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

Barthe, Christelle; Vimeux, Françoise; Risi, Camille; François, Sören

doi:10.5194/egusphere-2026-548

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https://doi.org/10.5194/egusphere-2026-548

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09 Apr 2026

| 09 Apr 2026

Status: this preprint is open for discussion and under review for Geoscientific Model Development (GMD).

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

Christelle Barthe, Françoise Vimeux, Camille Risi, and Sören François

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).

Received: 29 Jan 2026 – Discussion started: 09 Apr 2026

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Christelle Barthe, Françoise Vimeux, Camille Risi, and Sören François

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RC1:
'Comment on egusphere-2026-548', Anonymous Referee #1, 24 May 2026 reply
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:
suggest rephrasing "present-day phenomena such as squall lines..." as it suggests that a squall-line is something unique to present day

the abstract would benefit from more specifics regarding what type of model is presented in the paper: single-moment mixed-phase bulk, kinetic fractionation considered, deuterium and oxygen-18 isotopologues tracked, ...

Section 1:
despite defining isotopic delta only in eq. (9) - 5 pages later, it is already used on page 1/line 11, suggest rephrasing to (deuterium and oxygen-18)

line 25: "So far, modeling studies have relied on models with parameterized convection" seems to contradict later statement on existing cloud-resolving models or large-eddy simulation models

line 31: worth mentioning as well the DESCAM isotope module: Hiron & Flossman 2020 (DOI:10.1029/2019JD031753) and commenting on the type of microphysics description employed in each model (single-moment, multi-moment, bin; liquid- vs. mixed-phase, which isotopologues tracked, ...)

line 41: typo: "third", not "thirst"

Section 2:
first paragraph: among the 17 literature references provided for Meso-NH, several have no relevance for the presented development, suggest referring to references given within Lac et al. 2018, and focusing in this paragraph only on the model components which have further usage in the text;

line 60: does the per-mass-of-dry-air definition also apply to (specific) number concentrations?

line 69: use \textsuperscript{th} instead of math mode to avoid italics

line 89: would "ablate" instead of "decay" be a better suited word here? (see, e.g., DOI:10.1029/2019MS001689)

line 93: this line needs elaboration as it is not understandable without getting acquainted with the referenced paper and it is unclear what "suppressed" implies here?

Section 3:
despite the "implementation" in the title, there is no discussion of implementation in this section, it rather deals with model formulation, what about "Formulation of the Meso-NH-ISO water stable isotopologues model"?

line 98: suggest highlighting that the approach implies that enabling the isotopic fractionation dynamics does not alter any of the model results that would be obtainable with the fractionation disabled (IIUC)

line 101: "duplicated twice" sounds like quadrupled, but here you get three entities, right?

line 102: "tracked" or "considered" rather then "implemented"

eq. (7): nabla is missing

line 105: the paper deals with 2D simulation, hence suggest omitting "3D" here

line 108: "source species" sounds like referring to the source term, better "light species"?

line 118: VSMOW ratios are defined for atoms, not molecules, worth commenting on it (https://web.archive.org/web/20200729203147/https://nucleus.iaea.org/rpst/documents/VSMOW_SLAP.pdf)

eq. (9): inclusion of the 1000 factor in the definition is misleading - this is just a unit for reporting the values; analogously, the definition of the mixing ratio is "m_v/m_d" and not "1000*m_v/m_d" even though you report it in g/kg; and rain rate definition does not feature 60 or 1000 multipliers for second-to-minute and meter-to-mm conversions; I am aware that this is common in hydrological literature, but still suggest just commenting that the common unit is per mille (see also eq. (1) in the above-linked IAEA Reference sheet); same for unit of d-excess in Table 2 (should be dimensionless) and delta units in Tables 3 & 4 (dimensionless).

eq. (10): explain the "meteoric water line" concept, elaborate on the 8 factor (cf. page 14/line 316!) and refer to Craig 1961 (DOI:10.1126/science.133.3465.1702) where it was first given

eq. (10): suggest using a short dash instead of "minus" for typesetting "d-excess"

subsection 3.2: suggest highlighting the relation with vapour (super)saturation

subsection 3.2.1: the handling of the Bergeron process should also be elaborated on here, and the implication for isotopic fractionation highlighted

eq. (16): according to Table 2, D_v should be in SI unit m²/s, but the value 0.211 yields a result in cm²/s (it is correct in the code, see also a note code quality below)

eq. (23): the text above suggests that Pruppacher & Klett provided the isotopic formula, which is not the case, suggest simply referring to eq. (14) above or citing a work which covered isotopologue mass transfer (e.g., Gedzelman & Arnold 1994, DOI:10.1029%2F93JD03518)

line 206: "interact" -> "interacts"

Section 4:
line 223: suggest changing "fully observed" to "subject to comprehensive radar observations" (also in line 427)

line 224: "simulated with ... numerical experiments" -> "simulated in ... numerical experiments" (?)

line 230: "short-live" -> "short-lived"

line 233: it is unclear if 40% concerns mass or number

line 246: "open" contradicts "periodic", and this is important to define if the isotopes are simulated in an open or closed reservoir (also in the context of later discussion on lack of steady state in vapour isotopic composition)

line 246: clarify if "no lateral advection" refers to Y dimension?

line 251: elaborate on what are the implications of using a 3D turbulence scheme in a 2D simulation

line 254: is "Steady-state" a good title for a section concluding that "isotopic composition of water vapor does not reach equilibrium at the end of the simulation"

line 263: "moist and" -> "moist air"

line 276: the text suggests that there is information about ice in the figure, but given that "total condensed mixing ratio" likely sums all five non-vapour water species, it is hard to interpret the figure. Suggest plotting liquid and solid water separately, and adding plot with relative humidity wrt ice

Figure 3: y-axis label should be "r_v" not "q_v" if it is consistent with the caption, otherwise the caption is inconsistent with the text, and the symbol tables need addition of specific humidity

Figure 4 caption: "kJ" not "kj"

line 293: "Isotopic results" sounds awkward to me, and perhaps "Simulated isotopic composition of ambient vapour and rain water"?

line 312: "We also probably detect rain evaporation within downdrafts" suggests as if it was not possible to confirm, while running the simulation with a given process disabled should provide a clear way of confirming it. There are several features commented as occurring "probably" or "likely"; given the low computational cost of a 2D simulation, I expect that some of these are verifiable by exploring simulation results with the processes in question toggled on or off.

line 344: "minima" instead of "minimums" (as with datum->data)

line 409: use "\Delta t" for the time step, not "dt"

Section 5:
line 431: "non-isotopic steady state" is not clear (also line 434), please rephrase

line 433: rephrase "region on development"

line 436: "well captures" -> "captures well"

Table 1:
values for "c" and "C" are missing $\times$ symbols (3 occurences)

Supplement:
Figures in the Supplement are raster graphics (screenshots) not suitable for publication

according to GMD guidelines (https://www.geoscientific-model-development.net/submission.html): "Supplementary material is reserved for items that cannot reasonably be included in the main text or as appendices." (videos, very large images, maps, CIF files, as well as short computer codes...). Please incorporate the supplementary figures into the main body of the manuscript, especially that these are repeatedly mentioned in the text and required to follow the narrative.

Maths, symbols, nomenclature:
the term "isotope" applies to atoms, while "isotopologue" to molecules, hence suggest replacing "Water stable isotopes" with "water stable isotopologues" (incl. the title and keywords)

number concentrations have a per-mass definitions in the paper, which contradict the common per-volume understanding of the term, "specific concentration" would be a better name

use upright font for label-like non-value subscripts, e.g. $\rho_\text{a}$, not $\rho_a$, etc (D_v, k_a, L_s, L_v, m_r, N_c, N_r, r_c, r_r, r_v, R_c, R_i, R_l, ...)

both "isotope ratio" and "isotopic ratio" terms are used in the paper, please unify, see e.g. https://list.uvm.edu/cgi-bin/wa?A2=ind2110&L=ISOGEOCHEM&P=R40040

it is unclear from the text if "total water" includes all isotopic species or only the light isotopologue?

semiheavy water is sometimes labelled as HDO, and in other instances as HD¹⁶O, suggest unifying

Code:
the model name and version given in the paper title (Meso-NH-ISO v1.0) are not consistent with the code archive (MNH-V5-5-0), and are not mentioned at the linked website (http://mesonh.aero.obs-mip.fr/mesonh55); the code archive must include the same labelling as the paper title

the Zenodo archive Barthe 2026 lists Creative Commons as license, but the CC licenses are not applicable to software (https://creativecommons.org/faq/#can-i-apply-a-creative-commons-license-to-software), and the contents of the archive and paper text indicate CeCILL-C license.

the text indicates that the Zenodo archive contains "complete code of Meso-NH v5-5-0 including the water isotope scheme", but Zenodo metadata indicate: "Fortran routines to be inserted in Meso-NH V5-5-0 to use the water isotope scheme."

the Zenodo metadata indicate three authors: Barthe, Francois and Vimeux; the reference entry in the paper lists just one

the large archive size of 234 MB is due to inclusion of "convenience copies" of external libraries and tools, which is neither expected nor described anywhere, and I'd consider it a malpractice as the libraries are straightforward to obtain. Examples:
84.7 MB: MNH-V5-5-0/pub/ncl-6.4.0.tar.gz

18.8 MB: MNH-V5-5-0/src/LIB/netcdf-c-4.7.4.tar.gz

16.2 MB: MNH-V5-5-0/src/LIB/oasis3-mct_v3.tar.gz

12.0 MB: MNH-V5-5-0/src/LIB/hdf5-1.12.0.tar.gz

11.0 MB: MNH-V5-5-0/src/LIB/eccodes-2.18.0-Source.tar.gz

8.6 MB: MNH-V5-5-0/src/LIB/RAD/ecrad-1.4.0.tar.gz

6.5 MB: MNH-V5-5-0/pub/ga-5-0-3.tar.gz

Moreover, the MNH-V5-5-0/bin_tools folder contains 43 MB of binary files which are not portable (e.g. would work only on Linux systems with outdated libpng12.so.0 present), and also not expected to be included in a source archive.

aiming to double check the correctness of the diffusivity formula (16) constant value given with non-SI unit in the paper, and expecting a single code location for the constant, I have spotted a troubling amount of code duplication:
src/src_isotopes/isotopes_tendencies.f90:528: ZDV(:) = 0.211E-4 * (ZZT(:)/XTT)**1.94 * (XP00 / ZPRES(:))

src/src_isotopes/ice_adjust.f90:625: ZDV(:,:,:) = 0.211E-4 * (ZT(:,:,:)/XTT)**1.94 * (XP00 / ZPRES(:,:,:))

src/SURFEX/canopy_blowsnw_subl.F90:78: ZDV(:,JK) = 0.211E-4 * (PT(:,JK)/XTT)**1.94 * (XP00 / PP(:,JK))

src/MNH/lima_tendencies.f90:478: ZDV(:) = 0.211E-4 * (ZT(:)/XTT)**1.94 * (XP00 / PPABST(:))

src/MNH/ice4_tendencies.f90:430: ZDV(JL) = 0.211E-4 * (ZT(JL)/XTT)**1.94 * (XP00 / PPRES(JL))

src/MNH/lima_adjust.f90:530: ZDV(:) = 0.211E-4 * (ZZT(:)/XTT)**1.94 * (XP00 / ZPRES(:))

src/MNH/lima_adjust.f90:823: ZDV(:) = 0.211E-4 * (ZZT(:)/XTT)**1.94 * (XP00 / ZPRES(:))

src/MNH/lima_adjust.f90:935: ZDV(:) = 0.211E-4 * (ZZT(:)/XTT)**1.94 * (XP00 / ZPRES(:))

src/MNH/lima_notadjust.f90:353: ZDV(:) = 0.211E-4 * (ZZT(:)/XTT)**1.94 * (XP00 / ZPRES(:))

src/MNH/subl_blowsnow.f90:376: ZDV(:) = 0.211E-4 * (ZZT(:)/XTT)**1.94 * (XP00 / ZPRES(:))

src/MNH/rain_ice_elec.f90:2455: ZDV(:) = 0.211E-4 * (ZZT(:)/XTT)**1.94 * (XP00 / ZPRES(:))

src/MNH/ch_aqueous_tmicice.f90:1059: ZDV(:) = 0.211E-4 * (ZZT(:)/XTT)**1.94 * (XP00 / ZPRES(:))

src/MNH/lima_cold_slow_processes.f90:324: ZDV(:) = 0.211E-4 * (ZZT(:)/XTT)**1.94 * (XP00 / ZPRES(:))

src/MNH/lima_mixed.f90:440: ZDV(:) = 0.211E-4 * (ZZT(:)/XTT)**1.94 * (XP00 / ZPRES(:))

src/MNH/rain_ice_slow.f90:120: PDV(:) = 0.211E-4 * (PZT(:)/XTT)**1.94 * (XP00 / PPRES(:))

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:
fix DOI in Caniaux et al. 1994 (should be 10.1175/1520-0469(1994)051<2046:ANSOTS>2.0.CO;2)

fix DOI in Chang et al. 1993 (should be 10.1175/1520-0469(1993)050<0161:TDMSOS>2.0.CO;2)

fix DOI in Chong et al. 1987 (should be 10.1175/1520-0493(1987)115<0670:ATSLOD>2.0.CO;2)

fix DOI in Gamache & Houze 1981 (should be 10.1175/1520-0493(1982)110<0118:MAMAWA>2.0.CO;2)

fix DOI in Hall & Pruppacher 1976 (should be 10.1175/1520-0469(1976)033<1995:TSOIPF>2.0.CO;2)

fix DOI in Meyers et al. 1992 (should be 10.1175/1520-0450(1992)031<0708:NPINPI>2.0.CO;2)

fix DOI in Redelsperger & Lafore 1988 (should be 10.1175/1520-0469(1988)045<1334:ATDSOA>2.0.CO;2)

HTH

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Citation: https://doi.org/10.5194/egusphere-2026-548-RC1

Christelle Barthe, Françoise Vimeux, Camille Risi, and Sören François

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https://doi.org/10.5194/egusphere-2026-548-supplement

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Meso-NH v5-5-0 Meso-NH team http://mesonh.aero.obs-mip.fr/mesonh55/

Christelle Barthe, Françoise Vimeux, Camille Risi, and Sören François

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Short summary

We implemented water stable isotopes in the non-hydrostatic mesoscale atmospheric model Meso-NH. We validated this isotopic version (Meso-NH-ISO) with a simulation of a well-documented squall line in the Sahel region. In future works, simulations with Meso-NH-ISO will be done on real cases of cyclones or squall lines. The goal is to better interpret and quantify isotopic observations on the field in terms of atmospheric processes that drive development and intensity of those thunderstorms.


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