Self-enhanced aerosol&ndash;fog interactions in two successive radiation fog events in the Yangtze River Delta, China: A simulation study

Shao, Naifu; Lu, Chunsong; Jia, Xingcan; Wang, Yuan; Li, Yubin; Yin, Yan; Zhu, Bin; Zhao, Tianliang; Liu, Duanyang; Niu, Shengjie; Fan, Shuxiang; Yan, Shuqi; Lv, Jingjing

doi:https://doi.org/10.5194/acp-2022-833

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https://doi.org/10.5194/acp-2022-833

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24 Jan 2023

| 24 Jan 2023

Status: a revised version of this preprint is currently under review for the journal ACP.

Self-enhanced aerosol–fog interactions in two successive radiation fog events in the Yangtze River Delta, China: A simulation study

Naifu Shao, Chunsong Lu, Xingcan Jia, Yuan Wang, Yubin Li, Yan Yin, Bin Zhu, Tianliang Zhao, Duanyang Liu, Shengjie Niu, Shuxiang Fan, Shuqi Yan, and Jingjing Lv

Abstract. Abstract. Aerosol–fog interactions (AFIs) play pivotal roles in the fog cycle. However, few studies have focused on the differences in AFIs between two successive radiation fog events and the underlying mechanisms. To fill this knowledge gap, our study simulates two successive radiation fog events in the Yangtze River Delta, China, using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). Our simulations indicate that AFIs in the first fog (Fog1) promote AFIs in the second one (Fog2), resulting in higher number concentration, smaller droplet size, larger fog optical depth, wider fog distribution, and longer fog lifetime in Fog2 than in Fog1. This phenomenon is defined as the self-enhanced AFIs, which are related to the following physical factors. The first one is conducive meteorological conditions between the two fog events, including low temperature, high humidity and high stability. The second one is the feedbacks between microphysics and radiative cooling. A higher fog droplet number concentration increases the liquid water path and fog optical depth, thereby enhancing the long-wave radiative cooling and condensation near the fog top. The third one is the feedbacks between macrophysics, radiation, and turbulence. A higher fog top presents stronger long-wave radiative cooling near the fog top than near the fog base, which weakens temperature inversion and strengthens turbulence, ultimately increasing the fog-top height and fog area. In summary, AFIs postpone the dissipation of Fog1 due to these two feedbacks and generate more conducive meteorological conditions before Fog2 than before Fog1. These more conducive conditions promote the earlier formation of Fog2, further enhancing the two feedbacks and strengthening the AFIs. Our findings are critical for studying AFIs and shed new light on aerosol–cloud interactions.

Received: 13 Dec 2022 – Discussion started: 24 Jan 2023

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Naifu Shao et al.

Status: final response (author comments only)

RC1:
'Comment on acp-2022-833', Anonymous Referee #1, 16 Feb 2023

## MAIN COMMENTS
This manuscript presents a modelling study of two successive fog events in the Yantze River Delta region of China. It aims to show how the fog properties in the second event are influenced by the first. I find this a truly interesting topic and exciting approach. However, I struggled with the manuscript for the following reaons:
- The central message is that fog properties are influenced by aerosol as well as other boundary-layer conditions. The latter may be modified by a preceding fog event, resulting in fog property differences between both events. This simple -- and very interesting -- finding is hidden behind the phrase "self-enhanced AFIs", and thus took me more time to understand than would have been necessary. I would suggest to focus on the changes to the fog rather than "AFIs", and to speak about "aerosol loading" or "polluted conditions" to clarify the meteorological context. Also, "AFIs", which is modelled on the common abbreviation "ACI" for aerosol-cloud interactions should probably lose the "s" to make it consistent with ACI. A change of the title could also be considered to more closelz reflect the paper's focus, e.g. "Radiation fog properties in two consecutive events under clean and polluted conditions..." or similar

- The state of the art chapter does not seem complete. The central motivation, i.e. limited knowledge about AFI, is only briefly stated, and not explained (line 81). The fundamental premise that an event may be influenced by a previous event does not follow from the literature review presented at all. The focus, concepts and terminology of the first research question are neither derived from the literature, nor are they explained. What is a "stronger" fog scenario? What does "stronger AFIs" mean? What would you expect? Why? And why does it matter?

- In some places, aspects concerning methodology and interpretation remain unclear. How precisely is the validation performed? To what extent and under what conditions can the findings of this study be generalized? Instead of using "AFIs", in many places it would be more helpful to explicitly address the parameter of relevance, e.g. LWP, aerosol loading, droplet radius...

- While the paper is both legible and intelligible, it would profit from a linguistic revision.

## DETAILS
15 - "pivotal" is unclear here

15 - what is "the fog cycle"?

16: Why should they focus on these differences? What is special about successive events?

17: What knowledge gap exactly?

19: "AFIs ... promote..." -- Do you mean high/low aerosol loadings? Or the interaction (mechanisms) specifically?

22: "is defined as" -- you mean that you define it as, or is this taken from elswhere?

38: fog does not lead "to environmental pollution" - please clarify this statement

40: You state that the "physical processes of fog remain unclear". What exactly do you refer to? Can you provide a reference, please? I would think that the processes are pretty well understood.

47: First sentence is a repetition of statement in line 36.

52: What do you mean by "fog number concentration"? droplet number concentration in fog?

53: Can these numbers be generalized? How would they be expected to change given different environmental conditions? Is this continental radiation fog, sea fog, advection fog over land, ...?

70: That radiative cooling "is an important factor for temperature inversion, providing stable conditions for fog formation" is not a finding of the cited studies in the 2010s, but can be derived from very basic textbook knowledge.

81: In what respect is this knowledge limited? What is lacking?

83: Why do you think successive fog events are worth considering?

84/5: Why?

89: How do you define "stronger AFIs", what do you mean by this and why does it matter?

101: What aerosol species?

101: What is "massive"? Please be more specific.

140: What does this experiment consist of, and what sensitivities are tested for?

160: How do you compare observations and model? How do you define "consistent" in this regard?

161: Based on which parameters is HSS calculated?

172: Why this threshold?

Figure 5: I find it slightly confusing that the reference case is shown with 100% bars in all cases. I suggest leaving this out and only showing the polluted (a,b) or fog2 (c) situations.

185ff: Here, and in several other places, you assume that AFI lead to changes in fog2. In section 5 you state that fog2 is different because boundary-layer conditions are different after a previous fog event, and not specifically because of the aerosol. Please make sure your reasoning is consistent.

241: higher stability

Citation: https://doi.org/10.5194/acp-2022-833-RC1
- AC1: 'Reply on RC1', Naifu Shao, 28 Apr 2023
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-833/acp-2022-833-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2022-833-AC1
- AC4: 'Reply on RC1', Naifu Shao, 16 May 2023
  
  Dear Referee,
  We appreciate your positive and constructive comments. We have read these comments carefully and made revisions accordingly.
  Sincerely,
  Naifu Shao, Chunsong Lu*, and all co-authors.
  
  Citation: https://doi.org/10.5194/acp-2022-833-AC4
RC2:
'Comment on acp-2022-833', Anonymous Referee #2, 27 Feb 2023

This paper describes a case study of two fog events on two consecutive days in Nanjing, simulated by WRF-chem, and proposes that aerosol-fog interactions in the first fog promote aerosol-fog interactions in the second. Most of the hypothesis is reasonable: the first fog influences boundary layer turbulence for the second fog, and that influence is affected by aerosols. However, I am not yet convinced whether the hypothesis that aerosol-fog interaction in the second fog is affected by aerosol-fog interaction in the first is adequately demonstrated by the simulations in the paper.
Despite this, the paper describes a useful and interesting study of aerosol-fog interactions, which in itself is well worth publishing in ACP. It is also well structured and well written, in general. I recommend that the authors either perform additional simulations to test their hypothesis, they weaken their definition of self-enhancement, or they change the message of the paper to simply highlight aerosol-fog interactions in Nanjing. Either way, in my assessment the article needs major revisions, but assuming the major comments can be addressed, it would be suitable for ACP.
Major comments
1.The authors’ summary of their evidence for their hypothesis of ‘self-enhanced aerosol-fog interactions’ is that by increasing droplet concentrations and by postponing the dissipation of the first fog and promoting the earlier formation of the second, aerosols increase the fog thickness and prolong its lifetime.
Figure 7 shows the meteorological differences that arise during the first fog between clean and polluted conditions persist into the second fog. This figure is key. But would these meteorological differences still persist if the second fog, and the period between the fogs, were not also polluted? Can the authors demonstrate that direct aerosol-meteorology interactions during the clear-sky period between the two fogs do not lead to the meteorological differences in Figure 7 and the early onset of Fog 2?
Assuming the authors can demonstrate this, their theory is aerosol-fog interaction in Fog 1 changes meteorology which enhances aerosol-fog interaction in Fog 2. They show the first part of this in Figure 7: aerosol-fog interactions affect meteorology. It’s reasonable that this influences the formation time of Fog 2 in the simulations. But does it also influence aerosol-fog interactions in fog 2? The authors do show aerosol-fog interactions are stronger in Fog 2 than Fog 1 in their table 3. However, the authors don’t demonstrate a causal link between the increased strength of ACI from Fog 1 to Fog 2 and the ACI in Fog 1.
To show conclusively the aerosol-fog interaction is ‘self-enhancing’ in the simulations as per their own definition, I think the authors would need to show that the aerosols in the first fog affect the aerosol-fog interactions in the second fog. In principle, this could be done with a third simulation, in which the first fog was clean and the second polluted. In this simulation, if the AFIs were weaker in the second fog than in the simulation in which both fogs were polluted, I think the authors’ hypothesis would be confirmed.
The authors also need to show how the absolute PM2.5 concentration varies with time through the two fog events (preferably both in simulations and observations). Otherwise, the results in Table 3 are not useful, as the AFIs might get stronger simply because aerosol concentrations get higher. Furthermore, for the same reason, it would be useful to show the timeseries of aerosol number concentrations (perhaps > 100nm diameter) in the two simulations.
2. Figure 4 is very hard to interpret quantitatively. Is LWP from Himawari available as it is, for example, from MODIS, GOES or SEVIRI? Could it be used instead of the visible light images?
Minor Comments
In the abstract the authors say “AFIs in the first fog…result in higher [droplet] number concentration …in Fog 2 than in Fog 1. For this to be true, my first thought was that AFIs in the first fog would have to reduce scavenging of aerosol and result in higher aerosol concentration in the second fog than would have been the case if the first fog hadn’t formed. The authors don’t show this. They do show that Fog 1 changes meteorological conditions, which might indirectly affect droplet concentration in Fog 2 by changing LWC in Fog 2, but starting the list at line 21 with droplet concentration rather than LWC implies (to me at least) that the main mechanism is an aerosol one: aerosol-fog interactions in Fog 1 affect aerosols in Fog 2, which then change droplet concentration in Fog 2, which then changes LWC and lifetime (the classic ACI pathway). The authors don’t have any evidence for that (the mechanism is meteorology, not aerosols).
Line 45 “proven” – I would say “showed” – a ‘pivotal role’ is not a mathematical concept so it is not really ‘proved’.
Line 70 – what is the ‘critical turbulence coefficient’? The reader should not need to look up the literature unless they are very unfamiliar with fog.
Line 115 -the innermost simulation still has quite coarse spatial resolution. How well can this resolve the turbulence? Is there a sub-grid cloud parameterization in the model, or does the Grell 3D cumulus scheme lead to sub-grid variability in fog?
Line 165- what would be a perfect HSS score? Is the score calculated using each gridbox as input? Please be clearer about how this evaluation was done.
Figure 9: Is ‘fog optical depth per unit height’ the same as “average extinction coefficient through the fog”? It might help the reader to explain this in the caption.

Citation: https://doi.org/10.5194/acp-2022-833-RC2
- AC2: 'Reply on RC2', Naifu Shao, 28 Apr 2023
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-833/acp-2022-833-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2022-833-AC2
- AC5: 'Reply on RC2', Naifu Shao, 16 May 2023
  
  Dear Referee,
  Thank you for your positive and constructive comments. We have addressed your comments. Briefly, we have performed additional simulations, weakened the definition of self-enhancement and changed the message of the paper to simply highlight aerosol-fog interactions in Nanjing, according to your suggestions.
  
  With regards,
  Naifu Shao, Chunsong Lu*, and co-authors.
  
  Citation: https://doi.org/10.5194/acp-2022-833-AC5
EC1:
'Comment on acp-2022-833', Graham Feingold, 28 Apr 2023

Dear Authors,
I have reviewed the referee comments and the manuscript, which it appears you have already revised.
I note that nowhere in your manuscript do you discuss the role of shortwave radiation and its affect on fog lifetime. This is an important part of the discussion. With increasing liquid water path (LWP) and increasing drop concentration, SW heating increases.
There are also longwave aerosol-related effects: at low LWP (< ~ 25 g/m2); an increase in drop concentration will increase radiative cooling.
I also note that there is no quantitative information on LWP, only its response (e.g., Table 4). This is really important information if one is to understand the radiation interactions.
I would also like to reiterate a point made by at least one of the reviewers. The manuscript would really benefit from some careful English langauage editing. I am sympathetic to the fact that the authors are not native English speakers but it would be to our advantage to improve this aspect.
A sample of examples (please look for similar ones and change as necessary):
1) What are "conducive PBL conditions"? I think you mean "conducive to fog formation". Please make changes throughout.
2) change 'scenario' to 'event'
3) remove all “the” before "EXPn"
4) dissipation time (not dissipate time)
5) Fog 1 occurs under clean conditions (not Fog1 161 is under the clean condition)
6) “under clean and polluted conditions” (remove ‘the’)
7) What is ‘more remarkable AFI’? Do you mean stronger AFI? Please be clear.
8) “can enhance cooling” do you mean “enhances cooling”? Please use clear causal language if that’s what you mean. There are many instances "can affect", "can indicate".
9) You have a tendency to create new acronyms like AFI, FOD, TOD, which makes the manuscript less readable to the broader audience. The use of symbols significantly alleviates this problem (e.g., \tau_f, \tau_t). Even AFI might be unnecessary given the familiar ACI. (You could simply point out that ACI in fog has its own particular questions). Also, why N_f when N_d (drop concentration) or N_c (cloud droplet concetration) are widely used. And \tau_c would be better than \tau_f. As it is you use other standard cloud-related acronyms such as LWP, LWC.
Please address these comments in your revised version so that I can send the manuscript out to the referees.
Thanks,
Graham Feingold

Citation: https://doi.org/10.5194/acp-2022-833-EC1
- AC3: 'Reply on EC1', Naifu Shao, 16 May 2023
  
  Dear Editor,
  We are sorry for the late response. The Wiley Editing Services (https://editingservices.wiley.cn/) provide thorough English language editing, which delays the manuscript submission.
  Thank you very much for your decision letter and the referees’ comments on our manuscript "Radiation fog properties in two consecutive events under polluted and clean conditions in the Yangtze River Delta, China: A simulation study" (Manuscript ID: acp-2022-833) submitted to Atmospheric Chemistry and Physics. We have revised the manuscript according to the comments.
  Please find our uploaded revised manuscript and point-by-point responses to the referees’ comments. Briefly, we have addressed all the concerns raised by the referees in the revision. We believe consideration of their comments and suggestions has significantly improved the manuscript.
  
  Sincerely,
  Naifu Shao & Chunsong Lu*, and all co-authors
  
  Citation: https://doi.org/10.5194/acp-2022-833-AC3

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Short summary

Fog is an important meteorological phenomenon affecting visibility. Aerosols play critical roles in the fog life cycle. In this study, the self-enhanced aerosol–fog interactions (AFIs) are proposed in two successive radiation fog events (Fog1 and Fog2), defined as a phenomenon that AFIs in Fog1 enhance AFIs in Fog2. The AFIs delay Fog1 dissipation, leading to more conducive meteorological conditions and stronger AFIs in Fog2.


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