PCLM-D-21-00079

Global hydro-climatological indicators and changes in the global hydrological cycle and rainfall patterns

PLOS Climate

Dear Dr. Benestad,

Thank you for submitting your manuscript to PLOS Climate. After careful consideration, we feel that it has merit but does not fully meet PLOS Climate’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Three reviewers have evaluated your submission and have identified a number of aspects of the manuscript that require attention, including the clarity of the description of the methods and the nuance of the interpretation and discussion of the results. Please respond carefully to all of the reviewers' comments when preparing your revisions.

Please submit your revised manuscript by Mar 05 2022 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at climate@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pclm/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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• A rebuttal letter that responds to each point raised by the editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
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• An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

We look forward to receiving your revised manuscript.

Kind regards,

Jamie Males

Executive Editor

PLOS Climate

on behalf of

Xander Wang, Ph.D., P.Eng.

Academic Editor

PLOS Climate

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Additional Editor Comments (if provided):

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Does this manuscript meet PLOS Climate’s publication criteria? Is the manuscript technically sound, and do the data support the conclusions? The manuscript must describe methodologically and ethically rigorous research with conclusions that are appropriately drawn based on the data presented.

Reviewer #1: Partly

Reviewer #2: Yes

Reviewer #3: Partly

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: I don't know

Reviewer #2: Yes

Reviewer #3: Yes

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3. Have the authors made all data underlying the findings in their manuscript fully available (please refer to the Data Availability Statement at the start of the manuscript PDF file)?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception. The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: No

Reviewer #2: Yes

Reviewer #3: Yes

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4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS Climate does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: General comments:

This study explored Global hydro-climatological indicators and changes in the global hydrological cycle and rainfall patterns. This study is valuable and interesting to describe the state of Earth's global hydrological cycle. However, it is still difficult to get certain indicators accurately, example for the global area with daily precipitation. In addition, the excessive qualitative description in the paper also affects the quality of the paper. Although this paper is within the scope of this journal, in my opinion, It has not yet reached the publication level required by the journal. Therefore, this manuscript could not be acceptable for publication.

Specific comments:

1) Too much qualitative description and too little quantitative description reduce the quality of the paper, such as "decline", "decrease", "diminishing", "modest", etc.

2) expressed too little information on the Figures and Tables, this prevents the reader to obtain more information in the manuscript.

Reviewer #2: The manuscript performs a global hydro-meteorological analysis based on three indicators applied to ERA5 data. A wavelet analysis explains the energies related to the different precipitation systems ans reveals some interesting changes over time.

From my point of view it is an excellent manuscript and it was a pleasure to read it. Besides the analysis, I appreciate especially the discussion clearly listing problems and limitations of the analysis.

There are only two minor issues, authors should address in a new version of the manuscript before publishing:

1.) the approach of the trend analysis should be described more in detail, especially which trend test has been applied

2.) the discussion of assimilated data is good and very important (numerous authors using reanalysis data would benefit from this knowledge, e.g. in the statistical downscaling community). But from my point of view, several issues are little bit blurred:

a.) NCEP1 also does not include satellite data

b.) it needs to be distinguished if i) only satellite data are assimilated which are not affected by clouds and precipitation or ii) all satellite data are assimilated

c.) as far as I know there are difference with respect to the assimilation of station data into ERA5 between US and Europe

Though I know that it is difficult to figure out difference between reanalysis products, this part of the manuscript could benefit from more detail.

Reviewer #3: OVERVIEW

The paper investigates long term rainfall variability on a global scale by using reanalysis products (mainly ERA5). Specifically, the paper introduces 3 indices that can be used to investigate the long term global hydrological cycle.

GENERAL COMMENTS

The paper is fairly well written and clear. The topic is surely of interest for the readership of PLOS Climate. However, I believe that the results are not clear enough and an improved analysis is needed. Moreover, also the methodology requires some clarifications. The general comments are listed below, with the indication of their relevance.

1) MAJOR: The main problem of the paper is the huge variability of the results if different precipitation datasets are considered. Indeed, looking for instance at Figures S6 and S9, there are large differences between the area fraction of different datasets (TRMM, ERA5, NOAA 20CR, ERA20C, NCEP1), both in terms of magnitude and trend. Moreover, it is evident that the jump in ERA5 data from 1980 to 1990 is an artefact, not present in other datasets. Therefore, it is not possible to draw any conclusions from this analysis. The authors didn’t discuss enough this problem. In my opinion, the conclusion of the paper should be that the uncertainties in our capability to estimate global rainfall are too large to draw any meaningful conclusions on long-term trend and variability. A potential solution might be using ground-based observations (e.g., CRU, GPCC, GPCP) to try to obtain, at least over land, an agreement between the results.

2) MAJOR: The introduction of the 2-D Haar wavelet transform is an interesting point of the paper. However, I miss the description of this methodology, and particularly what is the physical meaning of, for instance, the normalised squared energy shown in Figure 6, and in the supplementary material.

3) MAJOR: Another important missing description is related to Figure 7. The figure is hardly readable, and it is not clear what kind of regression has been carried out between annual mean energy of each global wavelet component and global temperature. Moreover, what should be understood from the results shown in Figure 7? Consequently, it is not clear to me also how Figure 8 is built and which conclusion should be drawn from it.

4) MODERATE: Throughout the text, the supplementary material is referred but not specifically. I mean, the specific figures and tables reported in the supplementary material should be given (e.g., Figure S1, S2, …).

5) MODERATE: The introduction should be revised. The first part (page 2 and half page 3) is too general and it seems more appropriate for a book than for a paper. At page 5 (top) a link between global rainfall and evaporation is given but I do not understand why it is done. If we have precipitation data, and we want to investigate long term rainfall trend and variability, why do we need evaporation data?

6) MODERATE: The three indices used in the paper should be better described. The equations used for computing the indices should be given. The authors provide the code (that is very good), but also the equations should be provided.

SPECIFIC COMMENTS

Please, provide the line numbering, otherwise it is very hard to provide specific comments.

Page 6: “24-hr precipitation that they summarise”. To be revised.

Page 7, bottom: “global LAND area”.

Page 8: What is Gt?

Page 8 bottom: “implying an evaporation rate that increased by 5% per degree warming”. How is it computed? This sentence comes out of the blue.

Page 10: “can be explained in terms of both increased evaporation”. It is not shown. Why?

RECOMMENDATION

On this basis, I found the topic of the paper relevant and hence I suggest a major revision.

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6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

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Global hydro-climatological indicators and changes in the global hydrological cycle and rainfall patterns

PCLM-D-21-00079R1

Dear Dr. Benestad,

We are pleased to inform you that your manuscript 'Global hydro-climatological indicators and changes in the global hydrological cycle and rainfall patterns' has been provisionally accepted for publication in PLOS Climate.

Before your manuscript can be formally accepted you will need to complete some formatting changes, which you will receive in a follow-up email from a member of our team. 

Please note that your manuscript will not be scheduled for publication until you have made the required changes, so a swift response is appreciated.

IMPORTANT: The editorial review process is now complete. PLOS will only permit corrections to spelling, formatting or significant scientific errors from this point onwards. Requests for major changes, or any which affect the scientific understanding of your work, will cause delays to the publication date of your manuscript.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they'll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact climate@plos.org.

Thank you again for supporting Open Access publishing; we are looking forward to publishing your work in PLOS Climate.

Best regards,

Xander Wang, Ph.D., P.Eng.

Academic Editor

PLOS Climate

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Reviewer Comments (if any, and for reference):

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

Reviewer #3: All comments have been addressed

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2. Does this manuscript meet PLOS Climate’s publication criteria? Is the manuscript technically sound, and do the data support the conclusions? The manuscript must describe methodologically and ethically rigorous research with conclusions that are appropriately drawn based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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4. Have the authors made all data underlying the findings in their manuscript fully available (please refer to the Data Availability Statement at the start of the manuscript PDF file)?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception. The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS Climate does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: General comments:

To my point of view，the author have revised it all rather well, and the quality of the manuscript has improved substantially. It can be accepted for publication.

Reviewer #2: Dear authors,

all my comments have been addressed and I am looking forward to the publication of the article!

Reviewer #3: The authors addressed all reviewers' comments successfully.

I have particularly appreciated that the authors highlighted all the limitations of the study, mostly driven by the large uncertainties we face to estimate global precipitation over long time periods.

As a note, I still believe that the use of a ground-based dataset (e.g., GPCC, CRU) would have been useful to better constrain the obtained results, but of course it's up to the authors, I don't want to force them.

Therefore, I believe the paper can be accepted as is, congratulations to the authors.

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7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

Do you want your identity to be public for this peer review? If you choose “no”, your identity will remain anonymous but your review may still be made public.

For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

Reviewer #3: Yes: Luca Brocca

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