the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Nov 2024
Lightning declines over shipping lanes following regulation of fuel sulfur emissions
Abstract. Aerosol interactions with clouds represent a significant uncertainty in our understanding of the Earth system. Deep convective clouds may respond to aerosol perturbations in several ways that have proven difficult to elucidate with observations. Here, we leverage the two busiest maritime shipping lanes in the world, which emit aerosol particles and their precursors into an otherwise relatively clean tropical marine boundary layer, to make headway on the influence of aerosol on deep convective clouds. The recent seven-fold change in allowable fuel sulfur by the International Maritime Organization allows us to test the sensitivity of the lightning to changes in ship plume aerosol size distributions. We find that, across a range of atmospheric thermodynamic conditions, the previously documented enhancement of lightning over the shipping lanes has fallen by over 40 %. The enhancement is therefore at least partially aerosol-mediated, a conclusion that is supported by observations of droplet number at cloud base, which show a similar decline over the shipping lane. These results have fundamental implications for our understanding of aerosol-cloud interactions, suggesting that deep convective clouds are impacted by the aerosol number distribution in the remote marine environment.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-3236', Anonymous Referee #1, 07 Dec 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3236/egusphere-2024-3236-RC1-supplement.pdf
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RC2: 'Comment on egusphere-2024-3236', Anonymous Referee #2, 10 Dec 2024
This work follows a previous paper (Thornton et al, 2017) which showed a pattern in the lightning climatology around shipping lanes near South-East Asia. This work shows that the lightning in this region declined in the period following the 2020 IMO fuel sulphur regulations. The authors suggest that this decline is a consequence of reduced aerosol emissions from ships, supporting the conclusions of the previous paper that this lightning enhancement is due to an aerosol invigoration of convection.
While I think the authors are probably correct in their assessment, the presentation of the results could use a bit of improvement. In addition, I think some of the analysis could be improved to offer a more convincing picture of this change in lightning and how it relates to aerosol. This paper is otherwise in scope for ACP and I think would be suitable for publication after revisions.
Main pointsI find the overall pattern of change in lightning a little puzzling, which in turn casts a bit of doubt in the interpretation of the results. While there has been a decrease in lightning in the shipping lanes, there has also been a broad decrease in lightning across the study region (but with increases in some regions, Fig 1c). Given the pattern of lightning in the study region includes the shipping lanes, a large-scale reduction in lightning frequency across the region would also show a larger reduction in the shipping lanes. How do we know that the observed shipping lane change is not part of a larger change in lightning/convective cloud occurrence across the region?
It is not clear to me that the meteorological parameters used for regressing out the background meteorological state work well. The manuscript states that they only explain 33% of the variance in lightning occurrence. Fig. 3 suggests that they don't have a strong impact to lightning frequency (or enhancement), that there is a change in lightning occurrence across all meteorological conditions. I find this a little concerning, as we might expect the aerosol enhancement of convective clouds to depend on cloud/meteorological regime - why does it not in this case? This could be a potential indicator of some kind of confounding effect.
The manuscript also appears to be a little long for the ACP letters format (I don't have an exact word count, but a basic one suggests it is ~10% over the limit).
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Specific points
Abstract (and elsewhere) - while there have been changes to the aerosol size distribution, is the main effect a change to the number concentration? I might have expected this to be the first-order effect?
The ACP discussion format used to include line numbers, which are very useful for referencing specific locations during a review. References from here onward are 'Page - Paragraph - Line'
P1P3L3 - Williams et al, JGR 2002 might also be a relevant paper here
P2P3L6 - To me, 'declined' implies that this decrease is continuing - would 'decreased' here (and elsewhere) be clearer?
Fig 1 - A plot showing the current pattern might also be useful, as it is difficult to mentally subtract a linear scale form a log one.
P3P1L1 - Is assuming a linear ENSO effect sensible? There is significantly variation in the ENSO impact across regions.
P3P1L3 - Are these variables really explanatory? There seems to be a large pattern of variation across the study region, does regressing by these factors remove it?
Fig 2 - The colours for the lines in the right hand subfigure are within the colorbar. Different colors (e.g. black, green) might be easier to read.
Fig. 2 - As above, there appear to be a decrease in lightning far from the shipping lane in an approximately similar proportion (~25%). What is the reason for this?
P4P3L6 - Is a shallow-cloud Nd product suitable here? The bright core adiabatic assumption may be suitable for non-precipitating stratocumulus, but in more convective cases, it seems that it will pick out the precipitating locations (which are not adiabatic). Comparisons with aircraft data suggest it is not reliable in convective cases (Gryspeerdt et al, ACP, 2023).
P4P5L4(and elsewhere) - Nd misses a subscript.
Fig 3 - Naming the rows in the figure would make this easier to read
P6P2L1 - It is not clear to me that the Nd maintains previous levels in most regions. To the south of the shipping lane it appears there is a significant decrease in Nd post-IMO.
P6P2L6 - Double (
P6P3L2 - A 40% decrease is relatively small, given the ~80% reduction in ship fuel sulphur content expected from IMO2020?
P8P8L5 - Is such a high solar zenith angle common for MODIS retrievals in this region?
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PS2P5L4 - This is an very unusual referencing style, I would suggest the referencing in the SI should be the same as for the main manuscript.
PS2P6L1 - Why is reanalysis AOD used here? Previous studies (e.g. McCoy et al, JGR, 2017) suggest that reanalysis SO4 is a much better proxy.
PS4P1L1 - Is the background AOD really low in this region? The MODIS Terra mean in this region is around 0.2-0.3. While this is not high, it is significantly above the MODIS noise floor and not what I would have classed as 'low'.
Fig S2 - The SCS study region doesn't correctly cover the shiptrack in this diagram (or at least rendered with Acrobat Reader in Windows 11).
Fig. S6 - While not significant at any particular distance, there does appear to be a consistent enhancement in the AOD post IMO to the north of the shiptrack. What is driving this and does it affect the Nd results presented earlier in this work?
Citation: https://doi.org/10.5194/egusphere-2024-3236-RC2
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