the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Feb 2022
Reduced surface fine dust under droughts over the southeastern United States during summertime: observations and CMIP6 model simulations
Abstract. Drought is an extreme hydroclimate event that has been shown to cause the increase of surface fine dust near source regions, while the drought-dust relationship in regions predominantly influenced by long-range transported dust such as the southeastern US (SEUS) has received less attention. Using long-term surface fine dust observations, weekly US Drought Monitor (USDM), and monthly Standardized Precipitation-Evapotranspiration Index (SPEI), this study unmasks spatial disparity in drought-dust relationships where the SEUS stands out as being abnormous in that it shows a decrease in surface dust concentrations during drought in contrast to the expected increase in dust found in other contiguous US (CONUS) regions. Surface fine dust was found to decrease by ~0.5 µg/m3 with a unit decrease of SPEI in the SEUS, as opposed to an increase of ~0.15 µg/m3 in the west. The anomalies of elemental ratios, satellite aerosol optical depth (AOD), and dust extinction coefficients suggest that both the emissions and trans-Atlantic transport of African dust are weakened when the SEUS is under droughts. Through the teleconnection patterns of negative North Atlantic Oscillation (NAO), a lower than normal and more northeastward displacement of the Bermuda High (BH) was present during SEUS droughts which resulted in less dust being transported into the SEUS. At the same time, enhanced precipitation in Sahel associated with the northward shift of the Intertropical Convergence Zone (ITCZ) leads to lower dust emissions therein. Of the four selected models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6), GISS-E2-1-G was found to perform the best in capturing the drought-dust sensitivity in the SEUS. This study reveals the mechanism of how regional-scale droughts influence aerosol abundance through changing long-range transport of dust.
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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.
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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.
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Review of “Reduced surface fine dust under droughts over the southeastern United States during summertime: observations and CMIP6 model simulations”', Anonymous Referee #1, 05 Apr 2022
General Comments: The manuscript “Reduced surface fine dust under droughts over the southeastern United States during summertime: observations and CMIP6 model simulations” by Li and Wang explores the drought-dust sensitivities over the CONUS based on observations and four AerChemMIP models. The authors found negative drought-dust sensitivities over the southeastern US as opposed to the usual positive sensitivities found over the western US. They associate these anomalous sensitivities over the southeastern US to decreased emissions and trans-Atlantic transport of African dust. The manuscript is well written and the content is straight-forward with reasonable conclusions. I appreciate that the authors consider multiple observational products and analysis to add confidence to their findings. I recommend acceptance with minor revisions.
Specific comments: (1) In line 102-103, if I understand correctly, the EPA-CSN data is remapped from 1x1 (coarse) to 0.5x0.5 (fine) grids using bilinear interpolation? It is often recommended to use conservative remapping when regridding from coarse to fine resolution. This way we can avoid biases near high emission/near-source regions. (2) In figure 1a, does the southeast US region show significant regression slopes when the dust is constrained within SPEI<0.5 bins? I understand that it will contain a substantial amount of missing data, but I would be curious to see the spatial distribution of the slopes under dry conditions. Also, I do not see the point of p-value in Figure 1b (southeast case). I realize that there is a positive relationship between SPEI and dust concentration, but it is not significant based on regional averages. So, consider looking at the slopes at each grid box to show the spatial distribution of the SPEI-dust sensitivity with significance. (3) From line 207-209, please elaborate on why these exact thresholds (30% or 70% etc.) are used. Consider putting appropriate references. (4) In figure 4b, why does the difference figure show 2 contrasting bands? Is it possible that the pathway shifted northward? What does the spatial map look like for severe drought conditions? (5) Consider adding the number of realizations used in the CMIP6 model evaluation. I think the readers would be interested to see a model ensemble mean response as well in Figure 8 (even though there are only 4 models and probably 10 realizations).
Citation: https://doi.org/10.5194/egusphere-2022-18-RC1 -
RC2: 'Comment on egusphere-2022-18', Anonymous Referee #2, 08 Apr 2022
Review of “Reduced surface fine dust under droughts over the southeastern United States during summertime: observations and CMIP6 model simulations” by Li and Wang
Fine surface dust in the southeastern U.S. is known to increase during summer months due to long-range transport of North African dust to the region. This manuscript investigates dust-drought relationships in the region and changes in large-scale atmospheric variabiliy and teleconnections with drought and dust transport to the SEUS. Evaluation of global transport models against observations also elucidate the ability of models to capture these connections during severe drought periods. The manuscript is well-organized and written, and is an important contribution to the literature. I suggest publishing after minor corrections based on the comments below.
Line 97: Can the authors provide more details regarding completeness criteria for including data from these sites? How do the different sampling periods at some sites (6-day vs 3-day) affect daily interpolations? Also, additional sites come on-line during this period (2000-2019), did the authors only use sites that were operating during the entire period? Adding sites for different years could bias the results from year to year. How did the authors treat the bias between the CSN and IMPROVE dust concentrations when combining the data? (e.g., Gorham et al., 2021; Hand et al., 2012).
Line 117: From 1996 until when?
Line 134: If am I understanding correctly, under extreme drought conditions the data for each site could correspond to different days?
Line 157: The shifts in Figure S1 appear different for both CONUS (severe drought is shifted further) and for the SEUS, it is not shifted as much. Can the authors elaborate?
Line 170: I am not sure I follow the reasoning for conducting the linear regression only using the lowest for SPEI bins. It would seem that the reasoning for doing this should apply to both the west and the east. Otherwise, it appears the data points are being ignored to get the desired results.
Line 187: How did the authors determine how the southeast region defined with the box shown in Figure 1a? How did they decide on the lat/lon limits or sites to include?
Line 207: How were these particular limits chosen?
Line 207: It also helps clarity of writing to include the opposite description in the text and not just parentheses (here and other places in the manuscript and captions), such as “Regional severe drought (non-drought)…”. Unless there are page limit and space issues, it causes more effort to understand than to just write it out.
Line 212: Are these droughts limited to the SEUS region mentioned above?
Line 214: How was this AOD limit chosen?
Line 259: Can the authors comment regarding the years with severe droughts that aren’t associated? Such as 2000, 2007, 2019?
Line 319: Include units with 4.76.
Line 331: Typo: “unite”
Line 355: It may be less confusing to write this as that conditions that caused reduced transport of dust also correspond to period with drought conditions (so it doesn’t seem that drought conditions in the SE are somehow causing less dust transport from Africa). The description in line 359-361 clarifies this but it could be misinterpreted here.
References:
Gorham et al., 2021, “Comparison of recent speciated PM2.5 data from collocated CSN and IMPROVE measurements”, Atmospheric Environment, 244, 117877.
Hand et al., 2012, “Seasonal composition of remote and urban fine particulate matter in the United States”, JGR-Atmospheres, 117, D05209.
Figures
Figure 1: Include JJA in the caption for part (a). Are the results shown in part (b) also for JJA?
Figure 2: Typo for part (b): “reginal”. Do these results correspond to all time periods?
Figure 3: Why were only IMPROVE data used (not CSN)?
Figure 4: Type in legend of part (a): “sever”. Also typo in caption: “Carrabin”. Include the wavelength of AOD in the caption.
Figure 5: Are these changes based on severe droughts only in the SEUS region? What years are included in this figure?
Figure 6: Include time periods in caption.
Figure 7: Include time periods in caption.
Figure 8: Line 306 states JJA but this legend reads “all months”. What is the time period? Include time period in caption. Also include that the dashed line corresponds to 1:1.
Table:
Table 1: Include time period in caption
Citation: https://doi.org/10.5194/egusphere-2022-18-RC2 -
AC1: 'Responses to Reviews', Wei Li, 17 May 2022
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-18/egusphere-2022-18-AC1-supplement.pdf
Interactive discussion
Status: closed
-
RC1: 'Review of “Reduced surface fine dust under droughts over the southeastern United States during summertime: observations and CMIP6 model simulations”', Anonymous Referee #1, 05 Apr 2022
General Comments: The manuscript “Reduced surface fine dust under droughts over the southeastern United States during summertime: observations and CMIP6 model simulations” by Li and Wang explores the drought-dust sensitivities over the CONUS based on observations and four AerChemMIP models. The authors found negative drought-dust sensitivities over the southeastern US as opposed to the usual positive sensitivities found over the western US. They associate these anomalous sensitivities over the southeastern US to decreased emissions and trans-Atlantic transport of African dust. The manuscript is well written and the content is straight-forward with reasonable conclusions. I appreciate that the authors consider multiple observational products and analysis to add confidence to their findings. I recommend acceptance with minor revisions.
Specific comments: (1) In line 102-103, if I understand correctly, the EPA-CSN data is remapped from 1x1 (coarse) to 0.5x0.5 (fine) grids using bilinear interpolation? It is often recommended to use conservative remapping when regridding from coarse to fine resolution. This way we can avoid biases near high emission/near-source regions. (2) In figure 1a, does the southeast US region show significant regression slopes when the dust is constrained within SPEI<0.5 bins? I understand that it will contain a substantial amount of missing data, but I would be curious to see the spatial distribution of the slopes under dry conditions. Also, I do not see the point of p-value in Figure 1b (southeast case). I realize that there is a positive relationship between SPEI and dust concentration, but it is not significant based on regional averages. So, consider looking at the slopes at each grid box to show the spatial distribution of the SPEI-dust sensitivity with significance. (3) From line 207-209, please elaborate on why these exact thresholds (30% or 70% etc.) are used. Consider putting appropriate references. (4) In figure 4b, why does the difference figure show 2 contrasting bands? Is it possible that the pathway shifted northward? What does the spatial map look like for severe drought conditions? (5) Consider adding the number of realizations used in the CMIP6 model evaluation. I think the readers would be interested to see a model ensemble mean response as well in Figure 8 (even though there are only 4 models and probably 10 realizations).
Citation: https://doi.org/10.5194/egusphere-2022-18-RC1 -
RC2: 'Comment on egusphere-2022-18', Anonymous Referee #2, 08 Apr 2022
Review of “Reduced surface fine dust under droughts over the southeastern United States during summertime: observations and CMIP6 model simulations” by Li and Wang
Fine surface dust in the southeastern U.S. is known to increase during summer months due to long-range transport of North African dust to the region. This manuscript investigates dust-drought relationships in the region and changes in large-scale atmospheric variabiliy and teleconnections with drought and dust transport to the SEUS. Evaluation of global transport models against observations also elucidate the ability of models to capture these connections during severe drought periods. The manuscript is well-organized and written, and is an important contribution to the literature. I suggest publishing after minor corrections based on the comments below.
Line 97: Can the authors provide more details regarding completeness criteria for including data from these sites? How do the different sampling periods at some sites (6-day vs 3-day) affect daily interpolations? Also, additional sites come on-line during this period (2000-2019), did the authors only use sites that were operating during the entire period? Adding sites for different years could bias the results from year to year. How did the authors treat the bias between the CSN and IMPROVE dust concentrations when combining the data? (e.g., Gorham et al., 2021; Hand et al., 2012).
Line 117: From 1996 until when?
Line 134: If am I understanding correctly, under extreme drought conditions the data for each site could correspond to different days?
Line 157: The shifts in Figure S1 appear different for both CONUS (severe drought is shifted further) and for the SEUS, it is not shifted as much. Can the authors elaborate?
Line 170: I am not sure I follow the reasoning for conducting the linear regression only using the lowest for SPEI bins. It would seem that the reasoning for doing this should apply to both the west and the east. Otherwise, it appears the data points are being ignored to get the desired results.
Line 187: How did the authors determine how the southeast region defined with the box shown in Figure 1a? How did they decide on the lat/lon limits or sites to include?
Line 207: How were these particular limits chosen?
Line 207: It also helps clarity of writing to include the opposite description in the text and not just parentheses (here and other places in the manuscript and captions), such as “Regional severe drought (non-drought)…”. Unless there are page limit and space issues, it causes more effort to understand than to just write it out.
Line 212: Are these droughts limited to the SEUS region mentioned above?
Line 214: How was this AOD limit chosen?
Line 259: Can the authors comment regarding the years with severe droughts that aren’t associated? Such as 2000, 2007, 2019?
Line 319: Include units with 4.76.
Line 331: Typo: “unite”
Line 355: It may be less confusing to write this as that conditions that caused reduced transport of dust also correspond to period with drought conditions (so it doesn’t seem that drought conditions in the SE are somehow causing less dust transport from Africa). The description in line 359-361 clarifies this but it could be misinterpreted here.
References:
Gorham et al., 2021, “Comparison of recent speciated PM2.5 data from collocated CSN and IMPROVE measurements”, Atmospheric Environment, 244, 117877.
Hand et al., 2012, “Seasonal composition of remote and urban fine particulate matter in the United States”, JGR-Atmospheres, 117, D05209.
Figures
Figure 1: Include JJA in the caption for part (a). Are the results shown in part (b) also for JJA?
Figure 2: Typo for part (b): “reginal”. Do these results correspond to all time periods?
Figure 3: Why were only IMPROVE data used (not CSN)?
Figure 4: Type in legend of part (a): “sever”. Also typo in caption: “Carrabin”. Include the wavelength of AOD in the caption.
Figure 5: Are these changes based on severe droughts only in the SEUS region? What years are included in this figure?
Figure 6: Include time periods in caption.
Figure 7: Include time periods in caption.
Figure 8: Line 306 states JJA but this legend reads “all months”. What is the time period? Include time period in caption. Also include that the dashed line corresponds to 1:1.
Table:
Table 1: Include time period in caption
Citation: https://doi.org/10.5194/egusphere-2022-18-RC2 -
AC1: 'Responses to Reviews', Wei Li, 17 May 2022
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-18/egusphere-2022-18-AC1-supplement.pdf
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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
(1937 KB) - Metadata XML
-
Supplement
(220 KB) - BibTeX
- EndNote
- Final revised paper