Investigating the impacts of Saharan dust on tropical deep convection using spectral bin microphysics

Gibbons, Matthew; Min, Qilong; Fan, Jiwen

doi:https://doi.org/10.5194/acp-18-12161-2018

Articles | Volume 18, issue 16

https://doi.org/10.5194/acp-18-12161-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-18-12161-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 16

Research article

|

23 Aug 2018

Research article |

| 23 Aug 2018

Investigating the impacts of Saharan dust on tropical deep convection using spectral bin microphysics

Matthew Gibbons, Qilong Min, and Jiwen Fan

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Final revised paper (published on 23 Aug 2018)
Preprint (discussion started on 30 Aug 2017)

Interactive discussion

Status: closed

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

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RC1: 'Review of: Investigating the Impacts of Saharan Dust on Tropical Deep Convection Using Spectral Bin Microphysics.', Anonymous Referee #1, 09 Sep 2017
- AC1: 'Author Response to first Referee comments', Matthew Gibbons, 07 Dec 2017
- AC3: 'Revised Author's response to 1st Referee Comments', Matthew Gibbons, 21 Jan 2018
RC2: 'Referee Comment on Gibbons et al: "Investigating the impacts of Saharan dust on tropical deep convection using spectral bin microphysics"', Anonymous Referee #2, 11 Oct 2017
- AC2: 'Author Response to Second Referee comments', Matthew Gibbons, 07 Dec 2017
- AC4: 'Revised Author's response to 2nd Referee Comments', Matthew Gibbons, 21 Jan 2018

Peer-review completion

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

AR by Matthew Gibbons on behalf of the Authors (18 Feb 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (20 Feb 2018) by Corinna Hoose

RR by Anonymous Referee #1 (24 Feb 2018)

Suggestions for revision or reasons for rejection

Review of: “Investigating the Impacts of Saharan Dust on Tropical Deep Convection Using Spectral Bin Microphysics” by Matthew Gibbons, Qilong Min and Jiwen Fan
The revised manuscript is organized and written in a better and clearer way. In addition, the necessary details about the methodology were added. I have additional comments and suggestions for the authors.

Specific comments

I suggest to shorten the abstract and to focus only on the main conclusions here.

P3 L15: the increased evaporation of the smaller cloud droplet at the cloud periphery under polluted conditions can also increase the mixing between the cloud and the environment.

P14 L5: is it the same magnitude of forcing you get in the model results?

Technical comments

P2L12 (and other places): I suggest to be consistent and use “DCC” for “deep convective clouds” in all places.

P6 L19: at this point you still didn’t mention the use of a nested grid with 4 domains so the reader don’t know what does “the entire 4th domain” is referring to.

P10 L30: what is the scale of the exponential decrease in aerosol concentration with height?

P11 L2: you are talking here about wind shear but present values of wind speed.

P12 L28: “DCC cloud”, you can delete the second “cloud”.

P14 L23: correct: “From model hours 20 onwards, The…”

P19 L12 and P19 L25: change number to concentration – aggregation is sensitive to concentration and not to absolute number of partials.

P23 L14: suggest to add “(SBM)” after “spectral-bin microphysical”

I noted that you are referring to Fig. 6 only at the last paragraph of the manuscript. Isn’t it appropriate to refer to it at the end of P15 or begging of P16?

Figures: correct cm-3 to cm-3, L-1 to L-1 and so on.

Referee Report: PDF

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RR by Anonymous Referee #2 (03 Mar 2018)

Suggestions for revision or reasons for rejection

I accept the authors response to my initial comments. Below some further comments and questions are listed that I would like the authors to address.

Major points:

- The authors explain that (e.g. p23, l27/28) “Latent heat release in the heterogeneous nucleation regime is increased in the dust cases due to [..] smaller, more numerous particles”. I do not think that this really is an effect of particle size but rather a consequence of an increased glaciation of the cloud. I mean to say that the total amount of ice, independent of whether it is distributed to many small or a few larger particles, controls vapor deposition.

- I am struggling to reconcile Figs. 10 and 14 (d) with the authors conclusion on “less efficient graupel formation reducing convective rain rates” (e.g. p1, l24): Fig 14 (d) show that the reduction is dominated by a reduction around 6km height. In Fig. 10, riming seems to be about the same, while autoconversion is decreased and aggregation increased in this region - so for me it seems that a reduction in autoconversion is the cause of decreased precipitation. Could you clarify this, maybe with the help of difference plots in Fig. 10? Similarly, do you provide an explanation for your observation that “precipitation formation is shifted to colder temperatures” (e.g. p24, l2)? Comparing Fig. 14 (d) and Fig. 10 this seems to correspond to a shift from autoconversion to aggregation.

- I suggest to move section 4.2 “Radar Reflectivity” to the end of section 4. This would allow to discuss its findings using results from the foregone analysis. Also, it would reflect the structure of abstract and conclusion.

- The authors find that “reducing dust layer moisture content by 5% “ (e.g. p25, l1) inverts the convective radar reflectivity difference pattern. What does this sensitivity to layer moisture mean for the robustness of the results?

Smaller/technical comments:

- p1, l20: Do you mean: Before they are transported into the anvil?
- p6, l19: The “4th domain” is mentioned here without having been introduced.
- p7, l8: The sentence does not make sense as is.
- p13, l13: Where is the rain rate increased? Near the equator? Below the core it seems decreased?
- p13, l21: Could you also comment on the strong increase in snow radius below the freezing level?
- p17, l1: The black lines in Fig. 7 (a,b) do not look like more than 90% of ice formation occur above the -38 degree line. Could you clarify this?
- p 21, l16-19: I find this very hard to see - could you show/discuss this more clearly?
- p24, l2 and l 17/18: These sentences repeat each other.

- Fig 2: Why don’t you show difference plots here?
- Fig 10: Why don’t you use difference plots here? Can aggregate numbers be converted to rates so that there magnitudes can be compared to riming and autoconversion? Autoconversion rates in the warm region exceed the color scale for all dust levels so that they cannot be compared. Could you add a level to the color scale or use difference plots?

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ED: Publish subject to minor revisions (review by editor) (06 Mar 2018) by Corinna Hoose

AR by Qilong Min on behalf of the Authors (23 Mar 2018) Author's response Manuscript

ED: Publish as is (23 Mar 2018) by Corinna Hoose

AR by Qilong Min on behalf of the Authors (03 May 2018)

Short summary

The effects of dust aerosols on ice formation within a tropical Atlantic thunderstorm system were investigated using a 3-D weather model and compared with observations. Updated ice formation mechanisms directly connect available dust particles with ice particle formation. The resulting clouds were lower and narrower and produced less rain at the surface compared to cleaner conditions, due to ice formation occurring at warmer temperatures. These results agree well with observed changes.