| 26 Jun 2026
Contrasting impacts of Asian sulfate aerosols on regional summer extreme precipitation under different climate backgrounds
Abstract. This study investigates the impact of Asian sulfate aerosols on regional summer extreme precipitation under both pre-industrial and pre-industrial +1 K climate states, using the reduced-complexity climate model Fast Ocean Rapid Troposphere Experiment version 2 (FORTE2) with Systematic Regional Aerosol Perturbations simulations. Results show that increased Asian sulfate aerosols reduce extreme precipitation, characterized by lower frequency and weaker intensity over East and South Asia in both climates, but the suppression is more pronounced over northern India in the +1 K climate. Atmospheric energy budget analysis reveals this is primarily due to enhanced dynamic effects in the warmer climate. When aerosols are added to the pre-industrial climate state, an increased horizontal dry static energy (DSE) gradient partially offsets the aerosol-induced dynamic suppression over northern India. Under the +1 K climate, higher atmospheric static stability weakens the local meridional circulation induced by aerosol forcing, reducing the meridional temperature gradient and thus diminishing the offsetting effect, making the dynamic suppression more dominant. Further analysis indicates that this difference in precipitation responses between the two climates results from the nonlinear effect between the aerosol-radiation and aerosol-cloud interactions under the +1 K climate, whereas the two effects combine approximately linearly under the pre-industrial climate. This study suggests that continued global warming alone will exacerbate Asian sulfate aerosols’ weakening of precipitation, including extremes, over northern India. This implies that future reductions in aerosol emissions may unmask or amplify extreme precipitation increases in this region.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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General comments:
The manuscript by Luo et al. investigates the impacts of Asian sulfate aerosols on regional summer extreme precipitation and their underlying physical mechanisms using the reduced-complexity climate model FORTE2. The authors compare a suite of SyRAP-FORTE2 simulations under two different background climate states (piC and p1K) with different aerosol treatments, and further compare their findings against the PDRMIP multi-model simulations. The results demonstrate that sulfate aerosols generally suppress extreme precipitation across some Asian regions, primarily driven by large-scale dynamic effects, and that a warmer climate background enhances this suppression by increasing lower-tropospheric static stability. The study also suggests a linear ARI-ACI combination under pre-industrial conditions and nonlinear interactions under a +1K climate.
The topic falls within the scope of ACP and holds scientific value. The experiments are well designed, and the paper is generally written in a clear way. However, the Abstract and Conclusion need to be more concise and focused to effectively convey the core findings and broader implications. Furthermore, several methodological and analytical details require clarification. Therefore, I recommend a major revision before its final publication.
A general suggestion for plots: Please maintain consistency in the colorbars throughout the paper. For instance, blue represents positive values in Figs. 4, 7, 9, 11, 13 but negative values in other figures, which may confuse readers.
Specific comments:
Line 23: The Abstract should begin with a brief background and introduction to the topic, followed by a clear statement of the main scientific challenges/gaps and research questions this study aims to address.
Line 65: Please briefly explain the physical mechanism by which greenhouse gases induce precipitation increases (e.g., thermodynamics).
Lines 57-84: The current literature review on anthropogenic aerosols is overly general. Given that the focus of this study is specifically on sulfate aerosols, I suggest tailoring the literature review to discuss how sulfate aerosols specifically impact regional extreme precipitation, thereby making the transition to your study's focus more natural for readers.
Line 85: It would be more natural to go to a new paragraph when you introduce what this study will do.
Line 97: Please clarify the response of “what” to Asian sulfate aerosol perturbations, e.g., precipitation?
Line 100: The heading "methods" is too broad; consider renaming it for better clarity.
Lines 138-139: Please provide references or physical justifications for the choice of the 0.75 (σmin) and 300 hPa (pmin) thresholds.
Lines 155-156: This sentence is ambiguous. Do you mean that the spin-up period is 200 years, and the last 51 years are selected for analysis? Please clarify.
Line 163: While you introduced the ACI parameterization, the ARI parameterization scheme is missing. Please provide a brief description of how ARI is implemented.
Line 185: Please define what "10×" stands for in this context.
Line 213: Please provide a reference or justification for the 1 mm/day wet-day threshold.
Lines 241-243: Please state the units for each symbol in the energy budget equations.
Line 294: Please clarify how the precipitation responses to sulfate aerosols were calculated. For instance, for Fig. 4e, is it the relative difference in PRCPTOT between p1K and SO4_p1K in FORTE2?
Lines 302 & 307: The figure subplot citations appear to be incorrect. In line 302, it likely refers to Fig. 4a, b, e, f, i, j; similarly, please check the subplot numbers cited in line 307 (e.g., Fig. 4c–l).
Line 308: Please provide statistical evidence to support the claim of "high agreement".
Lines 389-401: Section 3.3 focuses on the differences between the two climate scenarios. Therefore, the analysis (e.g., “Relative to SO4_piC, SO4_p1K exhibits weaker cold anomalies over northern India…”) should be the primary focus of this paragraph, and the detailed description of SO4_piC for Fig. 10a–d can be toned down. Additionally, it would be clearer to directly show the difference between SO4_piC and SO4_p1K for the variables in Fig. 10, moving the previous Fig. 10a–h to the supplement.
Line 403: Does "response" denote the difference between the aerosol-added experiment and the baseline scenario (e.g., (ARI_p1K - p1K) / p1K for PRCPTOT)? If so, please explicitly define this in the figure caption, and clarify the sub-panel titles (e.g., using "ARI_p1K" directly can be misleading).
Lines 414-425: Figs. 11c, f, i, l and Fig. 12, along with the corresponding text, actually focus on the differences between the two climate conditions. This material should either be restructured into Section 3.3 or streamlined.
Line 437: Please specify what kind of "moisture" is being referred to (water vapor?).
Line 449: Replace “adding” with “the additional”
Line 453: Since precipitation is also evaluated in Section 3.1, it should be mentioned briefly here as well.
Line 469 The discussion regarding resolution issues does not seem well-integrated here; consider moving it to the Discussion section.
Line 447: The Conclusion should be more concise and compact, while outlining clear directions for future research. Currently, it largely repeats the results without offering future perspectives. Please sharpen the summary and add forward-looking statements stemming from your findings.
Line 492: The Discussion section should appear before the Conclusion. Also, note that the Introduction states Section 4 provides a summary and discussion, but Section 5 is labeled as Discussion. Please ensure consistency.