Evidence of the complexity of aerosol transport in the lower troposphere on the Namibian coast during AEROCLO-sA

Chazette, Patrick; Flamant, Cyrille; Totems, Julien; Gaetani, Marco; Smith, Gwendoline; Baron, Alexandre; Landsheere, Xavier; Desboeufs, Karine; Doussin, Jean-François; Formenti, Paola

doi:https://doi.org/10.5194/acp-19-14979-2019

Articles | Volume 19, issue 23

https://doi.org/10.5194/acp-19-14979-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

New observations and related modelling studies of the aerosol–cloud–climate...

https://doi.org/10.5194/acp-19-14979-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 23

Research article

|

11 Dec 2019

Research article |

| 11 Dec 2019

Evidence of the complexity of aerosol transport in the lower troposphere on the Namibian coast during AEROCLO-sA

Patrick Chazette, Cyrille Flamant, Julien Totems, Marco Gaetani, Gwendoline Smith, Alexandre Baron, Xavier Landsheere, Karine Desboeufs, Jean-François Doussin, and Paola Formenti

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Final revised paper (published on 11 Dec 2019)
Preprint (discussion started on 03 Jun 2019)

Interactive discussion

Status: closed

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

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RC1: 'Review', Michael Diamond, 30 Jun 2019
- AC1: 'Response to reviewer#1', Patrick Chazette, 17 Sep 2019
RC2: 'Review of "Evidence of the complexity of aerosol transport in the lower troposphere on the Namibian coast during AEROCLO-sA" by Chazette et al.', Anonymous Referee #2, 08 Jul 2019
- AC2: 'Response to reviewer#2', Patrick Chazette, 17 Sep 2019

Peer-review completion

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

AR by Patrick Chazette on behalf of the Authors (17 Sep 2019) Author's response

ED: Referee Nomination & Report Request started (20 Sep 2019) by Paquita Zuidema

RR by Anonymous Referee #2 (01 Oct 2019)

Suggestions for revision or reasons for rejection

Review of the revision to "Evidence of the complexity of aerosol transport in the lower troposphere on the Namibian coast during AEROCLO-sA" by Chazette et al.

I appreciate the authors' revisions in response to the two reviews, and in my opinion these revisions have improved the manuscript. In particular, adding the discussion of uncertainties was useful and the discussion of the potential transport of aerosol from South America has been resolved to my satisfaction.

I do have a lingering concern that some of the statements interpreting the lidar results, in particular, the lidar ratio, are over-interpreted. If my understanding of the retrieval technique is correct, the attenuated backscatter profiles are retrieved to extinction using (1) in the free troposphere, the lidar ratio that was assumed by CALIOP or CATS for their retrievals and (2) in the PBL, a lidar ratio selected from the small set of lidar ratio models of CALIOP or CATS, whichever one creates the best match to the sun photometer AOD. Since the lidar ratio in the free troposphere is merely assumed, it is not appropriate to discuss changes in the lidar ratio as if it is an observation. (Of course, the CALIOP and CATS algorithms do not choose it arbitrarily; it's based on other evidence, but it is not a direct observation. If some comment is to be made about it, it should be made about the evidence that was used in the lidar ratio selection, which is probably primarily the altitude of the aerosol layers and the satellite-observed attenuated depolarization.) In the PBL, since only a few lidar ratios are attempted, the precision or resolution of the lidar ratio is extremely coarse, and so the authors should likewise be very careful not to over interpret the results.

If the authors would consent to make the following changes, I would appreciate it. Other than this, I would be happy to see this manuscript published.

line number refer to v5

257: "significant changes in LR are also observed between P1/P2 and P3 with values in the FT evolving". This should be deleted. There is no observation (only assumptions) of lidar ratio in the free troposphere. If the authors want to keep something like this, I suggest something more like "CALIPSO and CATS retrievals suggest differences in the FT aerosols between P1/P2 and P3, with more occurrence of polluted dust in P1/P2 and polluted continental or smoke in P3."

259: and for the PBL, again, since the retrieval here is quite coarse, I suggest rewording to something like "In the PBL, the low value of lidar ratio required to reproduce the sunphotometer AOT is consistent with the presence of clean marine aerosols in the PBL. The higher lidar ratios required in P3 indicate the presence of other aerosol types, which may include smoke or a mixture of smoke and terrigenous aerosols."

268-269: The higher lidar ratio from this retrieval is not by itself indicative of dust. Any aerosol type other than marine aerosol or mixed with marine aerosol would produce a higher lidar ratio than the clean marine case; it does not require dust to explain this. I suggest deleting this sentence and simply waiting a few lines until the depolarization ratio is discussed before bringing up the hypothesis of a dust mixture.

893: I believe this description is still somewhat confusing and I think the Author Response made it clearer. Consider replacing "obtained" with "selected from the discrete set of lidar ratios shown in Table 2" (if, in fact, this is an accurate description of the procedure).

896: consider replacing "are representative of" to "are associated with" and delete "and consistent with the LRs from CALIOP for these aerosol types (compare Table 3 and Table 2)". If the lidar ratio were derived to a precision of a few sr, finding the lidar ratio to match the expected values for clean marine would be evidence of the presence of clean marine aerosol; however, since only a few lidar ratios are attempted, this evidence is rather weak. The statement that they are consistent with lidar ratios for CALIOP types seems to be merely redundant, since the retrieval used only those lidar ratios that correspond to CALIOP aerosol models (if I understand the response in the Author Response correctly).

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RR by Michael Diamond (03 Oct 2019)

ED: Reconsider after major revisions (03 Oct 2019) by Paquita Zuidema

AR by Patrick Chazette on behalf of the Authors (11 Oct 2019)

ED: Publish subject to minor revisions (review by editor) (26 Oct 2019) by Paquita Zuidema

This manuscript has improved a lot and reads very well. Below are some minor comments that are just meant to raise the level further; minor typos can be distracting to the reader and are worth removing.

abstract, line 22:- capitalize Lagrangian
P 2 line 37: correct St
P. 2 line 70: the aerosol radiative properties are held fixed in the Myhre and Stier papers. The model shortcomings are rooted more strongly in model diversity in the cloud fractions, e.g., Zuidema et al 2016 bams DOI:10.1175/BAMS-D-15-00082.1 ,and Stier et al. 2013, and this is what is affecting the sign of the regional DARE.
p.6 line 139: analyse -> analyze
P. 9 line 256: there seems to be some sort of latex error here.
p. 18, line 340: evidence the -> provide evidence of a
‘evidence’ isn’t a verb and is used incorrectly in the next sentence also. And line 359.
P. 18 line 363: ‘the’ before ‘regional’
p. 22 line 454: check the brackets. Also on line 458. And line 463. Line 489.
Figure 13, 14, 15: the figure labels and caption also have the same bracketing formatting problem.
p. 33, lines 610-613: given that lines 600-602 indicate an AOT contribution of 10-15% from south America, the last sentence seems indulgent. I think the last paragraph is very strong without this last sentence. The authors are correct that this is the first time that the vertical structure of the aerosol has been characterized over coastal Namibia and related to transport patterns. That is enough to make this a very interesting and useful contribution. This study is not in a position to assess precipitation influences, but my suspicion is that it is difficult for aerosol from south America to survive via the baroclinic mid-latitude disturbance route. It is beyond the scope of this paper to assess and that is fine.

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AR by Patrick Chazette on behalf of the Authors (27 Oct 2019) Author's response Manuscript

ED: Publish as is (28 Oct 2019) by Paquita Zuidema

AR by Patrick Chazette on behalf of the Authors (29 Oct 2019) Manuscript

Short summary

Evolution of the vertical distribution and optical properties of aerosols in the free troposphere is analysed for the first time over the Namibian coast, a region where uncertainties on aerosol–cloud coupling in climate simulations are significant. The high variability of atmospheric aerosol composition is highlighted using a combination of ground-based, airborne and space-borne lidar. Aerosols are mainly transported from Angola, but part of the highest aerosol layer may come from South America.