the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Aug 2024
A-year Continuous Observations of Near-Surface Atmospheric Water Vapor Stable Isotopes at Matara, Sri Lanka
Abstract. Atmospheric water vapor stable isotopes are crucial for understanding hydrological cycle processes under climate change. This study presents a year-long in-situ monitoring of atmospheric water vapor stable isotopes (δ18O, δD) at Matara, Sri Lanka, from March 2020 to February 2021 to assess how oceanic sources and moisture transport influence coastal atmospheric moisture isotopic composition. We identified clear seasonal patterns in the isotopic composition, with δ18O, δD, and d-excess showing substantial variation between the southwest and northeast monsoon periods. The primary moisture sources were the Arabian Sea and the Indian Ocean during the southwest monsoon (May to September), characterized by depleted δ18O from -20.4 ‰ to -9.1 ‰. During the northeast monsoon dominated period, the northern Bay of Bengal, the Indian subcontinent, and Southeast Asia were primary moisture sources, displayed enriched δ18O (-23.9 ‰ to -7.5 ‰) and higher d-excess values (up to 25 ‰). The study also identified significant influences of sea surface temperature and sea surface relative humidity, on the isotopic composition of water vapor. Additionally, outgoing longwave radiation (OLR) is a significant index used to gauge the intensity of convective activity. Lower OLR values, indicative of stronger and deeper convection, were associated with more depleted δ18O in air masses. These findings help to improve the understanding of influences of the monsoon and local meteorological condition on water vapor isotopes in tropical region and provide new dataset on enhancing water vapor isotopic modeling or atmospheric processes projection in coastal regions.
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RC1: 'Comment on egusphere-2024-1840', Anonymous Referee #1, 11 Sep 2024
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The paper by Wu et al. (2024) presents the results of a year-long measurement campaign of stable water isotopes and atmospheric conditions in Matara, Sir Lanka. To explain the variability of d18O and d-excess they investigate the contribution from different moisture sources (trajectory analysis), the impact of near-surface relative humidity conditions of different regions, and the influence of convective activity. They conclude that the isotopic composition of water vapor varies seasonally and is influenced by the different moisture sources during the southwest (Arabian Sea) and the northeast monsoon (Bay of Bengal). Near-surface evaporation significantly affects local d-excess while convective activity is significantly correlated to variations in d18O.
The scientific significance of the paper is absolutely given, as continuous measurements of stable water isotopes in water vapor covering at least one year are highly needed. However, the scientific approach and the presentation of the results are sometimes chaotic and confusing and need to be improved. In the following, I will point out the main critical points and make some suggestions.
## Scientific Quality/Approach ##
Classification into Monsoon Periods
The main point is the distinction between the different monsoon periods. You aim to explain the stable water isotopic variability of water vapor in Matara, Sri Lanka. For your subsequent analysis you divided the time series into southwest, northeast, and non-monsoon periods. However, the isotopic signal and the variability of the isotopic composition within these defined seasons vary greatly. In particular, the non-monsoon period starts with a phase of high d18O values with a low d18O variability while the subsequent phase shows highly depleted d18O values with a strong d18O variability. A similar pattern can be observed during the northeast monsoon period with depleted values at the beginning and enriched ones towards the end. Note, the strongest depletion and enrichment are recorded during in one monsoon period, the northeast monsoon. As a result, the subsequent analysis and the respective interpretation are not convincing.
I suggest using a different classification depending on the d18O signal.Definition of Regions
The definition of region a and region b (Line 570ff) is not defined in the text. Moreover, it is not clear, why you chose these regions.
Similarly, it is not clear why you chose a 5°x5° area for the spatiotemporal correlation (Line 625).
Used Variables
The number of considered variables is enormous. However, it is sometimes unclear where they come from (AWS, reanalysis etc.) and why you use all these variables as some are not relevant to your argumentation. For example, BLH and LCL are included in Table 1. Its label states that it contains the data from the station at Matara. In your data section BLH is from ERA5, which should be pointed out in the table and in the text. In contrast, the source for the LCL data is not given at all. Both variables are described in the text, but the results are not discussed in relation to the isotopic variability, neither in this paragraph nor elsewhere. Why do you present them here? Do you really need them?
Moreover, the analysis is not consistent in its use of variables (see Table below), meaning I cannot discern a clear and coherent line of argumentation. You present BLH and LCL at the beginning, but you did not consider or mention it afterwards. Why is the consideration of wind speed and wind direction necessary and why is it only included in Fig. 4 but not in Fig 2? In Fig 6, you consider dexcess and RHsst although in the rest of the paper you try to understand the d18O variability. You calculated the spatiotemporal correlation between precipitation and d18O of water vapor, but it is not clear to me why you did consider precipitation at this point. At the end, you analyse OLR although it was not considered before. Why you did use the NCEP-NCAR reanalysis for OLR although you used ERA5 for the other variables.Note: it seems that OLR and therefore convective activity is very important at your study site and agrees with the results of a recently published paper considering the tropical region of Ecuador, South America
- Landshuter, N., Aemisegger, F., & Mölg, T. (2024). Stable water isotope signals and their relation to stratiform and convective precipitation in the tropical Andes. Journal of Geophysical Research: Atmospheres, 129, e2023JD040630. https://doi.org/10. 1029/2023JD040630
I suggest including OLR at the beginning of the results section as time series (maybe at the end of Fig2). Additionally, you could consider generating spatial composites of OLR based on your defined periods.
Hum
(ppmv)
δ18O
δD
Dexcess
T
RH
Q
Press.
Precip
BLH
LCL
Wind
spe.Wind
dir
RH sst
OLR
Fig 2
x
x
x
x
x
x
x
x
x
Tab 1
x
x
x
x
x
x
x
x
Fig 3
x
x
x
x
Fig 4
x
x
x
x
x
x
x
x
X
Fig 5
x
x
Fig 6
x
x
Fig 7
x
x
x
Discussion and Interpretation
All the results are only sporadically compared to results of other studies. However, even more important, the results are not discussed and interpreted together to receive an overall conclusion of the results. So, how do all your results fit together? Consider putting a discussion at the end of a paragraph or section or include a section “Discussion”, which contains the discussion of your results with other studies and the interpretation.
Line 580-583: Here the comparison to another study occurs before presenting your own results. Consider splitting and moving the information of this sentence: The explanation of the relevance of RHsst should be at the beginning of the paragraph or section. A comparison of values from another study should be stated after the presentation of your results.
## Presentation Quality ##
The presentation of the study was often confusing for me. This is attributed to the overall structure of the results section and the use of an imprecise language within the paragraphs.
Figure Selection and Order
The overall structure was not easy to follow, which is linked to the already mentioned mix of analyzed variables, but also to the ordering of the figures and a missing explanation at the beginning of a section claiming why you did the respective analysis. For example:
-Figure 4: Why do you show the daily cycle of the variables? For presenting the new dataset? If so, consider switching Figure 3 and Figure 4 to keep the presentation of the new dataset together before going deeper into finding the explanation for the isotopic signal. Otherwise, I cannot see, why you included Figure 4. It does not contain new information that is already included in Figure 2 and is relevant for the subsequent argumentation/explanation.
-Figure 6: Why do you consider d-excess here? It is hard to follow your argumentation at this point. Do you use d-excess to support the d18O results? Or is it an analysis of a different isotopic variable? In the first case, you should be more specific and explain the link and how the d-excess results support your results of the moisture source analysis. In the second case, consider putting Figure 6 at the end of your analysis.
-Note Figure 2 shows humidity (ppmv) and pressure, which are not considered in the text. Remove those variables from the plot or include them if they are relevant for your argumentation.
Imprecise Language and Structure within Paragraphs
The language is often imprecise and does not only influence the readability of the manuscript but also the scientific quality. This mainly applies to the “Results” section. In the following, you find the most important cases.
Introduction
The introduction needs to be reconsidered. The first two paragraphs explain the influence of the monsoon on the Tibetan Plateau although the Tibetan Plateau is not considered in this study.
Line 105-112: This paragraph explains basic knowledge about stable isotopes and should be included before describing the results of recent studies (Line 81).
Data and methods
Line 201: Why did you use data from 2000-2020? Did you mean for the average annual precipitation and air-temperature? Is all the rest of the analysis restricted to the field campaign period?
Line 201ff: The structure is here not consistent, you present ERA5 data, then NCEP-NCAR and then you go back to ERA5 data. You say here that ERA5 is good for missing observational data. Did you use it for this purpose?
Line 332: It is unclear how you derived the specific humidity of the trajectories
Results
Line 348: Starting a paragraph by describing what a figure shows is not very helpful for the reader. Instead, I would shortly explain the structure of the section.
Line 348-350: Figure 2 does also show SST, precipitation, and humidity, which is not stated. Are all variables from the weather station? If not, please make it clear and include the source of the dataset in the text (and in the Data section).
Line 351: What kind of “average” values? Hourly, daily, monthly, annual? The consideration of different time scales is interesting, however, as in the given line, it is not clear which time scale you are considering. This is often not clear in the following paragraphs. It was confusing for me. To increase the readability of the text, I suggest being more precise regarding the time scale. Moreover, it would help to present all results of one timescale together or always in the same order.
Line 352: Lifting condensation level, where does the data come from? Be more specific and take care of the structure. First, consider AWS data and if necessary, consider variables from other datasets. However, in your case, LCL is not important for your argumentation as you do not mention it in the rest of your manuscript.
Line 356: No definition exists of the non-monsoon period: how is it defined? In Section 2.1 you introduce four monsoon periods but in your analysis you only have three.
Line 351-end of paragraph: You describe temperature, specific humidity, and relative humidity in an inconsistent way. Try to structure your paragraph either by presenting each variable or by going through the different months/seasons and compare the variables with each other, but always keep the same order.
Line 383-385: True, however, this does not fit to the time series and is related to the classification issue I mentioned before.
Line 358: In your text is no reference to Fig. S2. Overall, you should check that every figure is referred to in the text and that the reference has the correct number.
Line 363: You introduced the seasons in your text as monsoon seasons. For more clarity, try to avoid using other seasonal conventions such as winter, spring etc. throughout your text.
Line: 379: You present the results of d18O, dD, and d-excess. Nice. However, at some point you should state that you will only consider d18O in the following.
Line 394: State or explain (maybe already in the introduction) that high values of d-excess are related to moisture recycling.
Line 395-396: I cannot see it in the figure. Is it a result or an explanation/interpretation? In the latter case, please make it clear and use a reference e.g.,
- Graf, P., Wernli, H., Pfahl, S., and Sodemann, H.: A new interpretative framework for below-cloud effects on stable water isotopes in vapour and rain, Atmos. Chem. Phys., 19, 747-765, https://doi.org/10.5194/acp-19-747-2019, 2019.
- Aemisegger, F., Pfahl, S., Sodemann, H., Lehner, I., Seneviratne, S. I., & Wernli, H. (2014). Deuterium excess as a proxy for continental moisture recycling and plant transpiration. Atmospheric Chemistry and Physics, 14(8), 4029–4054. https://doi.org/10.5194/acp‐14‐4029‐2014
- Aemisegger, F., Spiegel, J. K., Pfahl, S., Sodemann, H., Eugster, W., & Wernli, H. (2015). Isotope meteorology of cold front passages: A case study combining observations and modeling. Geophysical Research Letters, 42(13), 5652–5660. https://doi.org/10.1002/2015gl063988
Similarly, Line 536-541, it is not clear, whether the stated information belongs to results of the study (please add a reference to the respective figure) or interpretation/background information from another study (please add reference).
Line 529: please order the factors in the way they appear in the following text.
Line 542: Did you really calculate water vapor sources as for example done by
- Sodemann, H., C. Schwierz, and H. Wernli (2008), Interannual variability of Greenland winter precipitation sources:
Lagrangian moisture diagnostic and North Atlantic Oscillation influence, J. Geophys. Res., 113, D03107,
doi:10.1029/2007JD008503.
or did you calculate the mean water vapor content of the clustered trajectories, or did you calculate the mean water vapor change along the clustered trajectories?
Line 549: It is unclear, what the d18O of the trajectories (Figure 5) means. It is probably not the mean traced d18O along a trajectory, but it is the calculated concentration-weighted d18O. How should this measure be interpreted?
Line 550: Results do not agree with the results of section 3.4
I hope my comments help to improve your manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-1840-RC1 -
RC2: 'Comment on egusphere-2024-1840', Anonymous Referee #2, 12 Sep 2024
reply
This manuscript presents a new record of stable water isotope measurements in water vapor from Matara, Sri Lanka. The isotope measurements are compared with meteorological measurements from a nearby station and ERA5 / NCEP reanalysis data from the surrounding region to identify the most important drivers of isotopic variability at the site. The authors find differences in isotopic signatures between the northeast and southwest monsoon seasons, which they attribute to different moisture source conditions and convective activity.
This dataset is very valuable, especially given the scarcity of isotope measurements in the region. The measurement protocol appears to be sound (but I am not an expert, I hope the other reviewer(s) can check this better), and the analysis is well done with some exceptions. My main concerns are the moisture source diagnosis (see major comment 1) and the structure, specifically in the introduction and the results (major comment 2).
Major comments:1) The trajectory analysis needs more explanation. For example, you wrote that you did a K-means clustering to calculate specific humidity along the trajectories. Why? Do you mean you did K-means clustering of the trajectories, and then calculate specific humidity along the resulting clustered trajectories?
Also you cannot simply assume that the end point of the trajectory is the moisture source. This is a very qualitative picture and does not provide more information than what is already known (i.e. moisture comes from the northeast during the northeast monsoon and from the southwest during the southwest monsoon). To get a more quantitative picture, you would have to look at moisture uptakes along the trajectories. For example, you could use positive changes in specific humidity, or locations where there is evaporation from the surface. There are several moisture source diagnostics that could do this, e.g. WaterSip (Sodemann et al., 2008), HAMSTER (Keune et al., 2022), UTrack (Tuinenburg & Staal, 2020). For all of these you will also need more trajectories to get a representative picture of the air masses. One trajectory every 6h is not enough. I would recommend to start trajectories from several heights and from different horizontal locations around the measurement site.2) The structure of the text could be improved, in particular the introduction and the results section.
The introduction now goes back and forth between monsoon, different isotope processes, and Sri Lanka. I would suggest restructuring it as follows: Motivate why the Indian Summer Monsoon is important for the Asian climate system (same as now), without mentioning isotopes yet. Then introduce stable water isotopes and why they are useful for studying the water cycle. Try to focus only on processes that are relevant for the study. Then write that there are not many studies on isotopes in the Indian Ocean and in particular Sri Lanka. Then introduce the new dataset.
The results section introduces many different figures and variables and it is not always clear why. I would suggest showing only figures/variables that are important for the story and lead to the conclusions. Also make sure to describe where you got variables from, if you show them.
Minor commentsTitle: „A-year continuous observations“ is grammatically wrong. Change to „One-year continuous observations“ or „One year of continuous observations“.
L27&30: This is a bit confusing, -20.4‰ to -9.1‰ does not seem more depleted than -23.9‰ to -7.5‰.
L29: displayed -> characterized by
L32: No comma after humidity
L35: The findings don‘t provide a new dataset (second part of the sentence), rather the other way around.
L37: „in tropical regions and provide a new dataset for enhancing...“
L44: There -> They
L45: Again, not really (cf. L27&30).
L46: The sea surface condition does not improve the understanding.
L178: Features -> Featuring
L182: „Most of the precipitation…“. With 8 out of 12 months attributed to either southwest or northeast monsoon, 70% is actually not so high (only 3.3% more than the average precipitation amount).
L186: derives -> forms/produces
L190: Similarly -> In contrast
L198: „Meteorological data are compared …“: I would move this sentence to the beginning of Section 2.2.
L202: Do you mean Fig. 1a&b?
L207: an -> the
L208: What does „averaged“ mean here? I thought you only have one year?
L213: Instead of the link, just cite NCEP? Does ERA5 not provide OLR?
L214: You do not really use all of these ERA5 variables, do you? I would mention only those that are used (and relevant). Also why 2000 to 2021, why not only 2020-2021?
L218: hourly -> one hour
Equation 3: This equation is not very clear. It looks like q_s is a function of sea surface salinity of 35 PSU, but what you mean (I assume) is that it is q_sat(SST) at a salinity of 0 PSU, while the left hand side is q_sat(SST) at a salinity of 35 PSU.
L231: I think here it would make more sense to take the atmospheric pressure (same as for q_sat(T_air)), because it is probably not constant, and the difference in pressure between 2m and the sea surface is negligible. Assuming a constant sea surface pressure might introduce artificial variations in RHsst.
L237: in conjunction with
L250: How far away from the AWS is the water isotope analyzer?
L250: is situated, is positioned, and consists
L251: Could you add the numbers describing the different components to Figure 1d?
L257: What is XX?
L264: Remove „are defined“
Equations 6 & 7: Actually the R values are the ratios of the isotopes rather than the isotopologues (Coplen, 1994), i.e. R_18O = 18O / 16O and R_D = D / H
L270: Add „respectively“ at the end.
L291: This sentence does not make sense (grammatically).
Section 2.4: For all of these models, please write somewhere which values are used for the different variables.
Equations 9&11: Please use a consistent notation for the equilibrium fractionation factor.
Equation 10: This is specific to HDO. I would either add a second equation for H218O, or make the first equation more general.
Equation 11: I would cite Craig & Gordon (1965).
L351: For the water isotopes the seasonal cycle is not very obvious from Figure 2. The hourly or daily variability is much larger than the seasonality.
L352: How did you get the LCL? And why is it relevant?
L357: The maximum temperature is much higher, isn‘t it?
Figure 2: I don‘t think it is necessary to show both humidity and specific humidity.
L414: emerged -> show
L442: Why do you compare your values to those from Greenland? It is a very distant site.
L466: What do you mean by precipitation leaching?
L472: “were” missing
L485: “and led…” does not fit here
L497: What do you mean by reversed? The diurnal variations go in the same direction, only the magnitudes are different.
L626: It is averaged in space, isn‘t it?
L662: Maybe write here that this is now for the simultaneous values.
L692: The highest? (Fluctuations)
SupplementL21: directedly -> directly
L23: of what?
L30: led -> leadFigure S3: Could you also mark the northeast and southeast monsoon months like in Figure 2?
Figure S4: Maybe add titles to the subfigures to make it clear which is which.
Figure S5: I don‘t see the yellow solid line. The figure resolution is not good.
Figure S7: Again, titles would help to know which subfigure corresponds to which season.What is the difference between Figures S7 and S9?
Figure S10: Why do you show only the northeast monsoon?
Table S1: Since VSMOW is there, add SLAP?
ReferencesCoplen, T. B. (1994). Reporting of stable hydrogen, carbon, and oxygen isotopic abundances (technical report). Pure and applied chemistry, 66(2), 273-276.
Craig, H., & Gordon, L. I. (1965). Deuterium and oxygen 18 variations in the ocean and the marine atmosphere. In Stable isotopes in oceanographic studies and paleo-temperatures (pp. 9–130). Lab. Geol. Nucl.
Keune, J., Schumacher, D. L., & Miralles, D. G. (2022). A unified framework to estimate the origins of atmospheric moisture and heat using Lagrangian models. Geoscientific Model Development, 15(5), 1875-1898.
Sodemann, H., Schwierz, C., & Wernli, H. (2008). Interannual variability of Greenland winter precipitation sources: Lagrangian moisture diagnostic and North Atlantic Oscillation influence. Journal of Geophysical Research: Atmospheres, 113(D3).
Tuinenburg, O. A., & Staal, A. (2020). Tracking the global flows of atmospheric moisture and associated uncertainties. Hydrology and Earth System Sciences, 24(5), 2419-2435.
Citation: https://doi.org/10.5194/egusphere-2024-1840-RC2
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