the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Jun 2025
Impact on Cloud Properties of Reduced-Sulphur Shipping Fuel in the Eastern North Atlantic
Abstract. The global reduction in shipping fuel sulphur that culminated in 2020 with an ~80 % reduction has enabled an inadvertent experiment on the role of aerosol-cloud interaction (ACI) in the climate system. We compare observations collected at the Atmospheric Radiation Measurement program's (ARM) Eastern North Atlantic site (ARM-ENA, 39.1 N, 28.0 W) during two June to September periods: 2016–2018 (pre-2020) and 2021–2023 (post-2020). We find a significant (~15 %) decrease in cloud condensation nuclei concentrations post-2020, which resulted in a decrease in cloud droplet number (Nd) and an increase in effective radius (re) of marine boundary layer clouds. However, cloud liquid water path (LWP) increased post-2020. The increase in LWP offset the increase in re, resulting in insignificant changes to the optical depth distribution. MODIS and CERES data in the vicinity of ENA during these periods produce similar results also with negligible change in the albedo and optical depth distributions. Regional cloud occurrence declined in line with changes in the large-scale meteorology. Our results point to a complicated interplay among the factors that modulate cloud feedback in the Eastern North Atlantic.
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Status: closed (peer review stopped)
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RC1: 'Comment on egusphere-2025-2075', Anonymous Referee #1, 26 Jun 2025
Mace et al. are interested in the impact of the shipping sulfur regulations imposed by the MLO in 2020 on cloud properties and the Earth radiation budget. They present a concise and useful analysis of systematic ground-based remote sensing at a maritime observatory station (yet another manifestation of the outstanding usefulness of the American Atmospheric Radiation Measurement Programme that around the world we would very much like to see continued). They complement this with an analysis of satellite retrievals and meteorological reanalyses.
The study is very well written, grounded in a concise and knowledgeable report about the state of the art. The results are diligently discussed. I only have a few minor suggestions for consideration as minor revisions.
l33 Not quite the exact definition of albedo, which rather is the ratio of reflected to incoming solar radiation
l45 This statement is a bit misleading. Diamond et al. estimate the -1 Wm-2 when considering all anthropogenic aerosol sources, not just the ones from shipping.
l88 why not also the unit for EIS in the caption
l92 Surface temperature is of course not a cloud-controlling factor, but it might still be interesting to also show it here. The reason is that warming in between pre and post might explain aspects of the cloud changes, such as the LWP increase
l100 is the change by 19 cm-3 relative to 160 cm-3 not 12%, rather than 18%?
l102 any idea why the change by 50% is much larger than the one for CCN?
l104 maybe it is worth noting this scaling is for vertically uniform droplet size distributions
l106 it might be interesting to examine (e.g. in Fig. 1) the temperature changes. Is perhaps the LWP increase (partly) consistent with an increase in adiabatic liquid water content in response to warming? if so, one might be able attempt to deconvolve the warming from the aerosol aspects
l113 very interesting
l170 just to clarify – this would be the expected signal. The accumulated precipitation over all intensity classes should change only in response to surface and atmospheric energy budget changes.
l173 I am not sure I understand this metric. first of all, where is the time unit, does one not need a rate? Second, why per horizontal cloud fraction and not rather something more related to LWP? Maybe a formula would help. Or maybe this calculation does actually not help the understanding.
Citation: https://doi.org/10.5194/egusphere-2025-2075-RC1 -
RC2: 'Comment on egusphere-2025-2075', Mark Miller, 03 Jul 2025
This is an interesting and informative analysis.
Line 54: “produce a warming of 0.2 W m-2: suggest “produce an increase in global surface radiation of 0.2 W m-2.
Line 81: Stronger surface winds may also lead to increased mesoscale organization in convective clusters.
Figure 2: Smaller effective radii, increased CCN, pre 2020 but increased precipitation relative to post 2020 seems counterintuitive. Does this suggest that LWP, which is greater in the pre-2020 period, is a stronger modulator of precipitation?
The 2021-2023 corresponds to an extended, strong La Nina. As you note in Line 160, changes in large scale circulation complicate any simple conclusions, but you might consider mentioning a possible link between the ENSO state and cloud coverage over the ENA.
Line 170: “While we find that the occurrence frequency of precipitation does increase slightly as the re increases, the occurrence of heavier drizzle (>0.1 mm day-1) decreases”. Heavy drizzle at ENA typically occurs when cumulus-coupled stratocumulus is present, hence this decrease in heavier precipitation may portend a reduction in cumulus-coupling events.
Line 176: “we speculate that the MBL cloud fraction would have perhaps increased due to the reduction in loss of cloud water to drizzle perhaps resulting in a negative feedback.” If drizzle evaporation in the sub cloud layer is reduced, sub cloud layer stability is decreased, which may also help to promote an increase in cloud fraction.
Line 195: “or if the small decrease in column water vapor and/or the weaker inversion strength acted to influence the distribution of drizzle rates”. As I noted in an earlier comment, at a process level, a change in the mode of the ENA cloud structure could also influence the distribution of drizzle rates over the ENA.
Citation: https://doi.org/10.5194/egusphere-2025-2075-RC2
Status: closed (peer review stopped)
-
RC1: 'Comment on egusphere-2025-2075', Anonymous Referee #1, 26 Jun 2025
Mace et al. are interested in the impact of the shipping sulfur regulations imposed by the MLO in 2020 on cloud properties and the Earth radiation budget. They present a concise and useful analysis of systematic ground-based remote sensing at a maritime observatory station (yet another manifestation of the outstanding usefulness of the American Atmospheric Radiation Measurement Programme that around the world we would very much like to see continued). They complement this with an analysis of satellite retrievals and meteorological reanalyses.
The study is very well written, grounded in a concise and knowledgeable report about the state of the art. The results are diligently discussed. I only have a few minor suggestions for consideration as minor revisions.
l33 Not quite the exact definition of albedo, which rather is the ratio of reflected to incoming solar radiation
l45 This statement is a bit misleading. Diamond et al. estimate the -1 Wm-2 when considering all anthropogenic aerosol sources, not just the ones from shipping.
l88 why not also the unit for EIS in the caption
l92 Surface temperature is of course not a cloud-controlling factor, but it might still be interesting to also show it here. The reason is that warming in between pre and post might explain aspects of the cloud changes, such as the LWP increase
l100 is the change by 19 cm-3 relative to 160 cm-3 not 12%, rather than 18%?
l102 any idea why the change by 50% is much larger than the one for CCN?
l104 maybe it is worth noting this scaling is for vertically uniform droplet size distributions
l106 it might be interesting to examine (e.g. in Fig. 1) the temperature changes. Is perhaps the LWP increase (partly) consistent with an increase in adiabatic liquid water content in response to warming? if so, one might be able attempt to deconvolve the warming from the aerosol aspects
l113 very interesting
l170 just to clarify – this would be the expected signal. The accumulated precipitation over all intensity classes should change only in response to surface and atmospheric energy budget changes.
l173 I am not sure I understand this metric. first of all, where is the time unit, does one not need a rate? Second, why per horizontal cloud fraction and not rather something more related to LWP? Maybe a formula would help. Or maybe this calculation does actually not help the understanding.
Citation: https://doi.org/10.5194/egusphere-2025-2075-RC1 -
RC2: 'Comment on egusphere-2025-2075', Mark Miller, 03 Jul 2025
This is an interesting and informative analysis.
Line 54: “produce a warming of 0.2 W m-2: suggest “produce an increase in global surface radiation of 0.2 W m-2.
Line 81: Stronger surface winds may also lead to increased mesoscale organization in convective clusters.
Figure 2: Smaller effective radii, increased CCN, pre 2020 but increased precipitation relative to post 2020 seems counterintuitive. Does this suggest that LWP, which is greater in the pre-2020 period, is a stronger modulator of precipitation?
The 2021-2023 corresponds to an extended, strong La Nina. As you note in Line 160, changes in large scale circulation complicate any simple conclusions, but you might consider mentioning a possible link between the ENSO state and cloud coverage over the ENA.
Line 170: “While we find that the occurrence frequency of precipitation does increase slightly as the re increases, the occurrence of heavier drizzle (>0.1 mm day-1) decreases”. Heavy drizzle at ENA typically occurs when cumulus-coupled stratocumulus is present, hence this decrease in heavier precipitation may portend a reduction in cumulus-coupling events.
Line 176: “we speculate that the MBL cloud fraction would have perhaps increased due to the reduction in loss of cloud water to drizzle perhaps resulting in a negative feedback.” If drizzle evaporation in the sub cloud layer is reduced, sub cloud layer stability is decreased, which may also help to promote an increase in cloud fraction.
Line 195: “or if the small decrease in column water vapor and/or the weaker inversion strength acted to influence the distribution of drizzle rates”. As I noted in an earlier comment, at a process level, a change in the mode of the ENA cloud structure could also influence the distribution of drizzle rates over the ENA.
Citation: https://doi.org/10.5194/egusphere-2025-2075-RC2
Data sets
Microwave Radiometer, 3 Channel (MWR3C) M. Cadeddu et al. https://doi.org/10.5439/1025248
Ka ARM Zenith Radar (KAZR2CFRGE) Y.-C. Feng et al. https://doi.org/10.5439/1891991
Balloon-Borne Sounding System (SONDEWNPN) E. Keeler et al. https://doi.org/10.5439/1595321
Cloud Condensation Nuclei Particle Counter (AOSCCN2COLASPECTRA). A. Koontz et al. https://doi.org/10.5439/1786358
Micropulse Lidar (MPLPOLFS) P. Muradyan et al. https://doi.org/10.5439/1320657
MODIS atmosphere L2 cloud product (06_L2), Terra, NASA MODIS Adaptive Processing System S. Platnick et al. https://doi.org/10.5067/MODIS/MOD06_L2.006
NASA/LARC/SD/ASDC. (2014). CERES Single Scanner Footprint (SSF) TOA/Surface Fluxes, Clouds and Aerosols Terra-FM2 Edition4A NASA Langley Atmospheric Science Data Center DAAC https://doi.org/10.5067/TERRA/CERES/SSF-FM2_L2.004A
NASA/LARC/SD/ASDC. (2014). CERES Single Scanner Footprint (SSF) TOA/Surface Fluxes, Clouds and Aerosols Aqua-FM3 Edition4A NASA Langley Atmospheric Science Data Center DAAC https://doi.org/10.5067/AQUA/CERES/SSF-FM3_L2.004A
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