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| 29 Jul 2025
Seasonal Characteristics and Trends in Precipitation Partitioning in the Arctic
Abstract. Driven by growing impacts of changing precipitation amounts and phase on the Arctic’s natural and built environment, we examine seasonal patterns and trends in Arctic precipitation and partitioning between its liquid and solid forms. Use is made of data from the ERA5 reanalysis, Automated Surface Observing System stations over land, and a climatology based on present weather reports over the Arctic Ocean. In the Atlantic sector of the Arctic, most precipitation falls as rain in all seasons in the extreme south, but snowfall is high over its northern parts. Annual precipitation over the dry central Arctic Ocean and terrestrial polar deserts almost always falls as snow. Even during the summer, typically 50 % of precipitation over the central Arctic Ocean falls as snow. Over land, nearly all summer precipitation falls as rain, except in the Canadian Arctic Archipelago where summer snowfall is still common. Annual precipitation has increased since 1979, primarily in the Barents Sea sector, accompanied by generally downward trends in snowfall and, hence, upward trends in liquid precipitation. Across much of the Arctic, the rainfall to total precipitation ratio has increased only in summer, while in the Atlantic sector, the rainfall to total precipitation ratio has increased in all seasons.
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RC1: 'Interactive comment for "Seasonal Characteristics and Trends in Precipitation Partitioning in the Arctic" by Cast et al.', Anonymous Referee #1, 12 Aug 2025
Summary:
This study explores patterns in precipitation partitioning (rainfall vs. snowfall) in the Arctic using ERA5 for 1979-2023. The authors provide a detailed breakdown of seasonal rainfall/snowfall occurrence, illustrating the more pronounced shift between winter snowfall and summer rainfall over land and the relatively high occurrence of snowfall over the central Arctic year-round. Temporal changes over the period are also explored, highlighting the shift towards higher liquid precipitation occurrence as temperatures have risen in recent decades.
The manuscript is very well written and provides a novel Arctic-wide analysis of precipitation and its partitioning between rain and snow, exploring both spatial and temporal patterns. The figures and discussion throughout are largely clear and fit well within the scope of the journal. I have several comments below that are primarily for clarification and increased discussion on a few points, particularly in the comparison of ERA5 to the other datasets included in the analysis.
General comments:
- For the ASOS data, where any missing data allowance thresholds applied? Figure 2 shows years of complete data – does this mean 100% coverage for the year, or is a threshold used to allow for years to be included if a small amount of data is missing?
- In Section 2.1, “liquid precipitation” and “rainfall” are discussed as separate variables – if I understand correctly, the rainfall is directly from ERA5 and liquid precipitation is calculated separately (as total precipitation minus snowfall), but this isn’t very clear in the text. Additionally, in the analysis they seem to be used interchangeably. For example, Figure 3 shows “Average rainfall/total precipitation ratio” in the bottom right panel, but the caption says “liquid/total precipitation ratio” and then Figure 6 uses the terminology “rainfall-to-total precipitation ratio”. The language should be made more precise so that it is clear when only rainfall is being analysed versus rainfall+mixed phase, or the definitions need to be made clearer if I have misunderstood the “liquid precipitation” defined in Section 2.1.
- Sections 3.3 and 3.4 are titled "Comparisons with ICOADS Records" and "Comparisons with ASOS Records", respectively, but there is little comparison of these datasets to the ERA5 results presented earlier. I think these sections would benefit from more direct comparisons to the ERA5 data (e.g., do the spatial patterns align? Do some variables (total precipitation, rainfall fraction, etc) see better/worse agreement than others?) to better illustrate agreement and disagreement between the different sources. In the conclusions, it is stated that ERA5 agrees well with these two independent datasets, but direct comparisons (e.g., a figure overlaying the observations over ERA5) or specific discussion of these comparisons are needed to support this statement.
- Please add a Data Availability Statement
Specific comments:
- Line 23: Suggest either removing “which has numerous causes” or listing some examples of causes – this statement lacks specificity but also isn’t completely necessary in the sentence.
- Line 45: suggest saying “rainfall versus snowfall” or “liquid versus solid” instead of “liquid versus snowfall” for consistency
- Line 59: Please clarify which variables are directly from ERA5 and what is calculated separately (see General Comment 2)
- Line 64: “ERA-5” should be “ERA5” for consistency
- Lines 79-81: What is the relevance of stating that most precipitation over the central Arctic Ocean comes at precipitation rates of < 1 mm/day? Suggest adding a statement of why this matters (e.g., difficulty observing or spurious drizzle in the reanalyses), or this sentence can be removed
- Figure 2: I assume that the colourbar indicates the fraction of observations per month, but it is labelled “observations per month”, please clarify
- Lines 124-125: Suggest including a reference for the statement about extratropical cyclones, since that isn’t analyzed/shown in this study
- Lines 142-144: References should be included for the discussion of the 2012 and 2019 warm air intrusions/Greenland melt events. Here are some potential references to consider:
- Hermann, M., Papritz, L., & Wernli, H. (2020). A Lagrangian analysis of the dynamical and thermodynamic drivers of large-scale Greenland melt events during 1979–2017. Weather and Climate Dynamics, 1(2), 497–518. https://doi.org/10.5194/wcd-1-497-2020
- Tedesco, M., & Fettweis, X. (2020). Unprecedented atmospheric conditions (1948–2019) drive the 2019 exceptional melting season over the Greenland ice sheet. The Cryosphere, 14(4), 1209–1223. https://doi.org/10.5194/tc-14-1209-2020
- Figure 3: what do the contours on the figures represent? Please clarify what the contours show on this figure and all following figures as I could not find definitions for the contours throughout.
- Line 158: "Wm-2" the exponent should be superscript
- Figure 4: the labels on each panel should specify the season instead of just “January” and “July”, unless this is actually only the individual months being shown
- Lines 179-180: Please provide a little more insight into why the summer latent heat flux is at the maximum values in Figure 5 across all land areas other than Greenland? It is stated that this helps support the higher summer precipitation amounts over land, but no explanation is explored.
- Line 200: Suggest including “of Norway” after “west coast” for clarity. Also, are the observations referred to here the ones being analyzed in this study? Otherwise, suggest including a reference.
- Line 209: missing a period at the end of this sentence (after “ERA5”).
- Line 210-214: What is meant by “early autumn” here? More discussion/clarification is needed for this paragraph as I do not understand what is meant by “precipitation phase shifts over the central Arctic Ocean are more prominent” as much larger changes are seen in other regions over the course of the year (e.g., Baffin Bay). Please clarify.
- Line 218: Is this referring to January and July (as in only those months) or winter/summer (as in Figure 4)?
- Figure 7/8 captions: “based on COADS data” – should this be ICOADS?
- Figure 9: I think having the varied sizes indicating station length is helpful to highlight stations with longer records! Would it be possible to make the circle sizes proportional to record length/years of data (instead of just 2 sizes), or would that make the plot difficult to interpret?
- Lines 260-265: This is interesting discussion of the rain-on-snow events – can the correspondence between rain events and occurrence of snow on the ground be included as a supplementary figure? I think it would be valuable to readers looking to connect this work more directly to rain-on-snow, but isn’t necessarily required in the paper
- Lines 305-307: Figure 11 is very interesting and is an effective way to illustrate the changes in liquid precipitation that may not be captured in a linear trend! I suggest adding a bit more discussion in this paragraph discussing Figure 11, perhaps highlighting the differences shown here that aren’t captured in the linear trend and why they differ.
Citation: https://doi.org/10.5194/egusphere-2025-3482-RC1 -
RC2: 'Comment on egusphere-2025-3482', Anonymous Referee #2, 20 Aug 2025
This article presents an analysis of the climatology and trends of Arctic precipitation partitioning from the ERA5 reanalysis product, with comparisons to a database of station-based terrestrial observations and historical interpolated observations over the Arctic Ocean. Regional climatological characteristics of precipitation are identified, with increased precipitation in the Atlantic sector consistent with the north Atlantic storm track. Precipitation is increasing overall, with increased summer liquid precipitation in all regions, and increased liquid precipitation in all regions in the Atlantic sector.
This article is well-written, and the scientific reasoning is generally sound, with just a few points requiring some clarification as per my comments below. The findings are well-motivated, with well-reasoned connections to physical and climatological explanations. The results are, to my knowledge, scientifically original, and fit well within the scope of the journal. The figures are generally clear and illustrative. I just have a few points where I think the article could benefit from some clarification and/or expansion, which I will describe in my comments below.
General comments:
This may be a matter of phrasing, but some of the points mentioned in the conclusion do not, in my view, follow from the analysis. ERA5 is stated as capturing trends in precipitation phase consistently with observations (cf lines 330-331), but in this work, I only see a comparison to climatological averages for both the AROSS data and the COADS data, with no observational comparison of trends or decadal differences. As such, I think that either the conclusion needs some rephasing for clarity, or some additional analysis needs to be conducted where observational trends or decadal differences are compared to the reanalysis data. (E.g. since some of the AROSS time series span both the 1979-1989 and 2013-2023 decades, a decadal comparison could potentially be carried out and compared with Fig 11.)
Furthermore, I would urge caution on reporting consistency with observations over the Arctic Ocean given the inconsistency in time periods examined (1950-1995 for COADS vs 1979-2023 for ERA5). I am very sympathetic to the challenges involved in validating reanalysis precipitation products over polar ocean regions where there is a general absence of station or observational campaign data, and I agree that reanalysis products are a good tool for assessing variability. However, I think care is necessary to not overstate the representativeness of reanalysis products in regions where there is little contemporary validation data. One option the authors may potentially consider is to add some brief discussion of how the results compare with results from contemporary examinations of precipitation phase, e.g. from remote sensing observations (such as Edel et al., 2020), similarly how results are discussed in comparison with QuikSCAT at line 260. Although these observational data products likewise have caveats, they can provide at least some additional contemporary observational basis for comparison; the results from Edel et al. (2020) also do appear generally consistent with the authors’ results in this manuscript.
The article is missing a data availability statement, and some figures are directly reproduced from other publications without clear attribution of the direct source (I expand on this in my line-by-line comments).
Some of the figure captions could benefit from more description; in general, I would appreciate if the authors would check that they specified which dataset is plotted in each figure (e.g. Figures 5 and 6 do not mention that the data shown is from ERA5). Additional description of how some datasets were averaged for plotting would also be helpful for reproducibility.
Finally, I think some discussion in the article could be expanded particularly with regards to caveats of some of the data products. For example, ERA5 has been found to have a warm bias over Arctic sea ice (Tian et al., 2024; Graham et al., 2019) and I am curious as to how this could introduce biases to the results. I would appreciate if the authors could comment on this in the text.
Line by line comments:
12: “extreme south”: how is this defined (e.g. south of which latitude)? This phrasing is vague to me.
34: The year should be 1996 for the Clark et al. reference, I assume
52: I think it would be worth mentioning here (or later in the article when results are being discussed) that precipitation is generally not assimilated into ERA5 (except over the continental United States region over a limited time period, if my recollection of the documentation is correct).
61: Please also mention that latent heat fluxes are taken from ERA5 (the ones in Figure 5)
64: Unnecessary hyphen in ERA5
Paragraph at line 71: here or later in the text, it may be worth mentioning the ERA5 warm bias over Arctic sea ice (as discussed in e.g. Tian et al., 2024, or Graham et al. 2019) which may impact the results of this study
79-81: Re: this drizzle effect, some studies apply a threshold to exclude these precipitation values, arguing that they are physically unlikely. Is there a reason why you opted not to apply this threshold?
94: Why is the data resampled to hourly frequency rather than 6-hourly frequency (for consistency with the reanalysis frequency)? Alternatively, why was hourly reanalysis data not used?
Figure 2: This figure is illustrative, but somewhat illegible due to size; consider scaling up the figure size so it spans the full page width. Please also specify in the caption that each horizontal band corresponds to a different station. Also, how are the bands in the plot organized within each country? (E.g. are the lower bands within Canada more northern regions?)
114: How coarse is the grid onto which data were interpolated? Please specify if possible
117: Perhaps consider mentioning that this old dataset is being used due to a general lack of long-term widespread in-situ measurements of precipitation type in the region
122: What is meant by “annual means (totals)”? I assume this refers to annual cumulative precipitation, rather than annual averages of daily-mean precipitation or similar, but please clarify.
144: Could you clarify what you mean by temperatures not being high enough for rainfall to occur? Was it a case of surface temperatures being warm enough for melt but precipitation falling as frozen snow, or a lack of coincident timing of precipitation during periods of warm intrusion (given that this is a low-precipitation region in general).
158: Superscript needed for Wm-2
Figure 5: Please specify how these fluxes were averaged (is this a monthly average of daily averages or something else?) Also please specify that these are from ERA5.
199-200: The point is well-made here, but please cite a reference for the statement connecting this infrequent snowfall to the North Atlantic Drift Current.
Fig 7 and 8: I was confused because these figures were cited as being reproduced from Clark et al. 1996, but I did not find them in that publication, although they do use data from that publication. Please additionally clarify the specific source from where the figures were reproduced. I’ve identified that these specific versions of the figures themselves are reproduced from “The Arctic Climate System” by Serrze & Barry (https://doi.org/10.1017/CBO9781139583817), having been reproduced there from an NSIDC special report (https://nsidc.org/sites/default/files/nsidc_special_report_4.pdf, pg. 93 & 99) which therein cites Clark et al. 1996. I am aware that one of the coauthors of this preprint is a coauthor of these publications, so I assume permissions are not an issue, but in the interest of data availability, I think more specific citations would be beneficial.
221-222: How is moderate-to-heavy precipitation defined (in terms of precipitation amount) for this dataset? Given how a large fraction of reanalysis precipitation is light precipitation (as mentioned above), this comparison may warrant some justification.
288: Missing word, did you mean “that increased Arctic precipitation” or similar?
294: “is in part convective precipitation”: this is somewhat vague, please be more specific. Is the increase in convective precipitation apparent when examining the ERA5 convective and large-scale precipitation products? I agree with the statement about convective precipitation moving into higher latitudes, but I would like to see this substantiated more. Perhaps consider including a supplementary figure with the convective precipitation, since this is mentioned a few times in the text.
299: Local increases poleward of Svalbard: are these likely also associated with the convective precipitation moving to higher latitudes? If so perhaps mention that.
Figure 11: I found this figure quite illustrative, could you include alongside this some analogous plots for the snowfall and total precipitation, following the format of Figure 10? (Since, as mentioned in the text, linear trends can be misleading.)
Figure 12: The grey over the continents makes it appear as if the entire ocean still has a weak trend; please either make grey the no-trend value for land and ocean, or make the no-trend continent values white as well.
330-331: I would suggest some caution in the phrasing of this statement. I agree that ERA5 is broadly consistent with observations where observations exist, and that this study has demonstrated consistency of ERA5 with independent datasets in the climatological sense. That said, the comparison is less robust over ocean/sea-ice regions since the observation-derived data cover a different time period than the reanalysis data. As such, I think caution needs to be taken with stating that ERA5 is representative in that region, particularly since this work did not validate with time-coincident observations in the region.
Furthermore, although this study did examine decadal differences and trends in ERA5, I did not see an examination of decadal differences or trends in this article for the AROSS database or for the other observational data to provide an observational comparison, so I do not think the claim can be made here that ERA5 captures precipitation trends consistently with observations, without additional analysis in which the trends (or at least decadal differences) in observed precipitation phase are compared to ERA5. Please rephrase these lines to clarify, or provide some additional analysis to support this statement.
References:
Edel, L., Claud, C., Genthon, C., Palerme, C., Wood, N., L’Ecuyer, T., & Bromwich, D. (2020). Arctic Snowfall from CloudSat Observations and Reanalyses. Journal of Climate, 33(6), 2093–2109. https://doi.org/10.1175/jcli-d-19-0105.1
Graham, R. M., Cohen, L., Ritzhaupt, N., Segger, B., Graversen, R. G., Rinke, A., Walden, V. P., Granskog, M. A., & Hudson, S. R. (2019). Evaluation of Six Atmospheric Reanalyses over Arctic Sea Ice from Winter to Early Summer. Journal of Climate, 32(14), 4121–4143. https://doi.org/10.1175/jcli-d-18-0643.1
Serreze, M. C., & Barry, R. G. (2014). The Arctic Climate System. Cambridge University Press. https://doi.org/10.1017/cbo9781139583817
Tian, T., Yang, S., Høyer, J. L., Nielsen-Englyst, P., & Singha, S. (2024). Cooler Arctic surface temperatures simulated by climate models are closer to satellite-based data than the ERA5 reanalysis. Communications Earth & Environment, 5(1). https://doi.org/10.1038/s43247-024-01276-z
Citation: https://doi.org/10.5194/egusphere-2025-3482-RC2
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