the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Strong aerosol indirect radiative effect from dynamic-driven diurnal variations of cloud water adjustments
Abstract. Aerosol-cloud interaction (ACI) is the critical yet most uncertain process in future climate projections. A major challenge is the sign and magnitude of cloud liquid water path (LWP) response to aerosol perturbations (represented by cloud droplet number concentration, Nd) at different temporal and spatial scales are highly variable, but potential microphysical-dynamical mechanisms are still unclear, especially at a diurnal scale. Here, robust observational evidence from geostationary satellite reveals that the diurnal variation of LWP adjustments is driven primarily by diurnal-related boundary layer decoupling and cloud-top entrainment. Strikingly, these diurnal adjustments exhibit a distinct regional pattern associated with cloud regimes. We find that the cooling effect of LWP adjustments would be underestimated by up to 86 % in study regions if neglecting their diurnal variations, leading to a further 45 % offset of Twomey effect, thus biasing aerosol indirect effect toward a warming direction. Our findings highlight the key role of diurnal variation of ACI in reducing the uncertainty in climate projections.
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Status: open (until 02 Feb 2025)
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RC1: 'Comment on egusphere-2024-3601', Anonymous Referee #1, 27 Dec 2024
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This study uses retrieved properties of marine low clouds based on the Himawari-8 satellite data to investigate the LWP adjustment to Nd during the full daytime part of the diurnal cycle. It claims a large positive effect of Nd on LWP, which was previously unaccounted for.
However, the methodology has a major flaw that causes an artifact to manifest as an indicated strong positive effect of Nd on LWP. After correcting this problem, the results are likely to be very different. Therefore, this paper has to be rejected or at least undergo a very major revision.
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Specific comments:
- The retrievals of cloud properties:
The native resolution of the AHI is 2 km. Why use a microphysical product of 4 km? The low resolution renders the retrieval very sensitive to errors due to partial pixel feeling in most cases, except for the fully cloudy scenes. Therefore, the effects of cloud cover are confounded with those on LWP.
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- Why sub-sampling the data? Why is it 8 km at the NH and 6 km at the SH?
- There is no justification for the threshold of re < 14 um. While larger re allows more water loss by precipitation, it may be more than balanced by less water loss due to less evaporation of the larger cloud drops.
- Furthermore, re increases with cloud geometrical depth (CGT) and LWP increases with CGT2. Therefore, excluding scenes by their re values is incurring bias, rendering the whole study questionable
- Line 131: The positive trend of LWP with Nd was previously documented to occur only at Nd<30 cm-3 (Figure 2 of Gyspeerdt et al., 2019). The clouds have to be very shallow with respectively small LWP for re<14 in clouds with Nd<30 cm-3.
In fact, the condition of re<14 um imposes an artifact of more LWP with larger Nd, because with larger Nd the cloud needs to grow deeper and have larger LWP for reaching re=14 um at the cloud top !!!
- Indeed, this study's maximum LWP is shifted from 30 (Figure 2 of Gyspeerdt et al., 2019) to nearly 100 cm-3. This is evident in Fig1 left panels, especially in the convective regime (AUW), where cloud thickness and, hence, LWP consistently increase with Nd. Â This artifact dominates the results of this study.
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Given the realization that the paper's main result is demonstrated here as an artifact of the methodology, I found little point in continuing to review the paper beyond this point, so I stopped the review here.
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Citation: https://doi.org/10.5194/egusphere-2024-3601-RC1 -
AC1: 'Reply on RC1', Jiming Li, 03 Jan 2025
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We would like to express our sincere gratitude to Reviewer #1 for the insightful and professional comments. We provide the following response to Reviewer #1's comments regarding the data and methods. Overall, the changes to the methods do not significantly affect our final results if we do not use re < 14 µm as suggested. The revised results is attached at the end of the response for reviewer’s reference. Please see the detailed reponses in the supplement!Â
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