the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 May 2024
Remote Sensing detectability of airborne Arctic dust
Abstract. Remote sensing (RS) based estimates of Arctic dust are oftentimes overestimated due to a failure in separating out the dust contribution from that of spatially homogeneous clouds or low-altitude cloud-like plumes. A variety of illustrations are given with a particular emphasis on claims of using brightness temperature differences (BTDs) as a signature indicator of Arctic dust transported from mid-latitude deserts or generated by local Arctic sources. While there is little dispute about the presence of both Asian and local dust across the Arctic, the direct RS detectability of airborne dust, as ascribed to satellite (MODIS and AVHRR) measurements of significantly negative brightness-temperature differences at 11 and 12 µm (BTD11-12) has been misrepresented in certain cases. While it is difficult to account for all examples of strongly negative BTD11-12 values in the Arctic, it is unlikely that airborne dust plays a significant role. One, much more likely contributor would be water plumes in the Arctic inversion layer.
The RS detectability of the impact of Arctic dust (notably due to Arctic dust from local sources) can, however, be of significance. Sustained dust deposition can substantially decrease (visible to shortwave IR) snow and ice reflectance albedo (pan-chromatic reflectance) and the signal measured by satellite sensors. Significantly negative BTD11-12 values would however only represent a limited area near the drainage basin sources according to our event-level case studies. The enhanced INP (Ice Nucleating Particle) role of local Arctic dust can, for example, induce significant changes in the properties of low-level mixed-phase clouds (cloud optical depth changes <~ 1) that can be readily detected by active and passive RS instruments. It is critical that the distinction between the RS detectability of airborne Arctic dust versus the RS detectability of the impacts of that dust be understood if we are to appropriately parameterize, for example, the radiative forcing influence of dust in this climate sensitive region.
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RC1: 'Comment on egusphere-2024-1057', Anonymous Referee #2, 14 Jul 2024
Review of: “Remote Sensing detectability of airborne Arctic dust” by Norman O’Neill et al.
This paper basically describes the effect that based on brightness temperature dust in the Arctic may have been overestimated and misclassified in the past. It is basically a response to the Bowen & Vicent (2021) nature paper and in this work O’Neill et al. argue that water (clouds) may lead to an systematic overestimation of dust in the Arctic. This is of course a relevant finding. However, I am not very happy with the presentation. Due to the lack of a dedicated result section and a long supplement, which is not ordered I found the paper a bit difficult to read. I recommend to re-structure the paper for better readability.
Some specific comments:
Line 42: (and later line 74) what do you mean by AOD 0.016 × 1.5^ ±1 and ~ 0.0023 × 1.2^ ±1 ? Is the exponent the Angstroem? If so: what does the +/- sign means? To what wavelength you are referring to? How did you derive these values?
Line 47: reference to S8: Tell here right away that MORTRAN has been used. Does this refer to an AOD of 1? Is the aerosol distribution log-normal (with which parameters?)
If you give AOD values without mentioning the wavelength, do you refer to 500nm?
Line 73: provide units for “Arctic dust mass”
Line 89: Ranjbar paper is it 2022 or 2021?
Given the general title of section 2, this paragraph is quite short. You may also relate to ground-based observations, which are in line with your argumentation, e.g.
https://acp.copernicus.org/articles/17/8101/2017/
line 152: “Nearly all of our negative BTD …” This sentence is important and you should be more specific on your data.
line 159: MODRAN simulations: you may list in the appendix the important values, assumptions etc. which you have used.
Otherwise change the title to … in the IR from satellite.
Table 1: I wonder, whether the relations between temperature lapse, emissivity slope and BTD pattern are always so easy and unique (if so: why?) what if there are several layers of dust and cloud in different altitudes? You must not fully answer this, but an idea of the assumptions and limitations behind the results of table 1 are important.
Table A2 is not completely clear to me. Can you please confirm or clarify: DOD_m in 3rd column is what you assume to calculate DOD(tau). The last column is what is needed to bring your results in agreement to KA. You are using eq A4 (not 4). If so: the DOD in last column is extremely variable (factor 16 between Resolute Bay and Eureka), I wonder how to interpret this. What are your critical assumptions here?
The value of appendix A6 and espec. Table A3 is not clear to me. I would be good to have a statement of the meaning in a broader context.
After these revisions I support a publication.
Citation: https://doi.org/10.5194/egusphere-2024-1057-RC1 -
AC1: 'Reply on RC1', Seyed Ali Sayedain, 20 Jul 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1057/egusphere-2024-1057-AC1-supplement.pdf
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AC2: 'Reply on RC1', Seyed Ali Sayedain, 12 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1057/egusphere-2024-1057-AC2-supplement.pdf
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AC1: 'Reply on RC1', Seyed Ali Sayedain, 20 Jul 2024
-
RC2: 'Comment on egusphere-2024-1057', Anonymous Referee #1, 19 Aug 2024
The paper discusses the remote sensing of dust aerosols over the Arctic and the question of the possible misinterpretation of dust identification when using the brightness-temperature differences at 11 and 12 μm as parameter. In particular the possible bias induced by clouds in is investigated. The paper provides an interesting discussion against recent literature and provides illustration based on specific cases. The paper topic is well suited for ACP and surely of relevance for the dust and remote sensing community. However, the presentation quality should be improved before publication. As general comment, in fact, the paper is quite hard to read as the presentation and the discussion is based on many references to other papers, including mention to literature figures, and reference to Appendix and Supplementary material of the paper itself. Several footnotes are also present in the text and could be avoided. Despite it is appreciable to have a concise manuscript, the many references to literature and additional material in the paper make the reading often difficult. The reviewer suggests to revise the presentation to make it more self-consistent.
Other specific comments:
Section 2 is quite short and not fully clear in particular since, as the introductory part, it relies on the reference to literature and supplementary material
Introduction and following sections: many literature measurements from diverse sites in the Arctic are discussed. It would be good to have the localisation of these sites either in the form of latitude and longitude (in Table or main text; these are mentioned for some sites in the Appendix section only) or as a map. A map could be useful to provide some contextualisation of the discussion for a non-Arctic specialized reader.
Appendix A, line 293: the value of 𝑟𝑒𝑓𝑓 of 2.7 μm id referring to a transported or locally emitted dust? As the dust diameter changes over transport time due to gravitational settling, is this assumed Reff value representative of source or long range transported dust? Please clarify in the text.
Footnote number 7 and Appendix B1: the OPAC database is quite outdated to represent dust infrared refractive index and the survey in Table B1 is missing several key works in the literature that investigated the infrared refractive index of dust aerosols. For this reason, I would either change the title of this section to clarify that this is not an exhaustive survey, or to extend the survey and take the variability of the refractive index of dust into account.
Citation: https://doi.org/10.5194/egusphere-2024-1057-RC2 -
AC3: 'Reply on RC2', Seyed Ali Sayedain, 12 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1057/egusphere-2024-1057-AC3-supplement.pdf
-
AC3: 'Reply on RC2', Seyed Ali Sayedain, 12 Sep 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-1057', Anonymous Referee #2, 14 Jul 2024
Review of: “Remote Sensing detectability of airborne Arctic dust” by Norman O’Neill et al.
This paper basically describes the effect that based on brightness temperature dust in the Arctic may have been overestimated and misclassified in the past. It is basically a response to the Bowen & Vicent (2021) nature paper and in this work O’Neill et al. argue that water (clouds) may lead to an systematic overestimation of dust in the Arctic. This is of course a relevant finding. However, I am not very happy with the presentation. Due to the lack of a dedicated result section and a long supplement, which is not ordered I found the paper a bit difficult to read. I recommend to re-structure the paper for better readability.
Some specific comments:
Line 42: (and later line 74) what do you mean by AOD 0.016 × 1.5^ ±1 and ~ 0.0023 × 1.2^ ±1 ? Is the exponent the Angstroem? If so: what does the +/- sign means? To what wavelength you are referring to? How did you derive these values?
Line 47: reference to S8: Tell here right away that MORTRAN has been used. Does this refer to an AOD of 1? Is the aerosol distribution log-normal (with which parameters?)
If you give AOD values without mentioning the wavelength, do you refer to 500nm?
Line 73: provide units for “Arctic dust mass”
Line 89: Ranjbar paper is it 2022 or 2021?
Given the general title of section 2, this paragraph is quite short. You may also relate to ground-based observations, which are in line with your argumentation, e.g.
https://acp.copernicus.org/articles/17/8101/2017/
line 152: “Nearly all of our negative BTD …” This sentence is important and you should be more specific on your data.
line 159: MODRAN simulations: you may list in the appendix the important values, assumptions etc. which you have used.
Otherwise change the title to … in the IR from satellite.
Table 1: I wonder, whether the relations between temperature lapse, emissivity slope and BTD pattern are always so easy and unique (if so: why?) what if there are several layers of dust and cloud in different altitudes? You must not fully answer this, but an idea of the assumptions and limitations behind the results of table 1 are important.
Table A2 is not completely clear to me. Can you please confirm or clarify: DOD_m in 3rd column is what you assume to calculate DOD(tau). The last column is what is needed to bring your results in agreement to KA. You are using eq A4 (not 4). If so: the DOD in last column is extremely variable (factor 16 between Resolute Bay and Eureka), I wonder how to interpret this. What are your critical assumptions here?
The value of appendix A6 and espec. Table A3 is not clear to me. I would be good to have a statement of the meaning in a broader context.
After these revisions I support a publication.
Citation: https://doi.org/10.5194/egusphere-2024-1057-RC1 -
AC1: 'Reply on RC1', Seyed Ali Sayedain, 20 Jul 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1057/egusphere-2024-1057-AC1-supplement.pdf
-
AC2: 'Reply on RC1', Seyed Ali Sayedain, 12 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1057/egusphere-2024-1057-AC2-supplement.pdf
-
AC1: 'Reply on RC1', Seyed Ali Sayedain, 20 Jul 2024
-
RC2: 'Comment on egusphere-2024-1057', Anonymous Referee #1, 19 Aug 2024
The paper discusses the remote sensing of dust aerosols over the Arctic and the question of the possible misinterpretation of dust identification when using the brightness-temperature differences at 11 and 12 μm as parameter. In particular the possible bias induced by clouds in is investigated. The paper provides an interesting discussion against recent literature and provides illustration based on specific cases. The paper topic is well suited for ACP and surely of relevance for the dust and remote sensing community. However, the presentation quality should be improved before publication. As general comment, in fact, the paper is quite hard to read as the presentation and the discussion is based on many references to other papers, including mention to literature figures, and reference to Appendix and Supplementary material of the paper itself. Several footnotes are also present in the text and could be avoided. Despite it is appreciable to have a concise manuscript, the many references to literature and additional material in the paper make the reading often difficult. The reviewer suggests to revise the presentation to make it more self-consistent.
Other specific comments:
Section 2 is quite short and not fully clear in particular since, as the introductory part, it relies on the reference to literature and supplementary material
Introduction and following sections: many literature measurements from diverse sites in the Arctic are discussed. It would be good to have the localisation of these sites either in the form of latitude and longitude (in Table or main text; these are mentioned for some sites in the Appendix section only) or as a map. A map could be useful to provide some contextualisation of the discussion for a non-Arctic specialized reader.
Appendix A, line 293: the value of 𝑟𝑒𝑓𝑓 of 2.7 μm id referring to a transported or locally emitted dust? As the dust diameter changes over transport time due to gravitational settling, is this assumed Reff value representative of source or long range transported dust? Please clarify in the text.
Footnote number 7 and Appendix B1: the OPAC database is quite outdated to represent dust infrared refractive index and the survey in Table B1 is missing several key works in the literature that investigated the infrared refractive index of dust aerosols. For this reason, I would either change the title of this section to clarify that this is not an exhaustive survey, or to extend the survey and take the variability of the refractive index of dust into account.
Citation: https://doi.org/10.5194/egusphere-2024-1057-RC2 -
AC3: 'Reply on RC2', Seyed Ali Sayedain, 12 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1057/egusphere-2024-1057-AC3-supplement.pdf
-
AC3: 'Reply on RC2', Seyed Ali Sayedain, 12 Sep 2024
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