Dear Authors

Thank you for providing the revised manuscript “The biomass aerosol influence on precipitation over the Central Amazon: An observational study” as well as your responses to the issues raised during the reviews.
Based on the reviewer’s comments and my own assessment I have concluded to reconsider your manuscript for publication in Atmospheric Chemistry and Physics after major revisions.
Many of the reviewer’s comments have been resolved in your response; however, issues remain in terms of the interpretation and presentation of your results. I would kindly ask you to address the issues listed below.
I am looking forward to your revised manuscript.
Kind regards,

Philip Stier
Co-Editor
Atmospheric Chemistry and Physics

General issues:

1. This study uses BC as “aerosol tracer”. It clearly is an aerosol - but is it sufficiently correlated with CCN to support this kind of analysis? Please present some evidence from measurements or related literature.

2. The language of the manuscript remains an issue. Some parts are very well written, e.g. most of page 4, others not so clear and need to be improved (e.g. page 2-3 or Conclusions). This has to be addressed.

3. The results sections should be limited to the presentation of the results and their discussion. In this manuscript they include a significant amount of speculative material. This should be clearly separated from the results and presented as hypotheses in discussions or conclusions.

4. The conclusions are very strong yet highly speculative. It is important to make clear what has been shown in this study, what has been robustly backed up by other evidence and what is speculation.

Specific issues:

1. Page 1: The affiliation numbering is incorrect.

2. Page 1: Abstract is missing

3. Page 1, L22-26: You refer to direct effects here but what you describe is a fast feedback commonly referred to as semi-direct effects. This should be clearly described and referenced.

4. Page 1, L28: You only focus on CCN. For deep convection you may want to discuss the role of IN early on.

5. Page 2, L6-11 The description and referencing of aerosol effects on precipitation is not very clear. Rain suppression is described fairly simplistically while there is quite a bit of controversy in the literature. Some of the references are not suitable as primary reference, e.g. any rain suppression in Nober is by construction of the study, which can hardly be cited as an evidence of this effect and Kaufman is an interesting paper but maybe not the strongest reference on cloud microphysics.

6. Page 2, L12: “Diehl et al. (2007) suggested that the ice phase could be an important factor in the rain process.” The importance of ice has been realised a long time before Diehl. Please use primary references.

7. Page 2, L17-24: The description of the hypothesis presented in Rosenfeld et al. (2008) is slightly confusing.

8. Page 2, L24-25: “Over a certain time, the cloud accumulates higher liquid water and ice contents, favoring more intense rainfall rates and increasing electrical activity (Graf, 2004).” This is neither very clear not sufficiently backed up to be presented as a fact.

9. Page 3, L19-21: “However, the CAPE dataset was evaluated and shown to have very useful information and capture the daily instability feature even though it was not recorded at the most appropriate time.” This is not very clear and needs to be better explained. What was evaluated and what was found?

10. Page 3, L31: “Therefore, due to its potential for ice nucleation, the presence of BC would favor the development of ice phase clouds, including deep convection.” This is very speculative and it seems very unlikely that BC regulates the occurrence of deep convection as the formlation suggests.

11. Page 3, L33- Page 4, L4: This is another insufficient description of the semi-direct effects / fast feedbacks. The description needs to be improved and should be merged with the above.

12. Page 4-5: The description of the indices RF, IRF, IF is insufficient. Explain their significance and their relation to the physical properties you try to represent. “1mm” is presented here without any explanation.

13. Page 6, line 15 and throughout manuscript: you use “elevation synonymous with upslope. This is not necessary the case. Discuss.

14. Page 6, L17: “At this distance, the lower radar elevation band is around 1 km high, which eliminates the ground clutter effect possibility. Thus, this area should receive special attention under forest protection policies.” I don’t understand this statement.

15. Page 6, L30-33: “Thus, the combination of a period of the year (wet season) with more rain and less aerosol concentration, and the other season (dry season) with high BC concentrations and smaller amounts of rain but more intense precipitation events allowed us to reach interesting results.” At the same time you point out one of the biggest difficulty to isolate any aerosol effect in your study – emissions, thus aerosols, are highly correlated with the wet / dry season. Any variation could be an effect of either. This needs to be discussed and considered throughout.

16. Page 7 L6: “After this value, the RF slightly increases.” It seems unclear how significant this increase is.

17. Page 7, L18-32: The attribution to a rain suppression mechanism here is fairly arbitrary. I cannot see any evidence to rule out scavenging as the dominant effect. This is certainly consistent with the higher rain fraction for lower BC concentrations in Fig. 4c. Please move speculative discussions from the results section and discuss all possibilities.

18. Page 7, L29-30: “As the RF decreases for elevated BC concentrations, the wet deposition seems not to be the dominant effect on much polluted atmospheres, which gives support to the rain suppression theory.” I don’t understand this conclusion. Decreasing BC with increasing RF is entirely consistent with scavenging.

19. The description of the applied test for scavenging is insufficient: “Thus, the concept of this test was based on eliminating from our statistics all samples whose precipitation was observed over the EUCAARI site. This elimination excluded the samples whose precipitation could have cleaned the atmosphere, throughout the scavenging process.” What data has exactly been excluded? The effectiveness of this screening will strongly depend on mixing time-scales (surface rain is not directly related to surface BC as the dominant scavenging mechanism will be activation scavenging, not below cloud scavenging). The scales of scavenging events are fairly small , introducing quite distinct gradients (c.f. Gryspeerdt et al., ACPD, 2015) so it is possible that this test will not work to screen out scavenging events. This needs to be considered.

20. Page 8, L21-25: “This result could be an indication that convection is invigorated by higher BBA concentrations (Lin et al., 2006; Graf, 2004; Rosenfeld et al., 2008; Altaratz et al., 2010; Koren et al., 2012). Considering that high CAPE values are associated with stronger updrafts, the aerosol effect on the rainfall and in the severity of the convective processes could depend on the intensity of the vertical motions.” I am not sure I am convinced by this argument. First, RF is purely geometrical it gives an intense rain fraction (horizontally). The link to cloud height or updraft strength is not obvious.

21. Page 8, L26-29: “The radiative effect that acts to stabilize the atmosphere is of a second order because even with high BBA the atmosphere is highly unstable, and thermodynamics, on this time scale, dominates over the radiative process. Also, the feedback effect due to the radiative effect, which increases droplet evaporation, does not seem to be the predominant mechanism.” I do not see any evidence or result corresponding to this statement. Please move from results section and provide evidence. Also, the effect of absorption is not limited to droplet evaporation. It could be an efficient mechanism to maintain convective inhibition for certain stratifications.

22. Page 8, L29-31: “Probably, the high instability (high updraft) and the large number of droplets inside the cloud ascend very fast, thereby reducing the evaporation.” This again is speculation. Possible, but it could be also very different, depending on the cloud structure. Turbulent entrainment is likely to increase with updraft velocity and will also increase evaporation. Please provide evidence for any such statements you make.

23. Page 8, L31-33: “Although impossible to quantify, the wet scavenging also seems to be of second order, and would act in the opposite direction through the fact that precipitation did not decrease BC
concentration at any point of the curve (Fig. 4b). “ This is an interesting observation but the question is if your sampling strategy would be able to detect scavenging from such low rain fractions of <2%.

24. Page 8, L33: “During the dry season only the upslope regions trigger convection and in the highly unstable cases it appears that BBA helps to increase ice nucleus, increasing precipitation.” I cannot see any results supporting this conclusion.

25. Page 9, L14-29: This section contains a large amount of speculation on effects for which there is no evidence. There is discussion of stratiform and convective clouds but no formal separation is made. The similarity of the ice fraction data to the rain fraction data seems suggests that ice fraction and rain fraction are highly correlated; this alone could explain the behaviour shown in Fig. 5. This needs to be discussed or explicitly excluded.

26. Page 9-10: Discussion of rain cell size: the plots closely follow the behaviour shown for RF and IF before so it appears that RF, IF and cell size may be strongly correlated. While the relationship appears strong, there is no evidence of causality, i.e. that it is that aerosols actually cause the observed relationship. Generally, high and low aerosol conditions occur in different synoptic situations with very different cloud regimes (c.f. Gryspeerdt et al., 2013,2014). A snapshot analysis can only reconfirm this relationship. The speculative part of this section should be separated from the results and be identifiable as hypotheses. Personally, I would require some evidence to be convinced by the proposed mechanisms.

Dear Authors

Thank you for providing the revised manuscript “The biomass aerosol influence on precipitation over the Central Amazon: An observational study” as well as your responses to the issues raised during the reviews.

I have assessed your revised manuscript and am pleased to inform you that the manuscript is now accepted for publication in ACP.

Thank you for choosing Atmospheric Chemistry and Physics.

Kind regards,

Philip Stier
Co-Editor
Atmospheric Chemistry and Physics