Above-cloud aerosol optical depth from airborne observations in the southeast Atlantic

LeBlanc, Samuel E.; Redemann, Jens; Flynn, Connor; Pistone, Kristina; Kacenelenbogen, Meloë; Segal-Rosenheimer, Michal; Shinozuka, Yohei; Dunagan, Stephen; Dahlgren, Robert P.; Meyer, Kerry; Podolske, James; Howell, Steven G.; Freitag, Steffen; Small-Griswold, Jennifer; Holben, Brent; Diamond, Michael; Wood, Robert; Formenti, Paola; Piketh, Stuart; Maggs-Kölling, Gillian; Gerber, Monja; Namwoonde, Andreas

doi:https://doi.org/10.5194/acp-20-1565-2020

Articles | Volume 20, issue 3

https://doi.org/10.5194/acp-20-1565-2020

© Author(s) 2020. 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-20-1565-2020

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

Articles | Volume 20, issue 3

Research article

|

07 Feb 2020

Research article |

| 07 Feb 2020

Above-cloud aerosol optical depth from airborne observations in the southeast Atlantic

Samuel E. LeBlanc, Jens Redemann, Connor Flynn, Kristina Pistone, Meloë Kacenelenbogen, Michal Segal-Rosenheimer, Yohei Shinozuka, Stephen Dunagan, Robert P. Dahlgren, Kerry Meyer, James Podolske, Steven G. Howell, Steffen Freitag, Jennifer Small-Griswold, Brent Holben, Michael Diamond, Robert Wood, Paola Formenti, Stuart Piketh, Gillian Maggs-Kölling, Monja Gerber, and Andreas Namwoonde

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Final revised paper (published on 07 Feb 2020)
Preprint (discussion started on 30 Jan 2019)

Interactive discussion

Status: closed

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

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SC1: 'Leblanc et al. discussion questions', A.M. Dzambo, 07 Feb 2019
- AC2: 'Discussion question response', Samuel LeBlanc, 21 Aug 2019
RC1: 'Review of LeBlanc et al.', Anonymous Referee #1, 28 Feb 2019
- AC1: 'Response to comments for Above Cloud Aerosol Optical Depth from airborne observations in the South-East Atlantic', Samuel LeBlanc, 21 Aug 2019
RC2: 'Review of LeBlanc et al., 2019', Anonymous Referee #2, 14 Mar 2019
- AC1: 'Response to comments for Above Cloud Aerosol Optical Depth from airborne observations in the South-East Atlantic', Samuel LeBlanc, 21 Aug 2019
RC3: 'Referee comment', Anonymous Referee #3, 09 Apr 2019
- AC1: 'Response to comments for Above Cloud Aerosol Optical Depth from airborne observations in the South-East Atlantic', Samuel LeBlanc, 21 Aug 2019
AC1: 'Response to comments for Above Cloud Aerosol Optical Depth from airborne observations in the South-East Atlantic', Samuel LeBlanc, 21 Aug 2019

Peer-review completion

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

AR by Samuel LeBlanc on behalf of the Authors (21 Aug 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (01 Oct 2019) by Jérôme Riedi

RR by Anonymous Referee #2 (21 Oct 2019)

Suggestions for revision or reasons for rejection

The paper written by Samuel LeBlanc et al. is focused on the Aerosol Above Cloud (AAC) properties measured by the 4STAR instrument during the ORACLES 2016 airborne campaign. The magnitude and spatial variability, as well as the vertical distribution of above-cloud aerosol optical depth (ACAOD) and Ångström Exponent (AE) are discussed. A comparison of 4STAR ACAOD with satellite retrievals from MODIS of clear-sky fine mode AOD and ACAOD is also made. Their results show that during the ORACLES 2016 campaign the aerosol properties sampled above the clouds are consistent with previous studies, which show high elevated thick layers of fine particles, typical to biomass burning aerosols. They also found that the largest ACAOD is found in the northern part of the sampling region, and that high AOD variability coincides with high AE variability. Both the satellite climatology over August and September and MODIS ACAOD retrievals collocated along the flight tracks consistently overestimate the 4STAR ACAOD. The final results show that the gap between the aerosol layers and the cloud tops is larger further away from the coast compared to near-coast samples.

The paper is now well documented and well written. The authors have made a wholesome effort in giving detailed responses to the reviewer’s questions and comments, modifying the text where needed, which increased the scientific value of the paper and concluded an in-depths study of aerosols above clouds over the Atlantic Ocean.

In my opinion, the paper is ready to be published in ACP.

Specific remarks:
Page 4, line 8: modify with Deaconu et al., 2019
Page 9, lines 12-14: Are the maximum values for the absolute range for ACAOD and full-column AOD correctly provided, since you mention: “The full column AOD is distinct from the ACAOD measurements as they necessarily require conditions without overlying cloud and thus will include the elevated biomass burning layer as well as any lower-level aerosol near the sea surface.”?

Deaconu, L. T., Ferlay, N., Waquet, F., Peers, F., Thieuleux, F., and Goloub, P.: Satellite inference of water vapour and above-cloud aerosol combined effect on radiative budget and cloud-top processes in the southeastern Atlantic Ocean, Atmos. Chem. Phys., 19, 11613–11634, https://doi.org/10.5194/acp-19-11613-2019, 2019.

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RR by Anonymous Referee #1 (04 Nov 2019)

Suggestions for revision or reasons for rejection

The authors have made substantial efforts to improve their paper and to take into account the reviewers’ comments. There is just the formulation of the new paragraph on page 2 (starting from line 22) that I do not totally agree on. The authors do a list of the uncertainties associated with the retrieval of the ACAOD from passive satellite instrument which I found to be unclear and partly incorrect.
(1) “the assumption that the spectral representation of aerosol absorption is assumed to be constant (e.g., Chand et al., 2009, Meyer et al., 2015)”; “aerosol properties don’t vary in a large spatial swath (e.g., Torres et al., 2012); and/or that the retrieved aerosol properties over highly reflective and opaque clouds are representative of all aerosol (e.g., Hu et al., 2007, Peers et al., 2015)”
All the retrievals based on the color-ratio effect (passive reflectance only measurements such as MODIS, OMI, VIIRS, SeaWiFS, SEVIRI) need to assume the aerosol microphysical properties (size distribution, spectral refractive index) as well as their vertical distribution, which means that the aerosol model is supposed to be representative of the whole region and for the whole biomass burning season. The single scattering albedo is the assumption which produces the largest uncertainty in the ACAOD retrieval.
(2) “aerosols only weakly impact polarized reflectances (e.g., Waquet et al., 2013b, Peers et al., 2015)”
Aerosols do impact the polarized reflectances. However, the algorithm based on polarized reflectances do consider that the impact of the aerosol absorption on the polarized signal can be neglected.
I would recommend the publication of this paper if these remarks are addressed.

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ED: Publish subject to minor revisions (review by editor) (04 Nov 2019) by Jérôme Riedi

AR by Samuel LeBlanc on behalf of the Authors (20 Nov 2019) Author's response

ED: Publish as is (21 Dec 2019) by Jérôme Riedi

AR by Samuel LeBlanc on behalf of the Authors (31 Dec 2019) Manuscript

Short summary

The southeast Atlantic during August–October experiences layers of smoke from biomass burning over marine stratocumulus clouds. Here we present the light attenuation of the smoke and its dependence in the spatial, vertical, and spectral domain through direct measurements from an airborne platform during September 2016. From our observations of this climatically important smoke, we found an average aerosol optical depth of 0.32 at 500 nm, slightly lower than comparative satellite measurements.