the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Aug 2024
Co-variability drives the inverted-V sensitivity between liquid water path and droplet concentrations
Abstract. Many studies using climatological data of liquid water path (LWP) and droplet concentration (Nd) find an inverted-V relationship, where LWP increases and then decreases with Nd. Our findings suggest that while these LWP responses to changes in Nd align with proposed causal mechanisms, such as entrainment evaporation feedback and precipitation suppression, the inverted-V is primarily driven by the co-variability between LWP and Nd. This co-variability arises from meteorological conditions and microphysical processes, each independently affecting LWP and Nd in opposite directions. We further demonstrate that the inverted-V relationship reflects the climatological evolution of Stratocumulus clouds (Sc). Therefore, background anthropogenic changes in Nd should, in principle, be manifested in changes across the entire Sc climatology along its evolution. Instantaneous LWP response to Nd derived from ship tracks, or other similar natural experiments, may therefore not accurately represent the climatological LWP response. This is because the local perturbations in Nd may not align with the plausible natural co-variability between LWP and Nd, which varies depending on the cloud state along the Sc evolution.
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RC1: 'Comment on egusphere-2024-2245', Anonymous Referee #1, 31 Aug 2024
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Review of: “Co-variability drives the inverted-V sensitivity between liquid water path and droplet concentrations” by Goren et al.
This excellent paper addresses LWP-Nd covariability, a highly relevant scientific question within the scope of ACP. The Authors explain in detail how LWP-Nd covariability shapes the LWP-Nd inverted-V relationship. They demonstrate that the inverted-V relationship reflects the climatological evolution of Stratocumulus (Sc) clouds downwind from the coasts. They focus on low-level marine clouds in Sc regions and use MODIS satellite data, ERA5 reanalysis data, and Sc regime classification based on a neural network algorithm.
In my opinion, the conclusions are very well supported by the analysis, and the methods are fully appropriate. I want to thank the Authors for this important work highly relevant to improving the understanding of aerosol impacts on clouds and assessing aerosol forcing of Earth’s climate. I strongly recommend the paper for publication in ACP and suggest a few minor points for the Authors to consider in revising the paper.
Comments:
I suggest highlighting in the introduction that the inverted-V served as an important line of evidence in Ch7 of the Sixth Assessment Report by the Intergovernmental Panel on Climate Change for positive radiative forcing by decreased LWP (Forster et al., 2021) following Gryspeerdt et al. (2019). Moreover, I suggest more explicitly highlighting in the conclusions (and abstract?) that the inverted-V should no longer be used as a line of evidence for positive radiative forcing through LWP responses to aerosols as it is largely explained by the covariability.
I suggest highlighting in the abstract the specific identified drivers of LWP-Nd covariability. Perhaps something similar to the first paragraph on page 12 under the conclusions?
What do you think about adding a table summarising the drivers of LWP-Nd covariability in Sc clouds (for two regimes, i.e. negative and positive LWP-Nd sensitivities)?
I like Table 1. However, besides Table 1, a map with the region boxes would have been most helpful. Perhaps a map would work best in the supplement? A simple base map or a map with Nd and LWP climatologies?
p3l63 Perhaps the method by Choudhury and Goren (2024) could be summarised by a single sentence.
p12l247 “Such Nd perturbations reflect the causal, instantaneous LWP response” I do not understand how a LWP response could be instantaneous. Do you mean with a short characteristic time scale, during which the steady state is not reached? Please note that “Instantaneous LWP response” is also mentioned in the abstract and p12l250.
p1l9 the local perturbations in Nd may not align with the plausible natural co-variability between LWP and Nd; I am not sure what you mean by “align” here
Technical corrections:
I would suggest adding the depicted parameter with units next to the colour bar in all figures with colour bars.
The titles of the main paper and supplement do not match:
“Co-variability drives the inverted-V sensitivity between liquid water path and droplet concentrations”
“Co-Variability, Not Causality, Drives Inverted-V Sensitivity Between Liquid Water Path and Droplet Concentrations”
p3l64 0.25cire by 0.25cire -> circ symbol missing?
Which geographical region does Fig 4. represent?
p3l63 we use a combination of MODIS-derived R and τc -> we use a combination of MODIS-derived R and τc to identify thin clouds
p4l85 Wood (2012) -> (Wood, 2012)
The caption of Fig4: Nd vs gm−2; please correct the units
Fig S4 Could add the region (SEP) in the title so it would be formatted similarly to other figures
Fig 1: What is depicted by thick black arrows? Meridians?
p12l249 “Glassmeier et al. (2021) who showed that such instantaneous Nd perturbations overestimate LWP adjustments” Please rephrase to be more explicit. E.g. you could say “underestimate the decrease in LWP”.
Abstract p1l2 decreases with Nd -> decreases with increasing Nd
p1l2 LWP responses to changes -> LWP responses to increases
p1l19 to changes in Nd -> to increasing Nd
p1l22 “The inverted-V indicates two opposite sensitivity regimes of the response of LWP to Nd” It might be good to rephrase so no causality would be indicated, e.g. “two opposite regimes for changes in LWP corresponding to increase in Nd?” readers might think of a causal response when you say response?
p2l44 “the response of LWP to Nd”; readers might think of a causal response when you say response?
p2l50 “especially when comparing models to observations” - perhaps it would be better to say something like: “and contribute significantly to the differences between models and observations”
p6l118 consistent increase of re with M -> consistent increase of re with less negative M
p9l154 “regime of LWP with Nd” perhaps this could be rephrased using a more formal wording?
Figure S4. “Correlations derived from the LWP-Nd joint histogram bins of the SEP.” Is “correlations” the best wording here? Relationships?
p2l46 “Here, we address and resolve this ambiguity.” I’m not sure if “resolve” is appropriate here, as the problem is likely not fully resolved. Although you identify important drivers of the co-variability, additional drivers may also play an important role…
Fig S3. Would you please add units for M
References
Forster, P., Storelvmo, T., Armour, K., Collins, W., Dufresne, J.-L., Frame, D., Lunt, D., Mauritsen, T., Palmer, M., Watanabe, M., Wild, M., and Zhang, H.: Chapter 7: The Earth’s energy budget, climate feedbacks, and climate sensitivity, 2021.
Gryspeerdt, E., Goren, T., Sourdeval, O., Quaas, J., Mülmenstädt, J., Dipu, S., ... & Christensen, M. (2019). Constraining the aerosol influence on cloud liquid water path. Atmospheric Chemistry and Physics, 19(8), 5331-5347.
Citation: https://doi.org/10.5194/egusphere-2024-2245-RC1
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