costs, and hindered opportunities for future developments.
References:
HTH