the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Dec 2023
Precipitation extremes in Ukraine from 1979 to 2019: Climatology, large-scale flow conditions, and moisture sources
Abstract. Understanding extreme precipitation events (EPEs) and their underlying dynamical processes and moisture transport patterns is essential to mitigate EPE-related risks. In this study, we investigate the dynamics of 75 EPEs (≥ 100 mm day-1) over the territory of Ukraine in the recent decades (1979–2019), of which the majority occurred in summer. The EPEs are identified based on precipitation observations from 215 meteorological stations and posts in Ukraine. The atmospheric variables for the case study analysis of selected EPEs and for climatological composites and trajectory calculations were taken from ERA5 reanalyses. Moisture sources contributing to the EPEs in Ukraine are identified with kinematic backward trajectories and the subsequent application of a moisture source identification scheme based on the humidity mass budget along these trajectories. The large-scale atmospheric circulation associated with EPEs was studied for a selection of representative EPEs in all seasons and with the aid of composites of all events per season. Results show that EPEs in summer occur all across Ukraine, but in other seasons EPE hotspots are mainly in the Carpathians and along the Black and Azov Seas. All EPEs were associated with a surface cyclone, and most with an upper-level trough, except for the winter events that occurred in situations with very strong westerly jets. Isentropic potential vorticity anomalies associated with EPEs in Ukraine show clear dipole structures in all seasons, however, interestingly with a different orientation of these anomaly dipoles between seasons. The analysis of moisture sources revealed a very strong case-to-case variability and often a combination of local and remote sources. Oceanic sources dominate in winter, but land evapotranspiration accounts for 60–80 % of the moisture that rains out in EPEs in the other seasons. Taken together, these findings provide novel insight into large-scale characteristics of EPE in Ukraine, in a region with a unique geographical setting and with moisture sources as diverse as Newfoundland, the Azores, the Caspian Sea, and the Arctic ocean.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-2594', Anonymous Referee #1, 02 Jan 2024
This paper presents a analysis of EPEs and their climatological drivers in Ukraine using a combination of advanced techniques and models. It identified the common trend of anomalies associated with EPEs in different seasons, and moisture sources of the EPEs. The findings like the important role of land evapotranspiration and the formation of an upper-level trough in all seasons expect winter are quite interesting and informational on the relationships between cyclones and flood related hazards. Overall I find the work to be interesting and sound, and the paper well presented. However, I believe the paper can be better if there are more physical background discussed either in section 3 or section 4. The findings are interesting and I believe that they are important, thus it's very natural for the audience to wonder what could be the physical explanations of the findings (such as geospatial patterns, relationships between the trough and precipitations, the difference between winter and other seasons). I would recommend the authors to have more of these discussed, better with references to former studies, together with the description of their findings.
Overall, I would recommend that this manuscript is suitable for publication in this journal ensuing to the authors addressing the major concern above, and my minor comments below.
- Line 45 – 48: Cite the data source in the corresponding format (newspaper, research paper, book, conference meeting, website, etc.)
- Line 54-55: It would be better if the authors can briefly talk about how the cyclones and blocking systems are relevant to EPEs.
- Line 68: ERA5 is an important data source in this paper, but it was not introduced properly. I would recommend the authors to introduce it before its first appearance in the paper.
- Line 114: I'm not quite familiar about this so I can be wrong, but I'm wondering if the threshold 0.025 g/(kg*h) is a common practice in this research field. If not, I would recommend the authors to briefly justify their choice or support it with former studies.
- Line 238: Add a comma before “southward in summer”.
- Line 264: The period from the former text paragraph seems to be after the figures and the caption.
- Line 338, 348, 359, 371, 391, 400: Use the standard citation format of ESWD. The citation needs to be both in text and in the reference list.
Citation: https://doi.org/10.5194/egusphere-2023-2594-RC1 -
AC2: 'Reply on RC1', Ellina Agayar, 04 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2594/egusphere-2023-2594-AC2-supplement.pdf
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RC2: 'Comment on egusphere-2023-2594', Anonymous Referee #2, 30 Jan 2024
The authors present a study of precipitation extremes in recent decades focussed on Ukraine, based on reanalysis data. Combining dynamical parameters with diagnostic fields for precipitation origin, the authors document commonalities in the atmospheric state between extreme precipitation cases, but also show pronounced variability, in particular with regards to the precipitation sources. Similar information has been provided before for neighboring areas, but not in the region studied here. The paper has a logical structure, is overall well-written, and the figures are in general of good quality. I have a few, mostly minor comments with regard to how the analysis could be strengthened further. These remarks concern the methods, presentation of the results, and their relation to published literature. I hope that these comments will be helpful for the authors in their preparation of a revised manuscript.
Minor comments
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The selection of the extreme precipitation events could be better justified. For example, to which percentile does the 100 mm day-1 threshold correspond to in different regions of Ukraine? With a constant threshold value, it seems that some events could be more extreme in some regions than in others. How sensitive are the results to this choice of threshold? This item is also connected to a better presentation of the climatological precipitation pattern in the study region (see below).
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Figure 1 does not provide a lot of information. It could be more informative to instead show for example the seasonal precipitation total in a 4-panel figure, and place the events with their maximum precipitation as text labels on top of the background.
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I find the results do nicely align with several other studies that have been done regarding the moisture sources of extreme precipitation in the Mediterranean and Central Europe, maybe also other regions, that are cited in the introduction. However, I found the discussion a bit brief, and more specific comparison could be done to the existing literature after presentation of the results. For example, the authors find, in agreement with above mentioned previous studies, that there is more structure/regularity in the upper-level circulation than in the moisture sources fields. Why is that so, and what does that imply? At least it could be stated as an overarching finding, and the question be raised, even if the authors do not want to speculate about possible reasons.
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I did not find Table 1 so useful, at least not in this location in the paper. Maybe this table should rather be introduced along with the seasonal results? There is also a lot to read in this table, which seems almost like a duplication of the writing in the results section. Maybe the table could be simplified, or some kind of coding of different "event types" could be devised, such that the table provides more comparable information at a glance?
Citation: https://doi.org/10.5194/egusphere-2023-2594-RC2 -
AC3: 'Reply on RC2', Ellina Agayar, 04 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2594/egusphere-2023-2594-AC3-supplement.pdf
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RC3: 'Comment on egusphere-2023-2594', Anonymous Referee #3, 31 Jan 2024
Overview
Overall, I found this manuscript to be clear, concise, and well-written. The study provides a novel climatological investigation of extreme precipitation events (EPEs) in Ukraine, documenting the synoptic-scale conditions in which these events occur and quantifying the moisture sources using a Lagrangian trajectory-based diagnostic. The findings help to address a gap in scientific understanding regarding EPEs in Ukraine.
While the paper is strong overall, I have a number of comments for the authors to consider. Once these comments are satisfactorily addressed, the manuscript may be acceptable for publication.
Major comments
- In my opinion, the manuscript is lacking in diagnostic analysis of the ingredients and processes resulting in extreme precipitation. Composite analyses and case studies are presented, and the circulation patterns are discussed, but it is still not entirely clear to me how the ingredients for heavy precipitation are established and maintained for these events. Are these events characterized by, for example, particularly anomalous moisture content or strong ascent? Do the key flow features tend to be particularly slow-moving? The study could be strengthened in this regard by inclusion of additional statistical/composite/case study analyses of key ingredients, such as dynamical forcing for ascent (e.g., quasi-geostrophic forcing or frontogenesis), moisture content/moisture flux, and convective instability (e.g., convective available potential energy). Such analyses would help to elucidate how the circulation features shown in Figs. 3 and 4 are linked to the ingredients for heavy precipitation in Ukraine, thereby providing a more complete picture of the synoptic-scale characteristics of the EPEs. Precipitation composites based on the ERA5 could also help to show where in the region the precipitation tends to be focused for the different seasonal groups of EPEs, thereby providing helpful context when interpreting the composite patterns.
- How was the 100 mm day-1 threshold selected, and how extreme is it for the various stations? I recommend quantifying where this threshold fits in the climatological distribution at each station. Would it be possible to identify EPEs as daily precipitation totals exceeding an upper percentile (e.g., 95th percentile) of the climatological distribution for each station instead of a fixed threshold?
- There is redundancy in showing maps of both 500-hPa geopotential height and near-tropopause PV. Both fields depict qualitatively similar structures and patterns of the upper-level flow. Is it necessary to show both fields?
Minor comments
Line 82: Do all of the stations have the same record length? Are they all available for 1979–2019?
Line 88: It would be more accurate to state that the reanalysis data were interpolated to a 0.5° grid; the actual ERA5 model resolution is finer than 0.5°.
Line 167: Is "intense" warranted here? What is the quantitative basis for this adjective in this context?
Comment on Figs. 3 and 4: I recommend the following changes to make these plots easier to read and interpret: (1) make the contours and arrows thicker, (2) make the outline of Ukraine thicker and perhaps plot it in a different color to make it more visible, (3) increase the font size for the lat/lon and color bar labels.
Line 225: Is the PV anomaly pattern in the summer composite perhaps a reflection of the occurrence of PV cut-offs?
Line 235: How much variability is there among the events in the composites with respect to the PV anomaly pattern? It might be worthwhile to also plot the composite standard deviation as a measure of the case-to-case variability.
Line 238: It is not clear to me what the authors mean by "northward", “southward”, “eastward”, and "westward" here. Do these descriptions refer to the direction of the PV anomaly gradient vector?
Lines 239–240: I encourage the authors to include a discussion of the possible implications of the composite PV anomaly patterns for forcing of vertical motion over Ukraine. In my opinion, a more direct link needs to be established between the composite flow patterns and the processes that caused the extreme precipitation.
Lines 405–406: When making statements for which direct evidence is not shown, I recommend including "(not shown).”
Lines 463–465: “The exception were winter EPEs…” This conclusion seems inconsistent with the 500-hPa Z and PV anomaly composites for the winter EPEs, which appear to depict strong troughs immediately upstream of Ukraine.
Typographical corrections
Line 229: I suggest removing "but not least" here.
Line 238: Change “consistently” to “consistent”
Line 285: Change “more important” to “greater”
Line 309: Remove “very”
Citation: https://doi.org/10.5194/egusphere-2023-2594-RC3 -
AC1: 'Reply on RC3', Ellina Agayar, 04 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2594/egusphere-2023-2594-AC1-supplement.pdf
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