The paper by Rittmeister et al describes the R/V Meteor transatlantic cruise which allowed the authors to study a 4000 km long transect of dust properties as measured by lidar. The authors' overall publication plan concerning that dataset, emerging from the information available to me, is as follows:
1. Kanitz et al (GRL, 2014) is a concise paper, and presents the dust layer optical properties, reporting near the Caribbean (dust aged ~ 10d) Angstrom exponents of 0.1, a LR of 45 sr, and a PDR of 0.2; and near the African coast (dust aged 2-3d) Angstrom exponents -0.5 (backscatter) and 0.1 (extinction), a LR of 64 (50) sr, and a PDR of 0.22 (0.26) at 355 (532) nm. The handheld sunphotometer data showed an AOD from 0.05 in clean conditions (marine-dominated) to 0.9 in a strong Saharan outbreak near Africa. A backtrajectory analysis suggested the influence of smoke in the dust layer, which was also explained in terms of optical properties. A comparison was made with COSMO-MUSCAT dust predictions.
2. Rittmeister et al (present paper): presents the optical properties of the different layers observed: marine (LR 17 sr, PDR 0.025, extinction 67 Mm-1) and dust (LR 43 sr, PDR 0.19, extinction 68 Mm-1, AOD 0.15). Handheld sunphotometer data are presented in conjunction with the lidar data. Backtrajectories are presented, and demonstrate the influence of smoke in the dust layer (this being the same conclusion than in the Kanitz paper, but a different conclusion than in the discussion version of the present paper). A conceptual model by Karyampudi et al (1999) is discussed in conjunction with the observations. The authors interpret the data also in terms of dust removal rates in the SAL and MAL, concluding that the sedimentation pace confirms previous observations.
3. Ansmann et al (Part 2, also in ACP): comparison of the observations reported in the above two papers with three dust models, and discussion on how well dust removal is represented. It is concluded that the models overestimate dust sedimentation beyond 2000 km, and as a consequence they underestimate the concentration of coarse particles.
During the online discussion, the risk of duplicating information with this overall plan was highlighted and rejection was recommended, with the suggestion of trimming down the information reported and possibly combine papers 2 and 3 together. The authors have given a detailed response and have rearranged the information in the present paper. The editor has communicated to be satisfied that the authors have provided justification that their article is significantly different, and therefore deserves consideration within the framework of the above overall publication plan. The present review will therefore not provide an opinion on the duplication of information or on the overall publication plan presented by the authors, and it will focus on how this paper is written.
GENERAL COMMENTS:
The paper is largely improved, showing a better presentation of the material and consequently it is easier to follow than in the first version (the first version seemed to have been assembled a little bit in a rush). The conceptual model being discussed at the beginning, with a clear definition of the layering considered (SAL, MAL, MBL), it is now possible to follow the authors' reasoning. Conclusions have been written (whereas they were almost unexistent in the previous version), although I think that they are not yet fully mature.
As opposed to Kanitz et al, the current paper presents quality assured final data. It would be good to highlight in section 2.2 what different processing and QA tests have been implemented compared to the previous paper.
The authors argue strongly against aging effects in the optical properties for the observed dust across the Atlantic, and some of their arguments appear convincing. On the other hand, however, they seem to confirm that the PDR would decrease in the case that the dust downstream were finer than upstream, and that the LR would be modified in the case that the dust were mixed with other species. I believe that there is a contradiction here: mixing with other species and modifications of the PSD are indeed aging mechanisms, which is something that the authors do not seem to recognise. I suggest therefore to reword the paper in such a way as to reflect these considerations.
Whereas PDR and LR can help identify aerosol type, and indeed aerosol typing algorithms are possible based on optical properties, it is not true that these algorithms always give the correct answer. Moreover, the "typical" optical properties for a same aerosol type may vary (e.g. dust from different source regions). I believe therefore that the authors' statements on this matter are too sharp. We should always bear in mind that these are not direct measurements of composition, and e.g. a same set of optical properties measured in a different area of the world (or when we know that different sources are active) could lead to different conclusions. I believe therefore that these statements could be softened, reflecting the fact that the optical measurements give indications, that need to be considered in conjunction with the understanding of sources and transport. The most experienced can understand these nuances even when they are omitted, but our papers are read by our students, and they learn from us. We want our students to develop a critical understanding of aerosol typing, rather than a simplified reality.
Similarly, sharp statements are made by the authors on the percent dust in a mixture with something else, without much explanation to the reader about how such quantities are determined, or what the underlying assumptions are. This type of quantification comes most probably from assuming a given depolarization for pure dust and a different one for the "other aerosol": these assumptions are fine, but like all assumptions, they must discussed and their consequences and uncertainties must be mentioned. One more assumption (too often overlooked, although it is fundamental) for this method to work is that the two aerosols are externally mixed, and this should be clearly stated.
The examination of the backtrajectories brings the most convincing indication concerning the mixing of dust and smoke in the observed aerosols. What the data available do not allow us to determine, is whether the mixture is internal (e.g. coating or adsorption of smoke on dust particles) or external (co-existing but separate dust and smoke particles). Although it will not be possible to resolve this matter from the data available, I think that it has to be mentioned as an area where further research can bring important insight.
Large depolarization ratios for the MAL near the African coast are shown in a figure and mentioned in the text, but they are not discussed although they are a very surprising result. I believe, however, that this may be due to an incorrect separation between the two layers using a fixed altitude set at 900 m. A variable boundary, inferred from the lidar dataset, could be used for a correct representation.
An effort has been made to differentiate this paper from the Kanitz et al paper. As such the figures have been rearranged by putting all those extracted from the Kanitz paper into a single multi-panel figure. Moreover, the text of the article has been restructured. The paper having been re-written in large part, I'll provide here a a new review for it, as it is difficult to follow changes from the previous version to the present one.
It seems from the answers to the previous review that the authors are reluctant to make any graphical modification to figures. Whereas this may represent some additional effort, in some cases it is necessary to ensure readability, and I recommend to update figures when issues are identified.
My recommendation is for a major review.
MAJOR POINTS:
1) To completely remove the impression that the current paper is the "long version" of the Kanitz paper, I would be more explicit in the abstract's opening sentence as follows: "We present here final and quality assured results of multiwavelength polarization/Raman lidar observations of the Saharan Air Layer (SAL) over the Atlantic. Observations were performed aboard the German research vessel R/V Meteor during the one-month transatlantic cruise from Guadeloupe to Cabo Verde over 4500 km from 61.5W to 20W (mean latitude ???) in April-May 2013 that was reported in a previous concise paper."
2) Whereas microphysical properties can be inferred from optical properties (indirect measurement), the type of lidar that the authors use cannot make a DIRECT measurement of microphysical properties. Therefore, omit "microphysical" on P3 L10.
3) P3 L34: the SAL is not only a dust layer; it is principally a hot and dry airmass. The way this is presented here seems to indicate that dust is the main property of the SAL, whereas this is incorrect. See also the next comment.
4) P4 L11-13: the authors report some geometric information on the SAL, but it is unclear where this comes from. Is this still extracted from Karyampudi et al? Note that this sentence is not very informative when it says that "the SAL top is assumed to lower due to [...] a general lowering of the dust-layer top". The authors also attribute the lowering of the SAL top to the depletion of giant particles, but (1) this seems to be in contradiction with the position that the authors take towards the absence of aging mechanisms for dust (the change in the PSD due to deposition would be an aging mechanism); (2) it is unclear how the sedimentation of the largest particles would affect the SAL layer top (top of the hot and dry layer); and (3) the radiosondes presented in the current paper clearly show that the airmass above the dust layer is different, hence the lowering of the SAL top is due to atmospheric dynamics rather than sedimentation. If sedimentation had been the driver, the air above the dust layer top would conserve its thermodynamic properties (hot and dry). I would assume that travel for a long time over the cold surface of the ocean could progressively reduce convective exchanges in the SAL and hence cause it to become shallower. Divergence of the air flow could potentially also reduce the SAL's vertical extent (airmass redistribution over a wider area and hence thinning of the layer for mass conservation).
5) P4 L14-16: I suppose that these considerations are about the potential of wet removal mechanisms associated with the stratocumulus deck at the top of the MBL: is it please possible to clarify? Clouds have some times also been observed at the top of the SAL: what about their ptential to trigger wet removal mechanisms?
6) P6 L4-21: As discussed in the general comments, I find this text to be contradictory. On one hand the authors take position against the existence of dust aging mechanisms, and on the other hand they admit that the PSD may shift towards smaller particles due to removal mechanisms, with a consequent reduction of the PDR. I feel that this is precisely an aging mechanism. Moreover, whereas the authors claim that fine dust's PDR is 0.14-0.18, I do not understand why all the intermediate values between 0.3 and 0.14 are not possible during different stages of the aging. Finally, how can aging due to deposition (PSD change) be distinguished by lidar from aging due to the uptake of water, smoke, or pollution, if the effect on the PDR is similar?
7) P7 L11: As discussed in the general comments, the large PDR of the MBL close to the African coast may possibly be ascribed to a methodological flaw, as the boundary between the two layers is set at a fixed height. Models often show a very shallow MBL height under the dust layer just off the African coast.
8) P7 L14-16: Soften statements! An AOD alone cannot be used in aerosol typing and other considerations are needed (lidar observations, meteorology, etc.), whereas the authors say sharply that an AOD of 0.05 automatically equates to marine conditions and one of 0.7 to a dust outbreak. L23: "indicates" --> "suggests" L25: "marine" --> "finer"
9) P8 L27-30: this is plausible, but from the analysis presented in this paper there is no proof of this airmass history. This should be made clear.
10) P9 L5: Unless there exists a specific Meteosat "wet deposition" product (which I am not aware of), I suppose that cloud cover was used as a proxy. Therefore please soften the statement: "potential impact of wet deposition by strong cumulus development" --> "presence of strong cumulus". L6: delete "wet deposition"
11) P10 L1-7: the statement that trajectories did not cross areas with biomass burning is contradicted by Fig. 5. Also, soften statements on the conclusions drawn from optical properties as follows: (1) specify how the estimated 10% non-dust extinction is computed; (2) Angstrom exponent indicates large particles; hence no external mixture with a non-dust component; but what about an internal mixture? An internal mixture would also display large particles and hence a low Angstrom exponent!
12) P10 L8: soften statement! "significant" --> "possible"
13) P10 L10-11: soften statement by being possibilistic on aerosol type and by explaining how the 50% non-dust is estimated.
14) P10 L15: soften statement! add "potentially" before "contained smoke"
15) P10 L20-21: soften statement!
16) P11 L1-9: these computations are valid for an external mixture. This should be stated. If dust is coated with "something else" then the Tesche et al (2009) method cannot be applied.
17) P11 L16: soften statement! "as obviously observed with the shipborne lidar" --> "the shipborne lidar observations are compatible with an aged smoke layer". As for above, no consideration for internal mixing is provided: please discuss.
18) P13 L2: see above (general comments) on using a fixed separation between layers at 900 m.
19) P13 L17: soften statement! "indicates" --> "suggests"; "smoke" --> "smoke or sea salt"
20) P13 L31: soften statements! "leads" --> "should lead"; "causes" --> "should exhibit"
21) P13 L33: "but this is not found". Vertical mixing within the SAL could explain the constant PDR across the layer, and only particles that make it to the bottom of the layer into the MAL would then be removed. Your radiosonde measurements could provide a unique insight on the SAL thermodynamics and ability to mix vertically. And if I read further to P14 L9-15, I see that you also explain this well. Therefore, the feeling that there is no new finding here, but a confirmation of previous findings. The whole way this is presented could be therefore reversed: initially present what other articles say, and then illustrate the position that your observations show. This would highlight that your contribution is to confirm given hypotheses. Note: another conclusion from this is that the next E-W Atlantic transect measurements would benefit from vertically resolved PSD measurements (although technology for this is just beginning to mature).
22) I feel that the conclusions are not yet mature for publication and I suggest that the authors spend a bit longer time to refine them. P 14 L21: I feel that it is an overstatement that the data available "permitted the study of dust removal aspects in large detail". This should be softened by saying that the data presented are compatible with previous papers (see previous point). L24-25: the paper does not present evidence on dust removal below the SAL. L26: "less efficient removal than expected": this is not correct, as literature that you also cite had already come to the same hypothesis; it is much better to say that your findings support those conclusions. L31: anthropogenic haze has not been discussed in this paper; "was" --> "could be". P15 L1: "was caused" --> "was estimated to be caused". L2: "grew" --> "may have grown". L26: mandatory? who has decided this?
23) I believe that the main points that the conclusions should focus on are (1) atmospheric layering observed; (2) potential mixing of dust and smoke; (3) similarities and differences of observations from Karyampudi's conceptual model; (4) similarities and differences of observations from different papers on sedimentation; (5) the need of more observations of the SAL over the tropical Atlantic; and (6) what other observations, in addition to the ones you had on the research vessel, would be desirable to answer the most compelling questions.
24) Figure 3: the gravitational settling (red arrow) and the exchange mechanisms between layers are not demonstrated in this paper, and the evidence presented by the authors seems to be against them. Therefore they should be omitted from this figure to reinforce the message.
25) Figure 6: I believe that the fixed separation at 900 m between SAL and MAL may cause interpretation errors. I believe that the effect of this is obvious in the plot of the MAL LR and PDR on 21-24 May.
OTHER POINTS:
26) Abstract P1 L13: add "at different longitude" after "20 nights". Exchange the order of the words MAL and SAL, as later on MAL is always presented before SAL. L17: add "typical" between "compared to" and "pure dust values".
27) P2 L1-2: add "in very specific circumstances" before "be lifted up to the tropopause"
28) P4 L7 "the observed optical properties": it is unclear which observations the authors refer to: the current paper? pre-existing literature?
29) P8 L9-10: "are predicted or described by" --> "confirm"
30) P8 L13: draw top of MBL in Fig. 2.
31) P8 L15: "dark blue layers" --> "layers showing a low depolarization"
32) P8 L24: "dissolve" --> "evaporate"
33) P9 L1-3: are the NE and E winds in the MAL and SAL confirmed in the radiosondes launched aboard the ship?
34) P11 L25: add at end of paragraph: "However, the data presented here are insufficient to either support or deny such influence".
35) P13 L15: "dust characteristics" --> "dust optical properties"
36) P13 L25: add "according to the conceptual model" before "gravitational settling is responsible for the removal of dust"
37) P13 L33: add "increasing" before "height"
38) P14 L22: cite the Karyampudi paper again
39) P15 L29: add "on board the research vessel" after "launched"
40) Figure 1 readability: continents in top panel, and grey areas in bottom panel: they are not visible.
41) Figure 2: draw MBL top height in the left panels; caption: "given in blue" --> "low depolarizations displayed in blue".
42) Figure 6 readability: grey areas are not visible on printed copy. |
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