Review Reports - Analysis of the Winter AOD Trends over Iran from 2000 to 2020 and Associated Meteorological Effects

Round 1

Reviewer 1 Report

This study looks at trends in AOD from the MODIS and MERRA2 reanalysis over the Middle East, with a focus on Iran. The time period selected is 2000-2020. This work is in scope for the journal. It builds on previous work looking at AOD and AOD trends, which is cited. The quality of writing and figures is generally ok (font sizes should be increased on most figures). I do, however, have concerns about the way the analysis was done, and some information is incorrect or missing. I, therefore, recommend minor revisions and would like to review the next version of this manuscript. These concerns are big-picture and may require rewriting much of the manuscript, so I would leave smaller typographical comments for the next version. My comments in support of this recommendation are:

The discussion of MERRA2 and its data assimilation is inaccurate and incomplete. For example, it does not assimilate bias-corrected MODIS AOD, it uses cloud-cleared MODIS reflectance and does a neural network AOD retrieval that is assimilated. The AVHRR assimilation is only over the ocean and, I believe, only for part of the time period. The AERONET assimilation is likewise only for part of the time period (I believe it stopped in the mid-2010s when the latest MERRA processing began, as the quality-assured AERONET data assimilated are not available sufficiently quickly, and of course, the available sites change dramatically over the study period from less than a dozen to several hundred). I am not sure if the MISR assimilation is ongoing in real-time, but in any case, MODIS and MISR only provide data from 2000 onwards. The point here is (a) the manuscript is not characterizing the assimilation correctly and (b) changes in the assimilation

Systems over the course of the period have the potential to change the error characteristics of the reanalysis when new sensors come in or drop out. Since this paper is a trend analysis, one implicit assumption here is that the reanalysis is equally reliable at all points in time. Yet if the observations are providing useful constraints on the model, then one would assume that the reanalysis is more reliable when it includes more observations. This immediately breaks the premise of using reanalysis to look at trends, and this isn't really discussed in the paper. We simply don't know how the reliability of the reanalysis changes throughout the time period. And, since AERONET is assimilated we can't easily use AERONET data to assess this (as the model is tuned to the AERONET sites assimilated). Beyond the question of AOD assimilation, the meteorological assimilation also changes in time which has implications for e.g. changes in the quality of modeled dust emission and aerosol aging and transport through the time period.

Author Response

Authors: Thank you for your valuable suggestion and recommended corrections in the text. The data sections including MERRA-2 and ERA5 reanalysis, MODIS and AERONET observations were improved in the revised manuscript. We added some few discussions regarding the data assimilation processes, we followed reviewer comments and corrected previous sentences, in a way to justify the reliability of the reanalysis datasets for assessing the long-term trends and correlations between aerosols and meteorological factors.

Reviewer 2 Report

The manuscript by R. Yousefi et al. presents an analysis of the winter aerosol optical depth (AOD) trends over Iran using the MERRA-5 and MODIS and two AERONET stations aerosol datasets from 2000 to 2020 depending on several meteorological parameter data (soil moisture, total precipitation, relative humidity, surface temperature, sea level pressure, and wind speed), taken from the ERA5 global reanalysis data for the 2000–2018 period. The authors showed that there are two time periods (2000 to 2010 and 2010 to 2018) with opposite behaviors (trends) of the winter AODs and dust concentration and the meteorological parameters may highly affect the dust activity over the Middle East (including Iran) even during wintertime, the season with lowest dust emissions.

The paper is comparatively well written, but English needs to be slightly proofread. The manuscript material and results fall within the scope of Remote Sensing journal and will be of interest to the remote sensing community, especially to those who deal with the Middle East.

My recommendation is to publish the paper in the Journal after minor revision. I have several technical comments that can be found below and in the highlighted version of the manuscript.

Specific comments and recommendations

1. Introduction (page 1, line 39). It should be “throughout a year” instead of “…throughout the year”.

2. Introduction (page 1, line 43). Perhaps, it would be better to write “…affecting cloud micro-physical processes” instead of “…impacting cloud micro-physical processes”.

3. Introduction (page 2, line 46). It should be “…related to the impact of aerosols on…” instead of “…related to impact of aerosols on…”

4. Introduction (page 2, lines 50–51). Citation: “...aiming to understand their role in atmospheric processes and…” I think it would be better to clarify whose role in atmospheric processes you are talking about here.

5. Dataset and Methods, 2.1. Study area (page 3, line 97). All Figures 1–10 and Table 1 should be center aligned.

6. Dataset and Methods, 2.3. Meteorology data (page 4, lines 133–134). It should be “surface temperature (ST)…” instead of “…surface temperature (T2M)…”

7. Results and discussion, 3.1. Aerosol trends (page 5, line 198). Please write “in [63]” or “by Yousefi et al. [63]” instead of “by [63]”.

8. Results and discussion, 3.1. Aerosol trends (page 5, line 201). Please write “during the 2000-2010…”

9. Results and discussion, 3.1. Aerosol trends (page 5, line 210–211). Please write “an insignificant positive trend…” I mean that the trend type must be explicitly specified here.

10. Results and discussion, 3.2. The wintertime AOD-meteorology interaction (page 9, line 299–301). These abbreviations have already been given in subsection 2.3. Meteorology data (see lines 132–134).

11. Conclusions (page 13, line 420). Please write “during the 2000-2010…”

12. Conclusions (page 13, lines 427–428). These abbreviations have already been given in subsection 2.3. Meteorology data (see lines 132–134)”.

13. References (pages 14–17). References [18, 20, 22, 23, 27, 31, 56, 64, and 72] are given incorrectly. See please the highlighted version of the manuscript for details.

Comments for author File: Comments.pdf

Author Response

Reviewer: 2

The paper is comparatively well written, but English needs to be slightly proofread. The manuscript material and results fall within the scope of Remote Sensing journal and will be of interest to the remote sensing community, especially to those who deal with the Middle East.

My recommendation is to publish the paper in the Journal after minor revision. I have several technical comments that can be found below and in the highlighted version of the manuscript.

Authors: We are really thankful to the reviewer for the consideration of our manuscript. We followed reviewer’s comments and suggestions and improved the scientific quality of the revised manuscript. Note that we took extra care and edited thoroughly the revised manuscript. Our point-wise response to all comments follows.

Specific comments

Introduction (page 1, line 39). It should be “throughout a year” instead of “…throughout the year”.

Authors: Thank you for your suggestion. “Throughout the year” was replaced by “throughout a year”

Introduction (page 1, line 43). Perhaps, it would be better to write “…affecting cloud micro-physical processes” instead of “…impacting cloud micro-physical processes”.

Authors: Thank you for your suggestion. “Impacting cloud micro-physical processes” was replaced by “affecting cloud micro-physical processes”

Introduction (page 2, line 46). It should be “…related to the impact of aerosols on…” instead of “…related to impact of aerosols on…”

Authors: Thank you for your suggestion. “Related to impact of aerosols on” was replaced by “related to the impact of aerosols on”

Introduction (page 2, lines 50–51). Citation: “...aiming to understand their role in atmospheric processes and…” I think it would be better to clarify whose role in atmospheric processes you are talking about here.

Authors: Thank you for your suggestion. This sentence was revised to “aiming to understand the role of these datasets in atmospheric processes”

Dataset and Methods, 2.1. Study area (page 3, line 97). All Figures 1–10 and Table 1 should be center aligned.

Authors: Thank you for your suggestion. All figures and Table were aligned.

Dataset and Methods, 2.3. Meteorology data (page 4, lines 133–134). It should be “surface temperature (ST)…” instead of “…surface temperature (T2M)…”

Authors: It was corrected throughout the revised manuscript, as suggested.

Results and discussion, 3.1. Aerosol trends (page 5, line 198). Please write “in [63]” or “by Yousefi et al. [63]” instead of “by [63]”.

Authors: Thank you for your suggestion. It was replaced.

Results and discussion, 3.1. Aerosol trends (page 5, line 201). Please write “during the 2000-2010…”

Authors: Thank you, “the” was added.

Results and discussion, 3.1. Aerosol trends (page 5, line 210–211). Please write “an insignificant positive trend…” I mean that the trend type must be explicitly specified here.

Authors: It was rephrased as suggested.

Results and discussion, 3.2. The wintertime AOD-meteorology interaction (page 9, line 299–301). These abbreviations have already been given in subsection 2.3. Meteorology data (see lines 132–134).

Authors: Thank you for your suggestion. Sentence was corrected just using the abbreviations.

Conclusions (page 13, line 420). Please write “during the 2000-2010…”

Authors: Thank you, “the” was added.

Conclusions (page 13, lines 427–428). These abbreviations have already been given in subsection 2.3. Meteorology data (see lines 132–134)”.

Authors: Thank you for your suggestion. Sentence was corrected just using the abbreviations.

References (pages 14–17). References [18, 20, 22, 23, 27, 31, 56, 64, and 72] are given incorrectly. See please the highlighted version of the manuscript for details.

Authors: Thank you for your suggestion, References were corrected.

Reviewer 3 Report

This is a well-written and interesting addition to the literature on dust in Iran. I only have a few comments:

Introduction: the authors are right to highlight dust effects on atmospheric processes, but I think they should also mention the numerous adverse impacts of atmospheric dust on society – it is a significant issue in Iran.

L62 & L365 I don’t accept the phrase “normal conditions”. What is normal I wonder?

L77 why the focus on winter? This should be justified.

L221 “Reversely” should be “Conversely”

L367 & L437 “anti-correlated” should read “inversely correlated”

Author Response

Reviewer: 3

This is a well-written and interesting addition to the literature on dust in Iran. I only have a few comments:

L62 & L365 I don’t accept the phrase “normal conditions”. What is normal I wonder?

Authors: With the term “normal” we refer to climatologically normal conditions in order to differentiate them from the extreme drought or dusty periods in the Middle East. In the revised manuscript, we rephrased the sentence in the first case L62 and we used the term “climatological normal conditions” for the other cases.

L77 why the focus on winter? This should be justified.

Authors: Thank you for your suggestion. The objectives and importance of this paper are now better underlined in lines 75-81 and 85-87 (at the end of the Introduction section). Briefly, we focused on the winter period because the dust trends were not well analyzed and known (as in spring and summer) and since during winter dust aerosols even dominate over the Middle East, while the meteorological conditions are much more variable, thus affecting significantly the dust activity, as well as dust emissions in the following spring season.

L221 “Reversely” should be “Conversely”

Authors: Thank you, it was corrected as suggested.

L367 & L437 “anti-correlated” should read “inversely correlated”

Authors: Thank you for your suggestion. “Anti-correlated” was replaced by “inversely correlated” throughout the manuscript.