the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Feb 2024
Impact of Biomass Burning Aerosols (BBA) on the tropical African climate in an ocean-atmosphere-aerosols coupled climate model
Abstract. The impact of biomass burning aerosols (BBA) emitted in Central Africa on the tropical African climate is studied using the ocean-atmosphere global climate model CNRM-CM, including prognostic aerosols. The direct BBA forcing, cloud feedbacks (semi-direct effects), effects on surface solar radiation, atmospheric dynamics and precipitation are analysed for the 1990–2014 period. During the June-July-August (JJA) season, the CNRM-CM simulations reveal a BBA semi-direct effect exerted on low-level clouds with an increase in cloud fraction of ∼5–10 % over a large part of the tropical ocean. The positive feedback of BBA radiative effects on low-level clouds is found to be mainly due to the sea surface temperature response (decrease of ~-0.5 K) associated with solar heating at 700 hPa, which increases the lower tropospheric stability. Over land, results also indicates a positive effect of BBA on the low cloud fraction especially for the coastal regions of Gabon and Angola with a potentially enhanced impact in these coupled simulations that integrate the response (cooling) of the SST. In addition to the BBA radiative effect on sea surface temperature, the ocean-atmosphere coupled simulations highlight that the oceanic temperature response is noticeable (about -0.2 to -0.4 K) down to ~80 m depth in the JJA between the African coast and 10°W. In parallel to low-level clouds, reductions of ~5–10 % are obtained for mid-level clouds over central Africa, mainly due to BBA-induced surface cooling and lower tropospheric heating inhibiting convection. In terms of cloud optical properties, the BBA radiative effects induced an increase of the optical depth by about ~2–3 south of the equator over the ocean. The result of the BBA direct effect and feedback on tropical clouds modulates the surface solar radiation over the whole Tropical Africa. The strongest surface dimming is over central Africa (~-30 W m−2), leading to a large reduction of the continental surface temperature (by ~-1 to -2 K), but the solar radiation at the oceanic surface is also affected up to the Brazilian coast. With respect to the hydrological cycle, the CNRM-CM simulations show a negative feedback on precipitation over the West African coast with a decrease of ~-1 to -2 mm per day. This study highlights also a persistent impact of BBA radiative effects on low-level clouds (increase in cloud fraction, liquid water content and optical depth) during the September-October-November (SON) period, mainly explained by a residual cooling of sea surface temperature over most of the tropical ocean. In SON, the feedback on precipitation is mainly simulated over the Gulf of Guinea with a reduction by ~-1 mm per day. As for JJA, the analysis clearly highlights the important role of the slow response of the ocean in SON and confirms the need to use coupled modelling platforms to study the impact of BBA on tropical African climate.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(23962 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(23962 KB) - Metadata XML
- BibTeX
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2024-496', Anonymous Referee #1, 27 Mar 2024
General Comments:
This study evaluates the coupled effects of biomass-burning aerosols from southern Africa on the regional climate, simulated by the CNRM-CM model over the period from 1990-2014. The large mass of emissions, combined with their steady presence for several months, exert strong effects on clouds, radiation, and the sea surface temperature. By comparing with a model run without these smoke emissions, the study shows that there are large and distinct effects in JJA (which is the majority of the annual burning period) and SON. These effects drive large local uncertainty in radiative balance, and this work provides an important analysis on when, where, and how BBA effects manifest. The paper is well-written and makes a convincing argument especially for the value of a dynamical ocean model in capturing regional trends outside of the main burning season. There are some points of clarification and background that do not detract from the overall work, and I recommend publication after minor corrections.
Specific Comments
Introduction:
As this study is a model application, studying the effects of toggling BBA emissions on and off, it necessarily can’t avoid inherent model biases. The authors show that the model represents smoke SSA well, but I would like to see some comment on model performance for other properties central to the study, where available, such as other smoke attributes or placement, cloud properties, or winds.
Lines 63-64: I understand that “few” is relative, but there are multiple recent studies overall analyzing the impact of African BBA on clouds, dynamics, and precipitation in the region. There have been several modeling studies addressing aspects of this question in the last several years with various methods, such as following the field campaigns AEROCLO-SA, ORACLES, CLARIFY, or LASIC. These may have important differences with this work, but they remain studies of this region on these topics. For example: Lu et al 2018, Gordon et al 2018, Diamond et al 2022, Perez et al 2023.
Methods:
Please add some physical description of the different size modes, such as the central diameter of each size bin. Aerosol optical and microphysical processes depend heavily on size ranges and this will give better context to other studies comparing to this work with different size schemes or parameters.
Line 132 and 142: Are nitrates and ammonium considered hydrophobic or hydrophilic, or something else?
Since precipitation changes are one of the focus topics of this work, I would like to see some mention of the impact of the missing second indirect effect as a standing uncertainty that could possibly modulate these results.
Line 149: Add a comment that defines the term “anomaly” used throughout the paper as in reference to the difference between these models, and exactly how it is being calculated.
Results:
193-194: Is the modeled SSA being 0.03-0.08 higher than observations playing a part in this heating differential?
Technical corrections
Figure formatting:
- Figure titles have “Anm” in the title but not defined.
- Several figure axes are labeled with the word ‘Presion’, which I believe should be ‘Pressure’
- The dashed grid lines for lat/lon should be labeled in most or all figures
- Since every model being used here is CPL_ndg, it isn’t necessary in figure titles since it doesn’t differentiate anything.
“Positive feedback” and “negative feedback” are used in multiple places when the context suggests the authors intend to mean ‘Positive/negative effect’ instead. The usage of feedback implies to me that the effect is self-reinforcing or self-destroying via some mechanism, rather than simply reporting an increase or decrease of some quantity. (examples at least at lines 5, 18, 415, 441, 451, )
Line 48: “indicate” should be “indicates”
Line 66: Should read “From the methodological…”
Line 95: Confusing sentence structure about what is causative and what is impacted- consider rewriting as “The overall effect on the solar surface radiative budget by both the BBA direct effect and changes in tropical clouds is also discussed."
Line 106: Is the second mention of “carbon cycle” redundant?
Line 108: missing right parenthesis )
Line 128: ambiguous usage of “supposed” - do you mean “assumed”? Or “intended”?
Line 128: “Externally mixed” does not refer to aerosol particles being separated by sources, but by species. I.e., a single particle is composed of a single species.
Line 152: Should read 26N, not 26S
Line 159: If this applies to every simulation used here, I don't see the need to specify a new acronym and put it in figure titles, as that would lead me to expect an un-coupled or un-nudged configuration to come up.
Line 162: Replace “thereafter” with “hereafter” or “below”
Line 165: Should read “...anomaly for JJA shows…” without the ‘the’
224: Change to “...anomaly is low west of 5°W…” rather than ‘above’
225: Specify what LW radiation means here - it seems to mean downwelling LW emissions from clouds, but is not clear the source.
272: ‘Atlantic coast’ should read ‘African coast’
288: ‘low cloud response’ is ambiguous. Does it refer to the response of low clouds? Or the relatively weak cloud response?
329: Should read ‘...the ocean modulates the BBA…” not ‘modulate’
337: The wording is confusing with “on the other hand”, since both results come from CNRM-CM. Perhaps change to “On the other hand, there is a moderate positive impact over northern Angola in CNRM-CM simulations.”
350: What do you mean ‘more important’?
428: What does precipitation ‘by day’ mean? Use either ‘per day’/’daily’, or ‘during the daytime’ depending on what you are saying.
429: change ‘on’ to ‘in’
Citation: https://doi.org/10.5194/egusphere-2024-496-RC1 -
AC2: 'Reply on RC1', Marc Mallet, 20 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-496/egusphere-2024-496-AC2-supplement.pdf
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RC2: 'Comment on egusphere-2024-496', Anonymous Referee #2, 24 Apr 2024
OVERALL COMMENTS:
Marc et al. used an coupled climate model to investigate the impact of biomass burning aerosols (BBA) on (1) cloud fraction, (2) ocean temperature and lower tropospheric stability, (3) AOD and surface solar radiation, (4) precipitation. While I appreciate the value of a coupled model, the quantitative results are not reliable and the attributions to processes are unconvincing given the limitations provided below. Besides, the presentation quality of the paper is very low. The use of terminology is inconsistent. Figures need panel titles, labels, and so on to improve clarity.
MAJOR COMMENTS:
Like many other global models, the model used in this study has significant low bias in low cloud fraction in the area of interest (Roehrig et al. 2020). The horizontal resolution is lower than many other GCMs/ESMs nowadays. How do these limitations affect all the quantitative results/arguments related to low clouds?
The model assumes the aerosols to be externally mixed. This is unrealistic for aerosols undergoing long distance transportation. In particular, there is no second indirect effect. Then it is not surprising that much of the impacts of BBA on several aspects of the model came from direct and semi-direct effects. How do these limitations affect the results?
MINOR COMMENTS:
Section 2.3: What emission inventory was used?
L169: "which could be due to ...": The authors listed so many possible causes that the sentence lost its value. Please narrow down the cause.
L193: "This difference is possibly attributed to ...": Can this hypothesis be tested using some offline radiative transfer model?
L210: The mechnism here is over simplified. The LTS increases in a broad area. But the regions with increases in Figures 3a, 3b, and 3c all show two stripes. In other words, an increase in LTS does not guarantee an increase in low cloud fraction or LWP. Please explain.
L230: "This could be due to": Please elaborate. How similar/different are the surface radiations in Solmon 2021 and current work?
L233: "local heating": What does "local" mean here?
L252: I assume there are many differences between the models used in these studies. Please explain why the treatment of the ocean and ocean-atmosphere coupling is believed to be the most important factor.
L260: "However, ...": Please be quantitative.
L267: "where the negative anomaly may exceed that identified between the surface and 20 m.": Would you please provide an explanation for this feature?
L286: "Over Central Africa, ...": Would you please provide an explanation for this feature?
L288: "due to the coupling between the ocean and the atmosphere being taken into account": Please elaborate.
L321: "the BBA radiative effects and solar heating": Isn't solar heating part of the radiative effect?
L355: "Indeed and as mentioned previously, ...": There are also regions with positive humidity anomaly but decreased precipitation. Please explain.
L362: The stongest positive precipitation anomaly occurred along the coast of Gabon and extended to the west. Why analyzing the moisture tendency anomaly for Angolan coast?
L385: "with a more pronounced effect": Would you please provide an explanation for this feature?
L386: "Contrary to JJA, ...": The statement in this sentnence is inconsistent with Figure 9a.
L393: The attribution of the strong signal outside of the Brazilian coast to the mission changes in Amazon is not convincing. Please elaborate.
TECHNICAL ISSUES:
Figure 2b: There is no need for the degree symbol with K.
Figure 6a: What is "medium-cloud"?
Figures 6b and 6c: Assuming the gray shading is terrain, why is it different in 6b and 6c, both averaged between 15 deg E and 25 deg E and shown for about the same latitude range?
Figure 7: Which one is 925 and which one is 850 hPa?
Figure A3: Are the panels in the left column for simulations with or without BBA emissions?
L346: Unit seems wrong.
Figure 8: Units not fully visible.
L380: What does "As for" here?
L420: What does "In parallel and as for the JJA season" mean here?
L422: "-0.3": Unit missing
The authors used "beyond" some longitudes in many places. Do they all mean "to the west of"? Please clarify. Also, in L235: "below the equator": Does it mean "to the south of the equator"?
Need to explain what is RegCM-SOM model and what are RegCM simulations. Are "RegCM-SOM" and "RegCM" the same thing?
Citation: https://doi.org/10.5194/egusphere-2024-496-RC2 -
AC1: 'Reply on RC2', Marc Mallet, 20 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-496/egusphere-2024-496-AC1-supplement.pdf
-
AC1: 'Reply on RC2', Marc Mallet, 20 Jun 2024
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-496', Anonymous Referee #1, 27 Mar 2024
General Comments:
This study evaluates the coupled effects of biomass-burning aerosols from southern Africa on the regional climate, simulated by the CNRM-CM model over the period from 1990-2014. The large mass of emissions, combined with their steady presence for several months, exert strong effects on clouds, radiation, and the sea surface temperature. By comparing with a model run without these smoke emissions, the study shows that there are large and distinct effects in JJA (which is the majority of the annual burning period) and SON. These effects drive large local uncertainty in radiative balance, and this work provides an important analysis on when, where, and how BBA effects manifest. The paper is well-written and makes a convincing argument especially for the value of a dynamical ocean model in capturing regional trends outside of the main burning season. There are some points of clarification and background that do not detract from the overall work, and I recommend publication after minor corrections.
Specific Comments
Introduction:
As this study is a model application, studying the effects of toggling BBA emissions on and off, it necessarily can’t avoid inherent model biases. The authors show that the model represents smoke SSA well, but I would like to see some comment on model performance for other properties central to the study, where available, such as other smoke attributes or placement, cloud properties, or winds.
Lines 63-64: I understand that “few” is relative, but there are multiple recent studies overall analyzing the impact of African BBA on clouds, dynamics, and precipitation in the region. There have been several modeling studies addressing aspects of this question in the last several years with various methods, such as following the field campaigns AEROCLO-SA, ORACLES, CLARIFY, or LASIC. These may have important differences with this work, but they remain studies of this region on these topics. For example: Lu et al 2018, Gordon et al 2018, Diamond et al 2022, Perez et al 2023.
Methods:
Please add some physical description of the different size modes, such as the central diameter of each size bin. Aerosol optical and microphysical processes depend heavily on size ranges and this will give better context to other studies comparing to this work with different size schemes or parameters.
Line 132 and 142: Are nitrates and ammonium considered hydrophobic or hydrophilic, or something else?
Since precipitation changes are one of the focus topics of this work, I would like to see some mention of the impact of the missing second indirect effect as a standing uncertainty that could possibly modulate these results.
Line 149: Add a comment that defines the term “anomaly” used throughout the paper as in reference to the difference between these models, and exactly how it is being calculated.
Results:
193-194: Is the modeled SSA being 0.03-0.08 higher than observations playing a part in this heating differential?
Technical corrections
Figure formatting:
- Figure titles have “Anm” in the title but not defined.
- Several figure axes are labeled with the word ‘Presion’, which I believe should be ‘Pressure’
- The dashed grid lines for lat/lon should be labeled in most or all figures
- Since every model being used here is CPL_ndg, it isn’t necessary in figure titles since it doesn’t differentiate anything.
“Positive feedback” and “negative feedback” are used in multiple places when the context suggests the authors intend to mean ‘Positive/negative effect’ instead. The usage of feedback implies to me that the effect is self-reinforcing or self-destroying via some mechanism, rather than simply reporting an increase or decrease of some quantity. (examples at least at lines 5, 18, 415, 441, 451, )
Line 48: “indicate” should be “indicates”
Line 66: Should read “From the methodological…”
Line 95: Confusing sentence structure about what is causative and what is impacted- consider rewriting as “The overall effect on the solar surface radiative budget by both the BBA direct effect and changes in tropical clouds is also discussed."
Line 106: Is the second mention of “carbon cycle” redundant?
Line 108: missing right parenthesis )
Line 128: ambiguous usage of “supposed” - do you mean “assumed”? Or “intended”?
Line 128: “Externally mixed” does not refer to aerosol particles being separated by sources, but by species. I.e., a single particle is composed of a single species.
Line 152: Should read 26N, not 26S
Line 159: If this applies to every simulation used here, I don't see the need to specify a new acronym and put it in figure titles, as that would lead me to expect an un-coupled or un-nudged configuration to come up.
Line 162: Replace “thereafter” with “hereafter” or “below”
Line 165: Should read “...anomaly for JJA shows…” without the ‘the’
224: Change to “...anomaly is low west of 5°W…” rather than ‘above’
225: Specify what LW radiation means here - it seems to mean downwelling LW emissions from clouds, but is not clear the source.
272: ‘Atlantic coast’ should read ‘African coast’
288: ‘low cloud response’ is ambiguous. Does it refer to the response of low clouds? Or the relatively weak cloud response?
329: Should read ‘...the ocean modulates the BBA…” not ‘modulate’
337: The wording is confusing with “on the other hand”, since both results come from CNRM-CM. Perhaps change to “On the other hand, there is a moderate positive impact over northern Angola in CNRM-CM simulations.”
350: What do you mean ‘more important’?
428: What does precipitation ‘by day’ mean? Use either ‘per day’/’daily’, or ‘during the daytime’ depending on what you are saying.
429: change ‘on’ to ‘in’
Citation: https://doi.org/10.5194/egusphere-2024-496-RC1 -
AC2: 'Reply on RC1', Marc Mallet, 20 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-496/egusphere-2024-496-AC2-supplement.pdf
-
RC2: 'Comment on egusphere-2024-496', Anonymous Referee #2, 24 Apr 2024
OVERALL COMMENTS:
Marc et al. used an coupled climate model to investigate the impact of biomass burning aerosols (BBA) on (1) cloud fraction, (2) ocean temperature and lower tropospheric stability, (3) AOD and surface solar radiation, (4) precipitation. While I appreciate the value of a coupled model, the quantitative results are not reliable and the attributions to processes are unconvincing given the limitations provided below. Besides, the presentation quality of the paper is very low. The use of terminology is inconsistent. Figures need panel titles, labels, and so on to improve clarity.
MAJOR COMMENTS:
Like many other global models, the model used in this study has significant low bias in low cloud fraction in the area of interest (Roehrig et al. 2020). The horizontal resolution is lower than many other GCMs/ESMs nowadays. How do these limitations affect all the quantitative results/arguments related to low clouds?
The model assumes the aerosols to be externally mixed. This is unrealistic for aerosols undergoing long distance transportation. In particular, there is no second indirect effect. Then it is not surprising that much of the impacts of BBA on several aspects of the model came from direct and semi-direct effects. How do these limitations affect the results?
MINOR COMMENTS:
Section 2.3: What emission inventory was used?
L169: "which could be due to ...": The authors listed so many possible causes that the sentence lost its value. Please narrow down the cause.
L193: "This difference is possibly attributed to ...": Can this hypothesis be tested using some offline radiative transfer model?
L210: The mechnism here is over simplified. The LTS increases in a broad area. But the regions with increases in Figures 3a, 3b, and 3c all show two stripes. In other words, an increase in LTS does not guarantee an increase in low cloud fraction or LWP. Please explain.
L230: "This could be due to": Please elaborate. How similar/different are the surface radiations in Solmon 2021 and current work?
L233: "local heating": What does "local" mean here?
L252: I assume there are many differences between the models used in these studies. Please explain why the treatment of the ocean and ocean-atmosphere coupling is believed to be the most important factor.
L260: "However, ...": Please be quantitative.
L267: "where the negative anomaly may exceed that identified between the surface and 20 m.": Would you please provide an explanation for this feature?
L286: "Over Central Africa, ...": Would you please provide an explanation for this feature?
L288: "due to the coupling between the ocean and the atmosphere being taken into account": Please elaborate.
L321: "the BBA radiative effects and solar heating": Isn't solar heating part of the radiative effect?
L355: "Indeed and as mentioned previously, ...": There are also regions with positive humidity anomaly but decreased precipitation. Please explain.
L362: The stongest positive precipitation anomaly occurred along the coast of Gabon and extended to the west. Why analyzing the moisture tendency anomaly for Angolan coast?
L385: "with a more pronounced effect": Would you please provide an explanation for this feature?
L386: "Contrary to JJA, ...": The statement in this sentnence is inconsistent with Figure 9a.
L393: The attribution of the strong signal outside of the Brazilian coast to the mission changes in Amazon is not convincing. Please elaborate.
TECHNICAL ISSUES:
Figure 2b: There is no need for the degree symbol with K.
Figure 6a: What is "medium-cloud"?
Figures 6b and 6c: Assuming the gray shading is terrain, why is it different in 6b and 6c, both averaged between 15 deg E and 25 deg E and shown for about the same latitude range?
Figure 7: Which one is 925 and which one is 850 hPa?
Figure A3: Are the panels in the left column for simulations with or without BBA emissions?
L346: Unit seems wrong.
Figure 8: Units not fully visible.
L380: What does "As for" here?
L420: What does "In parallel and as for the JJA season" mean here?
L422: "-0.3": Unit missing
The authors used "beyond" some longitudes in many places. Do they all mean "to the west of"? Please clarify. Also, in L235: "below the equator": Does it mean "to the south of the equator"?
Need to explain what is RegCM-SOM model and what are RegCM simulations. Are "RegCM-SOM" and "RegCM" the same thing?
Citation: https://doi.org/10.5194/egusphere-2024-496-RC2 -
AC1: 'Reply on RC2', Marc Mallet, 20 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-496/egusphere-2024-496-AC1-supplement.pdf
-
AC1: 'Reply on RC2', Marc Mallet, 20 Jun 2024
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Marc Mallet
Aurore Voldoire
Fabien Solmon
Pierre Nabat
Thomas Drugé
Romain Roehrig
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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