Contribution of local and remote anthropogenic aerosols  to a record-breaking torrential rainfall event in  Guangdong Province, China

Liu, Zhen; Ming, Yi; Zhao, Chun; Lau, Ngar Cheung; Guo, Jianping; Bollasina, Massimo; Yim, Steve Hung Lam

doi:https://doi.org/10.5194/acp-20-223-2020

Articles | Volume 20, issue 1

https://doi.org/10.5194/acp-20-223-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-20-223-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 1

Research article

|

06 Jan 2020

Research article |

| 06 Jan 2020

Contribution of local and remote anthropogenic aerosols to a record-breaking torrential rainfall event in Guangdong Province, China

Zhen Liu, Yi Ming, Chun Zhao, Ngar Cheung Lau, Jianping Guo, Massimo Bollasina, and Steve Hung Lam Yim

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Final revised paper (published on 06 Jan 2020)
Supplement to the final revised paper
Preprint (discussion started on 04 Oct 2018)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of “Contribution of local and remote anthropogenic aerosols to intensification of a record-breaking torrential rainfall event in Guangdong province” by Liu et al.', Anonymous Referee #1, 26 Nov 2018
RC2: 'Review of acp-2018-977 by Liu et al.', Anonymous Referee #2, 28 Nov 2018
AC1: 'Responses to the reviewers' comments', Steve Hung Lam Yim, 25 Apr 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Steve Hung Lam Yim on behalf of the Authors (25 Apr 2019)

ED: Referee Nomination & Report Request started (28 Apr 2019) by Johannes Quaas

RR by Anonymous Referee #2 (17 May 2019)

RR by Anonymous Referee #1 (18 May 2019)

Suggestions for revision or reasons for rejection

Second review of “Contribution of local and remote anthropogenic aerosols to a record-breaking torrential rainfall event in Guangdong province, China” by Liu et al.

1. The authors missed the point of my first major comment. My comment is about the regions with opposite precipitation response, which could be corresponding to the cold and warm sectors of the convective system, respectively. In their response, the authors chose two narrow areas (Box_N and Box_S), both of which have increased precipitation, to show the consistent features between those two boxes. Both areas are in the convergence zone based on Figure R1 and their cloud properties are of course similar. The cold section is probably northwest or northeast where decreased precipitation is seen (can be identified based on temperature field). Clouds at the cold sector would not be invigorated by aerosols so decreased precipitation can be seen as a result of suppressed conversion into rain or snow. Again, the point is that the authors need to explain the opposite precipitation response for different sections of the system, particularly for the 10Xrun, the decrease of precipitation is in a similar magnitude with the increase and occupies half of the simulation domain (Figure 19b). Based on Figure 19b, there is really no justification of only picking up the red box region to study.

2. The authors did not do a neat job in responses. Many responses have wrong line numbers and they also did not describe what changes they made (also did not copy the revised text to the responses), which made me have a hard time to check their changes.

3. There are quite a bit misunderstandings of cloud microphysical processes by reading the responses only (since I had a difficulty to find the changes in the manuscript due to incorrect line numbers). Here are examples, (1) a mistake in calculating cloud droplet number concentrations. They got unreasonably high (8e4 cm-3) cloud droplets (particularly for area mean, not a maximum value at gird-level) by using water density instead of air density to convert to number concentrations. What’s surprising me is that they still argue the reasonability of it. Such a high number concentration is only possible for aerosols (not droplets) in a very polluted condition. (2) the misunderstandings of BF process, latent heat, and precipitating particles (see my comments on #14 response below). (3) the primary driver of convergence (my comments on #16 response). All these aspects that they misunderstood are the key aspects for analyzing and interpreting the model results this study.

4. For many comments on clarifications, the authors responded but did not clarify in the manuscript, such as comment #4,

5. The writing is a little sloppy. There are typos and many statements are confusing. Here are a few just in a short abstract:
Abstract:
Line 28, “cloud property changes also resembled that in the control run” does not make sense. Changes means the differences between the 10xrun and control run, how can the changes resemble control run?
Line 29, “The precipitation average over Guangdong province decreased by 1.0 mm but increased by 1.4 mm in the control run” does not make sense either. Looks like you are describing an increase or decrease in the control run. Then what are you comparing with? Generally, the description should be the increase or decrease by comparing with the control run.
Line 30, “reinforced” should be removed. Also, downsteam of what? Urban city or aerosol source?
Last sentence in Abstract: Be specific about “the cloud invigoration effect”, which is different from convective invigoration. Cloud invigoration refers to larger and/or taller clouds. Convective invigoration refers to stronger storm intensity which usually leads to more extreme rain, more lightning, etc.

6. Detailed comments on responses
(1) #4 response: the description in the manuscript is still confusing. In the manuscript, you said BC is also scaled by a factor of 0.1 for domain 2. Since BC for domain 2 should be from domain 1 simulation, how can you scale it? About “In D2 experiment, the IC, BC, and emissions were scaled by 0.1 for domain 1. The IC and emissions were kept as same with the control run at the same time”, Isn't the second sentence contradicted with the first one? I am still confused about what you wanted to say in the second sentence.
(2) #8 response: Need to clarify in the manuscript (such as in the figure caption).
(3) #10 response: Line number is not correct so it is difficult to identify the text you revised for this comment. But I found there is a mistake in P8 Line 24, how can the cloud top for deep convection only extends up to 1 km?
(4) #11 response: Need to clarify in the figure caption that only cloud ice is considered.
(5) #12 response: The authors made a mistake in calculating the droplet number concentrations. They used water density (1 g cm-3) instead of air density (~1e-3 g cm-3 at low levels) for the calculation. The area mean value should ~ 80 cm-3 as I mentioned in the previous round, not 8e4 cm-3 that is not totally reasonable.
(6) #13 response: I do not understand how more cloud droplets are lifted to freeze can be named as "interim processes". Why not directly describe the process instead of using a term that is not known?
(7) #14 response: there are a few fundamental misunderstandings about cloud microphysical processes: (a) BF process. This process only occurs in the limited regime where Sw<0 but Si>0. In deep convection, most of updrafts are strong enough to make Sw>0. In that situation, both droplet and ice crystal will grow. In addition, this process only increases ice crystal mass, not ice crystals as authors claimed. (b) latent heat. The statements “the magnitude of snow and graupel mass is ten times of that of rain water. The latent heat release due to deposition in cold cloud is stronger than that due to condensation in warm cloud even though the latter is also important” have problems. It is conceptually wrong to discuss latent heat magnitude based on the mass for different phase of hygrometers. snow and graupel are not mainly formed from deposition. Riming is the process for graupel forming which converts a lot of liquid mass to solid phase. The latent heat release from riming may be small only because the latent heat release for converting per unit liquid to ice is only about 1/8 of that converting per unit of water vapor to liquid. I'd want to know in detail how you calculate latent heat for each process in the model. Currently it is just said “diagnosed”. If it is diagnosed from the mass like described here. Then it is not correct. (c) It is also not correct to say “most of the ice crystals fall as precipitation”. Ice crystals would not fall as precipitation. Snow and graupel are the precipitating particles. (d) The figure R10 is confusing. How can warm cloud have deposition and freezing? How do you define warm clouds? Also, why not show the values below 3 Kd-1, which is significant in differences? Please clarify "anomalies that exceed 90% significance level". First, there is no observations so please define anormaly here. Second, how the significance test is done since data between two simulations can not be compared in pairs in grid level because very different clouds could form. If the test is conducted based on mean values, are there enough data for such a test?
(8) #16 response: The convergence should be primarily because the dry cold air meet with warm humid air as a result of large-scale dynamics. Microphysics might enhance the convergence, but it is not the cause of the convergence over the large region. In addition, moisture is increased in the red box domain, which need to discuss where the source is.
(9) #18 response: (a) I do not understand “the persistent convective system makes the impact last for longer time”. (b) The authors missed the point about my question “how the changes in domain 1 impact the results over domain 2”. When emissions and aerosols are changed in Domain 1, the methodological field including temperature and moisture would be changed too. Those changes would impact domain 2 simulation since BC is from domain 1.

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ED: Reconsider after major revisions (20 May 2019) by Johannes Quaas

AR by Steve Hung Lam Yim on behalf of the Authors (01 Jul 2019) Manuscript

ED: Referee Nomination & Report Request started (02 Jul 2019) by Johannes Quaas

RR by Anonymous Referee #1 (20 Jul 2019)

Suggestions for revision or reasons for rejection

This the third around review. The authors did not take good use of the chances to address the concerns I raised. Besides they still missed some points (examples below), they even did not try to make effort to organize the paper and present the most important points. The paper gets so lengthy and appears lack of organization (see the comment #1 below for an example below). They now got 20 formal figures and 15 supplemental figures. It appears they added lengthy text and figures to address comments but did not think of how to better organize and only present the most important points. Another evidence showing the lack of effort in presenting the study is that the figures are out of orders, for example, it is jumped from Figure S2 to Figure S6 in referencing figures. The first appearance of Figure S3 and S4 is after Figure S7. The first appearance of Figure S5 is after Figure S7 but before the first appearance of Figures S3 and S4. The first reference to Figure 1 is also after Figure 2.

I did not have time to read the whole paper but only read their response and changes and the following corrections and clarifications are needed:

1. To address my first comment of the second round review, the authors conveniently only added a few figures to the end of the supplemental materials and discussed it in the Summary and Discussion section, which does not address the point. I emphasized before that the point is to explain the opposite precipitation response at the different sectors of the system. So, to address this point well, it is equally important to describe both the increase of the precipitation at the convergence zone and warm sector and the decrease of the precipitation in the cold sector. Then explain the reasons causing the increase and the decrease, respectively. Therefore, the changes should be started from the first paragraph in P8 where Figure 3 is discussed.

2. Also, the authors argued “There are lots of ice crystals with cloud ice extending up to 16 km, indicating strong deep convection”. At the cold sector of a frontal system, generally there is no mechanism to form deep convective clouds. It should be deep stratiform clouds, not strong convective clouds. If this case is different from the general understanding, then needs to present evidence such as large CAPE or large low-level upward motion to support the argument of deep convective clouds.

3. “The reduction of rain water and ice crystals (particularly in graupel) suggest that both the warm rain and cold rain are suppressed” - wrong statement. As I emphasized previously, ice crystals are non-precipitating particles which does not suggest precipitation information, but graupel is precipitating particle but it is not ice crystal, it is just one type of ice particles. They made such mistakes in terminology in many places throughout the paper. When talking about specific hydrometeors types, you may use ice crystal (or cloud ice), snow, graupel. When you want to take about ice particle in general covering different types, use “ice particle” not “ice crystal”. This need to be changed throughout the paper.

4. The suppressed precipitation for cold-based clouds should be mainly because the reduced warm rain formation at the early times and reduced graupel formation at the later time period based on Figure S12. This is a typical response of deep stratiform clouds to CCN since the most dominant changes by aerosols for this type of clouds are collision processes including autoconversion and riming, which are less efficient due to smaller droplet size. This should be the major argument for the suppressed precipitation. The three reasons listed by authors are mainly for deep convective clouds.

5. The sentence in the abstract that I pointed out previously still did not make sense. As I asked previously, how can the changes of precipitation between a polluted and clean condition resemble that from control run since the changes mean the differences?

6. In response of #7 comments in the last round, I read their changes and still have the following problems,
(1) “The warm rain is still suppressed before 15Z on December 15 (Figure 6c) even though with strong latent heat release through cloud water formation”: Warm rain will be always suppressed with the two-moment bulk scheme with the parameterization of autoconversion. This is very different from the treatment of in bin microphysics used in Fan et al. 2018. This needs to be clarified. Also, warm rain means the rain formed from autoconversion. Rain mass below 0 C level can be contributed by the melted particles. You only can discuss the warm rain at the times when there are no ice particles at all above 0 C level in Figure 6c.
(2) “To further analyze the source of this latent heat release, following Fan et al. (2018), the latent heat released from condensation, deposition, and freezing during cold and warm cloud processes are diagnosed”, I do not think you can say “following Fan et al. (2018)”. The author’s response did not give a clear description about how the latent heat is calculated. They should not have sent a bunch of codes, instead, it should be easily described with words about how the latent heat is calculated for each process. I think the latent heat calculation should be the part of Morrison scheme since this is the feedback to temperature that a full coupled model should consider. The authors should not need to add additional code for such calculation (probably only need to find the right variable name to output it). Because the authors had wrong statements about latent heat and also said they diagnosed it in the code before, I asked these details to check if this important part was done and interpreted correctly. I did not get the answer from their response.
(3) P10 Line 5-28, the lengthy statements they added need to revised due to misunderstandings. First, it is the basic cloud microphysics that latent heat from freezing is not a major component deep clouds as I explained last round. Condensation and deposition are always the very important condensate forming process in deep convective clouds. Second, in those past studies that the author mentioned, when they discussed the effect from freezing changed by aerosols, it is not just about the latent heat from freezing only, instead about the latent heat changes from all the processes due to the change of freezing induced by aerosols. For example, when there are more freezing, more ice crystals form, then riming and deposition will change as well.

With through rounds of review, I have tried hard to correct many basic knowledges and results about cloud microphysics and aerosol-cloud interaction processes to improve the quality of this paper. I urge the authors to take the opportunity to do a careful job in writing and organizing the results so that the paper can reach a certain level of qualify for publication.

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ED: Publish subject to minor revisions (review by editor) (07 Aug 2019) by Johannes Quaas

AR by Steve Hung Lam Yim on behalf of the Authors (29 Aug 2019)

ED: Publish subject to technical corrections (04 Sep 2019) by Johannes Quaas

AR by Steve Hung Lam Yim on behalf of the Authors (05 Sep 2019) Author's response Manuscript

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OH and HO₂ radicals are important trace constituents of the atmosphere that are closely coupled via several types of reaction. This paper describes a new laboratory method to simultaneously determine OH kinetics and HO₂ yields from chemical processes. The instrument also provides some time resolution on HO₂ detection allowing one to separate HO₂ produced from the target reaction from HO₂ arising from secondary chemistry. Examples of applications are presented.