Impact of Aerosol Absorption and Scattering on Winter Fog Lifecycle: Insights from NWP Simulations over Indo-Gangetic Plains

Bhati, Shweta; Anurose, Theethai-Jacob; Jayakumar, Aravindakshan; Gordon, Hamish; Jones, Anthony; Mohandas, Saji; Prasad, Vijapurapu Srinivasa

doi:10.5194/egusphere-2025-6088

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https://doi.org/10.5194/egusphere-2025-6088

Preprints

03 Feb 2026

| 03 Feb 2026

Impact of Aerosol Absorption and Scattering on Winter Fog Lifecycle: Insights from NWP Simulations over Indo-Gangetic Plains

Shweta Bhati, Theethai-Jacob Anurose, Aravindakshan Jayakumar, Hamish Gordon, Anthony Jones, Saji Mohandas, and Vijapurapu Srinivasa Prasad

Abstract. The Indo-Gangetic Plains (IGP) of India frequently experience widespread and dense winter fog with substantial health and economic consequences. This season also coincides with elevated aerosol loading. This study investigates the influence of aerosols, particularly aerosol-radiation interactions (ARI), on the development and evolution of dense fog over the IGP, using high-resolution numerical experiments. While aerosol-cloud interactions related to fog have been extensively studied, the role of ARI has received relatively less attention, due to the limited prevalence of absorbing aerosols in many other parts of the world. However, the increasing number of absorbing aerosols over IGP prompted this study to isolate and examine the contributions of both scattering and absorbing components of ARI. The results of numerical experiments reveal that disabling aerosol absorption led to nearly doubling the area affected by dense fog, increasing fog height by ~20 %, and delaying fog dissipation by about two hours. In contrast, turning off the scattering reduced fog coverage by 18 %. Satellite-derived absorbing aerosol indices corroborated the model’s representation of strong absorption over the region. Another key insight is the contrasting influence of absorbing versus scattering aerosols on the vertical development of fog. Scattering effects were spatially uniform, promoting the vertical growth of fog. Conversely, absorption had a spatially variable impact, enhancing or suppressing fog height depending on whether the absorbing aerosols resided within or above the boundary layer. These findings emphasize the need to accurately represent ARI in numerical weather prediction models for improved fog forecasting over aerosol-rich regions like the IGP.

Received: 06 Dec 2025 – Discussion started: 03 Feb 2026

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Shweta Bhati, Theethai-Jacob Anurose, Aravindakshan Jayakumar, Hamish Gordon, Anthony Jones, Saji Mohandas, and Vijapurapu Srinivasa Prasad

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-6088', Anonymous Referee #1, 26 Mar 2026
Peer Review of Bhati et al.,

Impact of aerosol absorption and scattering on winter fog lifecycle: Insights from NWP simulations over Indo-Gangetic Plains

General Remarks
This manuscript uses a series of sensitivity tests, conducted in a regional NWP model, to disentangle the relevant impacts of aerosol absorption, scattering, and indirect effects on fog formation and characteristics. The experiment is well-posed and the analysis is generally robust. However, the manuscript is incomplete. Section 2.2 is unfinished. Numerous citations are missing from reference list and some do not appear to exist at all; I find the implications of the latter deeply concerning. Finally, this draft does not engage sufficiently with the existing literature on aerosol-fog interactions. I thus recommend resubmission after major revisions, at which point I would be willing to review the manuscript again.

Major Comments
L59: “While the role of ACI in fog microphysics has been relatively well studied…” Can you provide a brief (~1 sentence) overview of the consensus here? Presumably this is the same as any other aerosol-cloud interaction but it would be good to specify.

L61-66: You highlight other studies that have looked at the impact of absorbing aerosols on fog. What are the remaining knowledge gaps? How does your study differ from theirs?

L89: Section 2.2 is incomplete.

L95: Can you clarify the AI experiment? By how much do you reduce the emissions of aerosols and aerosol precursors in order to obtain PM2.5 < 5ug/m2? How did you determine the necessary emission reductions?

L132: I’m not sure I agree that the model compares particularly well with observed LWP. The observed distribution is spatially heterogeneous with a number of moderately-high-LWP regions. In contrast, the model shows quite low LWP everywhere except for a single plume over NW Haryana which contains LWP values substantially higher than are seen anywhere in the observed domain. Although the spatially-averaged LWP is probably similar, I suspect that the dynamics are fairly different.

Fig 1: I am having difficulty reconciling the contents of the table (1a), which shows the model simulating dense fog every day except one, with the figure (1b) where the model never reaches below “moderate fog.” It looks like the shaded envelope (is that the min-max range across time?) maybe dips into dense fog once, but the line (median?) certainly does not. Please also expand the caption to describe the meaning of the lines vs shaded envelopes. It may also be helpful to indicate your study period (0530-0830 IST) on Fig 1b.

Figs 2-3 / L144 and L150: why does your definition of fog-affected change from LWP>0 to q1>0.01 g/kg?

Fig 5: It will be more clear when the grey shading is applied to all panels (see minor comments) but there are differences in the temporal evolution of the 4 quantities plotted. Can you comment on the relative behaviour of these quantities, e.g. the fact that LWR anomalies grow smoothly from 2200-1000 but the T and RH anomalies begin sharply at 0700, the difference in temporal evolution of T anomalies amongst the different experiments, and the complex behavior of the BLH anomalies? For BLH, please comment in particular on the change in sign of the ARII-A anomaly at ~1400?

L255-256: “Notably, in the AI experiment (Fig. 7e), fog does not form at all…” It clearly does form, see also Fig. 3. Part of why it appears lower in Fig 7e is that you changed the colour scale in panel e compared to the others; this does not appear to be necessary and leads to confusion, so I recommend using the same colour scale in all panels.

Section 3.4: Would this section be better placed with the model evaluation than with the results? The fact that absorbing aerosol is present in observations, as well as in your model, provides evidence that your experimental setup is reasonable but does not directly corroborate your results which are entirely simulated.

L327: “primarily via aerosol-radiation interactions” This is incorrect, your results indicate that ARI is important but that ACI dominates (relative magnitude of anomalies in ARII vs AI).

L346-347: The presence of absorbing aerosol in observations does not directly demonstrate that those aerosols influence fog.

Discussion is missing any comparison with previous studies. Please elaborate on how your results compare with others who have investigated the impacts of ARI on fog.

Minor Comments
L29: Zhou et al. (2019) is not included in the reference list; is this the best reference for aerosol-radiation interactions?

L35: Sarkar and Panda (2024) is not included in the reference list, and the only Sarkar and Panda (2024) I can find online discusses chromatography for biotechnology.

L48: I believe IGI is the airport and you want IGP here?

L63-64: “ARI-induced aerosol absorption” should be corrected

L64: “both emissions and ARI significantly modulate fog processes” what do you mean by emissions here? Also, in the context of this paragraph, be careful not to conflate “ARI” and “absorption.”

L88: These citations are not included in the reference list.

L109: cite MODIS data product.

L148-149: Although the decision to plot the diurnal variation from 1800 one day to 1700 the next makes sense when cross-referencing Figs 1 and 3, it would be helpful to mention in the text that you are doing this to centre the period with lowest visibility / highest fog.

L150: this citation is not included in the reference list.

L156-157: these citations are not included in the reference list.

L161: “48% increase in fog extent by around 48%”

Fig 4: Caption might be more clear as “Spatial distribution of the mean difference in fog liquid water path…” Also, perhaps it is just my screen but I find this colourscale challenging to interpret. It looks like the negative values go green – blue – darker green – darker blue, but my eye wants to read greens first and then blues, making the figure unintuitive. Would it be easy to modify?

Fig 5: I would recommend the following: (i) shade the hours without fog (1400-2100) instead of the hours with; (ii) include this shading on all panels; and (iii) expand the caption to explain the meaning of the shaded regions. This will make both the figure and its explanation in L193-196 easier to interpret.

L220: The presence of enhanced fog…

Fig 6: Please improve the kerning in the orange boxes to make it easier to read. Also please clarify in the description of the control experiment where the absorbing aerosols cause warming (elevated, not surface).

L261: as L48, should this read IGP not IGI? Also it should be “over the”

L264-265: The fact that regions with surface altitudes >450m were excluded from your analysis was not previously mentioned. Please do so and elaborate on the reasoning for this decision.

L272: “In certain regions, …” It would be better to specify that of the two regions you isolated for further analysis, the one with a negative fog height response to ARII-A had its BC concentrated near the surface.

Fig 7: Please use the same colour scale for all panels. The ranges are close enough that I do not see a strong reason for using a different scale for panel e. Please also clarify whether these are heights above the surface in each gridcell or relative to some common reference.

Fig 8: The text said 03 UTC / 0830 IST, but the caption says 00 UTC / 0530 IST.

Fig 9: It would be more typical for this colour scale to be reversed, with positive values in green/yellow.

L326: “fog formation persistence” should this be formation and persistence?

L331: replace “spatially” with “horizontally”

L335: remove “furthermore”

L340-341: “though not necessarily of dense character” is this phrase misplaced?
Citation: https://doi.org/10.5194/egusphere-2025-6088-RC1
RC2: 'Comment on egusphere-2025-6088', Anonymous Referee #2, 26 Apr 2026

This manuscript examines how aerosol absorption and scattering influence the winter fog lifecycle over the Indo-Gangetic Plains using DM-Chem1.0 sensitivity experiments for 1–10 January 2023. The topic is important, and the attempt to separate absorbing and scattering components of aerosol-radiation interactions within an operational fog-forecasting framework is useful. At the same time, some conclusions appear stronger than the current evidence supports, and the paper would benefit from clearer positioning relative to recent IGP studies, more cautious interpretation of the sensitivity experiments, and a fuller discussion of observational and model uncertainties.
Specific comments:
1. The novelty should be stated more clearly and placed better in the context of recent aerosol-fog studies over the IGP. Previous work has already shown that aerosol-radiation feedback and absorbing aerosols can affect fog timing, intensity, and spatial structure. The main contribution here seems to be the explicit separation of absorption and scattering effects within DM-Chem, and the authors should make this distinction clearer.
2. The experimental design is useful, but some conclusions should be phrased more cautiously. The AI experiment and the ARII experiments are idealized sensitivity tests, and they do not directly represent realistic emission-control or policy scenarios. The authors should clarify how the aerosol reductions were applied, and avoid implying that the results quantify a real-world source-response relationship.
3. The observational and model evaluation should be strengthened. The current evaluation relies mainly on visibility at one airport and a limited LWP comparison, while the spatial structure of modeled LWP and fog may not be fully consistent with observations. It would also help to present the aerosol absorption datasets as supporting evidence rather than direct validation of the simulated ARI response.
4. The interpretation of aerosol effects needs clearer separation between ACI and ARI. The results suggest that aerosols are important for fog formation mainly through their role as CCN, while ARI modifies fog growth, depth, and dissipation. The authors should avoid wording that gives the impression that ARI is the primary driver of fog formation, and should clarify the relative roles of ACI, absorption, and scattering.
5. The broader interpretation should better acknowledge remaining uncertainties and regional controls. The analysis covers a short period in January 2023, and IGP fog is also strongly affected by meteorology, land-surface moisture, irrigation, and boundary-layer structure. The manuscript should discuss how these factors may interact with the aerosol-radiation signal, and it should also ensure that the methods section is complete.

Citation: https://doi.org/10.5194/egusphere-2025-6088-RC2

Shweta Bhati, Theethai-Jacob Anurose, Aravindakshan Jayakumar, Hamish Gordon, Anthony Jones, Saji Mohandas, and Vijapurapu Srinivasa Prasad

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Short summary

This study shows that aerosols that absorb or scatter sunlight play a crucial role in the growth and sustenance of fog, with the Indo-Gangetic Plains (IGP) as a case study. Five simulations a numerical weather prediction model exhibited that absorbing aerosols have a greater influence on shaping fog over the IGP compared to scattering aerosols. These insights can help with fog forecasts and pollution-control policies over IGP, which are known for high pollution and dense fog episodes in winters.


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