the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Mar 2026
Isotopic Evidence for Ice Growth by Riming in Precipitation
Abstract. In this study, we investigate the impact of riming on the relative composition of oxygen and hydrogen isotopes or d-excess in precipitation. Riming, where supercooled liquid droplets freeze directly on ice particles, is an important process of precipitation formation in mixed-phase clouds but has long been assumed to occur without isotopic fractionation. We used an independent indicator of riming, the terminal fall velocity of snow particles, and correlated it with the d-excess of snow or rain precipitation (ranging from –23 to +45 ‰) in polar (Arctic, Antarctic), mid-latitude and tropical regions. Our results show an inverse correlation of d-excess with terminal fall velocity, which increases with riming, indicating that lower d-excess reflects variable extents of riming during precipitation formation. The lower d-excess of rimed ice results from a partial loss of the accreted liquid by evaporation, and possibly splashing and shedding, before freezing is complete. This contrasts with a higher d-excess of ice that grows by the vapor deposition process. We conclude that low d-excess from riming can explain the spatial variations of Greenland and Antarctica surface snow that were previously attributed to changes in source moisture origin. Our results also help to explain the wide range of d-excess observed in daily precipitation compared to a much narrower range in surface snow or ice cores. Further, spatial or temporal differences in d-excess would allow for the estimation of variations in the rimed mass fraction, which, in turn, can be used as observational constraints for improving microphysics schemes in climate models.
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Status: final response (author comments only)
- CC1: 'Comment on egusphere-2026-687', Harald Sodemann, 16 Apr 2026
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RC1: 'Comment on egusphere-2026-687', Anonymous Referee #1, 23 Apr 2026
Review of "Isotopic Evidence for Ice Growth by Riming in Precipitation" by Aggarwal and co-authors, Manuscript egusphere-2026-687.
This manuscript speculates that riming in precipitating, mixed-phase clouds has a strong control on the deuterium excess (d-excess) of precipitation and could potentially explain spatial and temporal variations in paleoclimate records in ice cores, in addition to process-driven short term variability in d-excess in the present day. Relationships are first established between the deuterium excess of precipitation and fall speeds of ice-phase precipitation using both in situ measurements (with MASC) and radar retrievals of doppler velocity, with lower deuterium excess in precipitation with greater riming. These relationships seem to hold more broadly across regions, and the relationship between the doppler velocities above the melting layer with riming makes the connection between riming and d-excess. A potential mechanism for evaporative fractionation of partially-frozen droplets during the riming process is sketched out with some support from past literature. The manuscript claims that the riming effect on d-excess explains many of the d-excess signals seen in paleoclimate datasets.
Assessment: Major revisions
I find the manuscript provocative and interesting if speculative. This study presents empirical evidence for the fractionation effect of riming but spends little time enumerating/exploring other explanations for the trend, aside from rain evaporation. How much of the relationship between riming and precipitation d-excess comes from the differing isotopic composition of cloud liquid and precipitating ice? Riming presumably increases the precipitation efficiency so that more cloud liquid falls out as precipitation than in the absence of riming. How much would such an effect contribute to the riming d-excess relationship independent of any hypothesized evaporative fractionation of partially-frozen droplets? How much of the riming d-excess relationship might be explained by changes in the condensation temperature, relative humidity over ice during deposition, subcloud relative humidity during rain evaporation and/or changes in water vapor isotopic composition during the storms studied here?
In my view, a revised version of this manuscript should ideally do two things: 1) explore other potential controls on the d-excess (condensation temperature, relative humidity over ice above the melting layer, a fuller accounting for potential changes due to subcloud rain evaporation/equilibration, changes in the water vapor isotopic composition) to make a more compelling case for isotopic fractionation during riming, and 2) explain more clearly the distinction between isotopic changes due to fractionation during the riming process and the isotopic impact of having more cloud liquid included in precipitation. If the authors view item (1) above as beyond the scope of the manuscript, I would at least ask them to characterize the riming-d-excess relationship as a hypothesis and note that the observational analysis is primarily based on correlation.
Major comments:
1. For me, the most obvious impact of riming on precipitation isotopic composition is simply that cloud liquid might have a different isotopic composition than precipitating ice, and precipitation with different fractions of cloud liquid might have different d-excess. Why isn't this emphasized in the paper, or at least explored and discarded? Why do we need the hypothesized, ephemeral liquid layers to do so much work isotopically when this simpler explanation exists? Why would those liquid layers experience such strong fractionation when the surrounding air is close to liquid saturation (since cloud liquid droplets are present)? How could those liquid layers induced by riming have such a strong impact on the overall isotopic composition of precipitation at the ground when only a small fraction of the precipitation mass is in the liquid phase during the hypothesized fractionation?
2. While the precipitation fall speed above the melting layer is hypothesized to be the main control on precipitation d-excess here, the potential for meteorology or changes in the water vapor isotopic composition to affect the d-excess signal seems to have been mostly ignored. Could correlations between the presence of riming and other factors explain some of the riming-d-excess relationship in many of the figures? The condensation temperature, supersaturation during vapor deposition, subcloud relative humidity during rain evaporation and possibly the isotopic composition of water vapor may be available from radiosonde data, reanalysis or vapor isotope observations. Why not explore such alternative explanations for the signals seen here?
3. The deuterium excess of ice formed by vapor deposition varies nonlinearly with increasing distillation, even for an equilibrium process (e.g., Figure 7 of Kopec et al, 2018, https://doi.org/10.1029/2018JD028750). The main point here is that other processes (e.g., vapor deposition onto ice) can cause precipitation d-excess to fall without a riming signal. Such behavior is also seen when vapor deposition occurs under ice supersaturated conditions (e.g., Dutsch et al, 2019, https://doi.org/10.1029/2019MS001764, Figure 1).
Minor Comments (5/112 means page 5, line 112):
2/39: Newer reconstructions of local and remote temperatures from ice core data (Markle and Steig, 2022, Clim. Past, https://doi.org/10.5194/cp-18-1321-2022) seem to show somewhat more modest changes in ocean evaporation temperature of ~3 K. Their model relies on a nonlinear definition of the deuterium excess, which may be important when interpreting d-excess signals in polar regions.
2/49: When I look at plots of ocean evaporation d excess in Pfahl and Sodemann (2014), I find ranges of about 8-13 per mil (Figure 1b) and about 0-25 (Figure 2), a wider range than stated in the present manuscript. Why do those ranges differ from the one included in the present manuscript?
2/57-59: During the WBF process, the liquid saturation ratio is unlikely to vary substantially from one. As a result, one can make a clear estimate of the supersaturation over ice (as the ratio of e_sat_liquid(T) over e_sat_ice(T)) which varies from 0 at 0 degC to ~50% at -40 degC. If isotopic equilibrium between vapor and liquid is assumed and one chooses values for the isotopic composition of vapor, the deuterium excess difference between the ice and liquid phase can be computed explicitly, which results from both kinetic fractionation over ice and from the equilibrium fractionation factor differences between liquid and ice.
Looking at supplemental figure S1, point C with 200% supersaturation over ice seems unrepresentative of conditions in earth's atmosphere, where the threshold for homogeneous nucleation of ice is roughly 100% at the tropical tropopause. Emphasizing the extreme supersaturations possible at colder temperatures is a bit misleading because the deuterium excess of precipitation depends on the mass-weighted isotopic composition integrated across all hydrometeors that contribute to that precipitation. Relatively little deposition is possible at low temperatures where the saturation vapor pressure over ice is small.
2/63: "Most precipitation _events_ over land and oceans, ..." Those papers are focused on precipitation events rather than accumulated precipitation.
sec. 2.3.3 (Doppler velocity at the study locations): I wish the list in this section and section 2.2.X could be merged. This would help the reader stay engaged with the material rather than having to wade through a second set of details about the various datasets. This could probably be accomplished by labeling section 2.2 as "Study data" and have that include both "isotopic data" and the contents of sections 2.3, 2.3.1 and 2.3.2. Then, section 2.3 could detail the data available from the various sites.
Alternatively, some of this could also be moved to an appendix or the supplement, with a smaller summary remaining in this section.
16/figure 3: Both the mean doppler velocity and d-excess are uncertain, so total least squares is more appropriate here than linear regression. The same is true for the plots later in the paper. Arguably the mean doppler velocity and rime fraction are more uncertain than the d-excess.
16/413-417: It's difficult for the reader to assess this argument without more information.
20/sec 3.3 on subcloud evaporation: Echoing major comment 2, why isn't subcloud relative humidity considered in the discussion of subcloud evaporation? Presubambly such information is readily available from radiosondes adjacent to the radar sites, in addition to reanalysis.
22/Fig 5 caption: Please define Vr and Vs here.
24/632-633: "This implies that the isotopic analysis of precipitation sampled on an hourly or daily scale would essentially provide a ‘process-weighted’ value of d-excess." I agree with this sentence. Such an approach might be used to identify the potential role of fractionation by riming.
29/Data availability: Gathering the references in a single place is useful for the reader who would like to take advantage of those data. Please do that.
Typographical/wording Suggestions (in most cases, suggested changes are _between the underscores._):
9/255: "... at the inflection point _where the MDV begins to increase from its more nearly uniform values aloft (Fig. S2)." I find the language about the "vertical" gradient a bit confusing.
18/483: Suggested re-wording: "The average MDV in the snow region above the melting layer and the d-excess values in surface precipitation ..."
28/725: "While riming _is more common_ in warmer conditions, ..."
Citation: https://doi.org/10.5194/egusphere-2026-687-RC1 -
RC2: 'Comment on egusphere-2026-687', Anonymous Referee #2, 29 Apr 2026
Isotopic Evidence for Ice Growth by Riming in Precipitation
This paper compares isotope ratios in precipitation with doppler velocity measurements and photographs of snow and precipitating ice. An anti-correlation between doppler velocity and the precipitation deuterium excess (d-excess) is presented. The paper argues that this anti-correlation indicates that d-excess in precipitation can be thought of as primarily reflecting the extent to which precipitation particles are rimed, and that any effects of moisture source region are essentially overwritten.
While I find the correlations presented fascinating, I do not feel that they substantiate the claim that d-excess is primarily an indicator of riming. I recommend reconsidering how the correlation is presented and using it as an opportunity to suggest future research steps. Furthermore, I strongly recommend that the paper cites works that have already used numerical modeling approaches to quantify the contributions of moisture source and microphysical processes to the d-excess signal. Finally, I suggest revisiting the introduction, as some of the descriptions of d-excess sensitivity do not match the literature consensus. I also strongly feel that one must be cautious in drawing parallels between hailstones over the US Great Plains and the smaller rimed particles likely to be encountered in Antarctica and Greenland.
Specific major comments
A. d-excess sensitivity. The first part of the introduction discusses the response of d-excess to evaporation and depositional growth. Many statements in this section appear to contain errors or lack specificity.
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- For example, evaporation increases (it does not lower) the d-excess in the vapor compared to the liquid.
- Also, when describing ice crystal growth by WBF, it would be helpful to specify with respect to what is the d-excess higher (particles grown by other processes? The vapor?).
- Starting on line 61, the paper claims that the increase in d-excess growth during vapor deposition is much larger than the range of values resulting from moisture source variability; thus moisture source may only have a limited influence. However, there is no citation backing this.
- I strongly recommend looking at papers by Zhengyu Xia (https://doi.org/10.1029/2021GB007245, https://doi.org/10.1029/2022GL101901), which attribute variations in d-excess explicitly to moisture source variations and microphysical processes.
- Another possibly helpful resource is Gao et al. 2025 (https://doi.org/10.1029/2024JD043047).
B. Hailstones vs. rime. The second part of the introduction attempts to link higher delta values and lower d-excess values to riming conditions. Much of this background is pulled from older experimental studies modeling isotopic variations in hailstones, and I worry that much of it is irrelevant to the study at hand.
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- For example, I’m concerned that the conditions in which hailstones form are quite distinct from the Antarctic and Arctic conditions in which most riming discussed in this paper occurs. Specifically, I would imagine that the extent of latent heating, splashing, and evaporation of liquid water during hailstone production is much larger.
- Similarly, are wet and dry growth regimes as relevant for polar rime production?
- Keep in mind that none of these early hailstone works were able to measure the supercooled liquid water in cloud for comparison. Lowenthal et al. 2016 did in fact measure both snow and cloud condensate, however, they mostly found insignificant differences between these two reservoirs. In fact, they assumed that snow would look more like cloudwater isotopically as a way to define the extent of riming.
- Perhaps more to the point, can the data evaluated here even distinguish whether rime has a lower d-excess than the cloud material from which it formed?
C. Correlation not causation. Figures 3 and 4 are the primary scatter plots arguing for a casual relationship between d-excess and riming (where doppler velocity is the proxy for riming). This relationship is fascinating; however, I am not convinced that these analyses alone allow us to claim that riming is the primary driver of differences in d-excess. For consideration….
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- How does the height of particle growth change with riming?
- Is riming more customary with warmer, wetter conditions? (See Wedum, Pettersen et al. 2026 as an example https://doi.org/10.1029/2025JD044309.)
- What synoptic conditions favor riming? J-L Bonne et al. 2015 found that warmer and moister conditions at Greenland (not only promote riming but) can also accompany significant changes in moisture source.
- Finally, to what extent does below cloud exchange matter?
- I don’t feel that the paper can rule out these other possible factors. I thus recommend that the paper revisit how it presents its results, backing off some of the bolder claims.
- It also feels like the paper largely ignores the seasonal variation in Greenland: the winter values suggest no correlation (or maybe a slightly positive one?) between d-excess and doppler velocity. How does this finding influence confidence in the results?
D. Photograph patterns. I appreciate that the paper looks for specific evidence of riming in precipitation photographs, but I have a hard time understanding how the table of photos supports the claim that d-excess decreases primarily with riming (MDV, or doppler velocity). Row B and F show two very different particle types with similar MDV but very different d-excess. Row D, the moderately to heavily rimed dendrites, have large MDV but medium d-excess. Based on the scale bars, is it possible that there is a stronger connection with particle size, with smaller particles resulting in lower d-excess?
Citation: https://doi.org/10.5194/egusphere-2026-687-RC2 -
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Comments on "Isotopic evidence of ice growth by riming in precipitation" submitted to ACP by Aggarwal et al.
The authors present work that proposes to revise the interpretation of the deuterium excess in precipitation and ice cores in terms of riming versus deposition processes in clouds. Based on analysis based on Greenland summer precipitation, a global relationship is constructed (using existing published datasets) between fall velocity and d-excess. The authors propose that the riming process dominates over other factors that may influence the d-excess, such as moisture sources and moisture transport history.
It is my impression that the authors bring a potentially important factor to light that could influence the d-excess in global precipitation. The manuscript also does a great job in compiling some of the literature to relate microphysical pathways to the isotopic composition of precipitation. However, I think that the evidence presented here is not convincing enough to dismiss or rule out that other factors than the one investigated here still are more important for the d-excess in precipitation.
In particular the proposed relevance for the interpretation of ice cores is in my view stretched too far, as many other factors, including post-depositional effects such as homogenisation of the top snow layers, vapour-snow exchange, and ablation effects can play into ice core signals, but none of these is discussed. In my view, the manuscript would benefit from not claiming that riming vs. deposition is the single most important factor, and rather presenting it as another factor that should be taken into account. This would open up an alley towards further quantification of the proposed effect from laboratory and field studies.
Specific comments
The cited evidence for isotope fractionation during riming appears to rely on one single laboratory study (Bailey et al., 1969). This study and the accompanying modelling study by Jouzel (1985) were done in the context of hail formation. While hail can be seen as an end member of the processes leading to graupel formation, there is thus no direct evidence for isotope fractionation during riming for smaller hydrometeors. The proposed process that involves fractionation due to evaporation of accreted liquid before freezing is complete should thus be presented as hypothetical. It would make the manuscript overall stronger (more valuable) if this knowledge gap is pointed out throughout the interpretation, and in the conclusions.
The analysis presented in Table 3 and Figure 2 is interesting, but appears difficult to reproduce. Some statistics should be given of how many snow flakes have been analysed and how much variation was observed. The selection of these few selected snow samples does also not rule out that other factors, such as the d-excess induced during equilibrium fractionation (Sodemann et al., 2008; Dütsch et al., 2017) or the moisture source conditions (Pfahl and Sodemann, 2014; Sodemann et al., 2024) influenced the d-excess if this has not been explicitly investigated.
In L. 61 and the paragraphs before, the authors claim that "the increase in d-excess during ice growth by vapour deposition is much larger than the range of lower values resulting from source moisture difference". However, this conclusion is made from an uneven paring between the global mean d-excess variation and local supersaturation near liquid droplets. Indeed, similar or even larger super- and sub-saturation regimes (RH<30%) can exist over oceans, for example when dry and cold arctic air is advected over the ice-free ocean and then warmed up in strong-wind conditions (Duscha et al., 2022; Sodemann et al., 2024). During such situations, the d-excess from water vapour evaporating under strong non-equilibrium conditions can reach 30 ‰ or more, quite comparable to the numbers cited for deposition growth.
The introduction recounts different interpretations of the d-excess, but lacks a clear description of how the d-excess arises from differences in diffusion speed between isotopologues during supersaturated conditions. It would be useful to include such a description to facilitate the physical interpretation of the proposed fractionation due to riming. Note that also equilibrium fractionation can induce a d-excess signal, arising from the definition of the delta scale (Dütsch et al., 2017).
The evidence presented in Fig. 3 is puzzling. There appears to be a relation for the summer data, but not for the winter data. The regression line is not related to the depicted summer or winter data, but taken from Fig. 2. A regression line and confidence bounds for the winter and summer data would be instructive here. However, the fact that the winter data appear to show no or even an inverse relation substantially weakens the evidence for the proposed process put forward here. If there is no consistent relation across seasons at the same site, why should the signal translate to ice cores, or be dominating at other global sites? The explanation proposed in L. 410 does not really address or remedy such concerns.
The evidence based on various datasets from different climate zones presented in Fig. 4 is intriguing. Importantly though, the figure leaves the questions of how important other factors could be in shaping this relation unaddressed. Each of the sites has different moisture sources, for example, and may be dominated by different transport pathways and fractionation conditions during transport. Accordingly, the contribution of such factors to the d-excess signal observed in the precipitation can not be ruled out, unless it has been investigated. Again, the Greenland data is evidence that other factors are important for the d-excess than fall velocity (as a proxy for riming). I don't think this is a problem in itself, but again the study would be significantly more impactful if such deficiencies are acknowledged, and the implications in the abstract and conclusions revised accordingly.
In L. 612, it is stated that the Greenland winter data is excluded from the regression. This choice seems to require a more rigid justification.
The relative location of all data points in Fig. 5 and Fig. 6 appear identical, except for the horizontal axis having a different scale. Does it make sense that the RMF obtained from the method of Kneifel and Misseev (2020) is just a linear scaling of the MDV?
My impression is that Sec. 4.2 is not needed and opens quite a lot of topics that would need to be assessed in more detail, also taking current interpretations into account. This includes the effect of moisture source and transport changes with different climates, and post-depositional effects in low-accumulation regions of Greenland and Antarctica. Given the weak links in some of the argumentation towards the importance and general validity of the proposed riming effect in the isotope composition, as indicated in the comments above, a more tentative interpretation, taking into account such uncertainties, would appear to make for a more impactful paper, as this could trigger new research to corroborate this new hypothesis.
References
Duscha, C., Barrell, C., Renfrew, I.A., Brooks, I. M., Sodemann, H., and Reuder, J.: A Ship-Based Characterization of Coherent Boundary-Layer Structures Over the Lifecycle of a Marine Cold-Air Outbreak. Boundary-Layer Meteorol 183, 355–380 (2022). DOI: https://doi.org/10.1007/s10546-022-00692-y.
Dütsch, M., Pfahl, S., and Sodemann, H., 2017: The impact of nonequilibrium and equilibrium fractionation on two different deuterium excess definitions. Journal of Geophysical Research: Atmospheres, 122, 12,732–12,746. https://doi.org/10.1002/2017JD027085.
Sodemann, H., Weng, Y., Touzeau, A., Jeansson, E., Thurnherr, I., Barrell, C., et al. (2024). The cumulative effect of wintertime weather systems on the ocean mixed-layer stable isotope composition in the Iceland and Greenland Seas. Journal of Geophysical Research: Atmospheres, 129, e2024JD041138. https://doi.org/10.1029/2024JD041138.
Sodemann, H., Masson-Delmotte, V., Schwierz, C., Vinther, B. M. and Wernli, H., 2008: Inter-annual variability of Greenland winter precipitation sources. Part II: Effects of North Atlantic Oscillation variability on stable isotopes in precipitation, J. Geophys. Res., 113, D12111, doi:10.1029/2007JD009416.