Evaluation of climate model aerosol seasonal and spatial variability over Africa using AERONET

Horowitz, Hannah M.; Garland, Rebecca M.; Thatcher, Marcus; Landman, Willem A.; Dedekind, Zane; van der Merwe, Jacobus; Engelbrecht, Francois A.

doi:https://doi.org/10.5194/acp-17-13999-2017

Articles | Volume 17, issue 22

https://doi.org/10.5194/acp-17-13999-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-17-13999-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 17, issue 22

Research article

|

24 Nov 2017

Research article |

| 24 Nov 2017

Evaluation of climate model aerosol seasonal and spatial variability over Africa using AERONET

Hannah M. Horowitz, Rebecca M. Garland, Marcus Thatcher, Willem A. Landman, Zane Dedekind, Jacobus van der Merwe, and Francois A. Engelbrecht

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Final revised paper (published on 24 Nov 2017)
Preprint (discussion started on 11 May 2017)

Interactive discussion

Status: closed

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

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RC1: 'This is a more a model evaluation paper than a paper informing on aerosol climatology', Anonymous Referee #1, 23 May 2017
- AC1: 'Response to reviewer 1', Hannah Marie Horowitz, 12 Sep 2017
RC2: 'Referee comment', Anonymous Referee #2, 29 May 2017
- AC2: 'Response to reviewer 2', Hannah Marie Horowitz, 12 Sep 2017

Peer-review completion

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

AR by Hannah Marie Horowitz on behalf of the Authors (12 Sep 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (12 Sep 2017) by Kostas Tsigaridis

RR by Anonymous Referee #1 (13 Sep 2017)

Suggestions for revision or reasons for rejection

I reviewed a previous version of this paper. In general, the authors have addressed my concerns, and I commend them for their efforts in the revision. I have a small number of remaining comments, but after that feel the paper can be accepted for publication in ACP. I am happy to read a future revision if the Editor would like, but I don’t feel that it is a necessity. The comments mostly relate to the presentation of some of the Figures, where what is shown is not clear from what is written.

General: I had suggested adding an evaluation of model Ångström exponent to the paper. The authors respond that the model does not provide this, only the AOD at 550 nm. That’s fine, but I suggest noting this clearly somewhere up front (like at the end of the introduction and/or the start of section 2.3). Otherwise a reader may wonder if you chose to ‘hide’ model Ångström exponent or something.

Page 3 line 20 and page 5, line 25: the authors cite both Holben et al (1998) and Dubovik et al (2002) for the AERONET network here. The Holben paper is the correct reference for the network as a whole. Dubovik presents some climatological results for AERONET’s aerosol inversions, which are not the data set the authors use (they use the direct Sun data). So this is not really relevant. I suggest the authors cite only Holben et al (1998) in these two places.

Page 7, lines 18-19: “The Ångström exponent values presented here are based on aerosol extinction.” I suggest deleting this sentence. It seems out of place, and anyway this is the standard definition which was already given in Equation 2.

Figures 3 and 4: I understand now that this is AERONET data, not model simulations. If this is the seasonal cycle and multi-annual means and standard deviations, then why are there standard deviations for sites and months where only 1 year contributed? For example in KAUST there are data for March-June, the numbers above the points indicate 1 year contributed to each month in the seasonal cycle, but there are large standard deviations. If there is only 1 year contributing then the standard deviation can’t be calculated, by its definition, because you only have 1 data point going into the mean. Is this an error or is this not in fact the mean seasonal cycle? Perhaps it is calculated as a mean of instantaneous data or of daily means instead? In that case the standard deviations would include both day to day and year to year variability (which is not what is implied). Either the plot needs correcting or the caption updating to reflect what is shown. My preference would be to take the AERONET daily product and average it to monthly means. Then, calculate the annual cycle (multi-year mean and year-to-year standard deviation) based on the monthly mean values. In this case sites like KAUST will end up without standard deviations, for that reason. But the current presentation of the figure is not consistent with what it says. Or have I totally misunderstood?

Figure 5: this appears before Figures 3 and 4 in the manuscript file uploaded (acp-2017-250-manuscript-version5.pdf).

Figure 6: the same comment about averaging for Figures 3 and 4 may apply here, it is not clear from the description.

Figure 9: it would be good to have the top and bottom rows on the same scale (top is 0-1.5 now, bottom is 0-1) to more easily compare the two time periods. Maybe putting both on the range 0-1.2 is a good compromise?

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ED: Reconsider after minor revisions (Editor review) (13 Sep 2017) by Kostas Tsigaridis

AR by Hannah Marie Horowitz on behalf of the Authors (23 Sep 2017) Author's response Manuscript

ED: Publish as is (28 Sep 2017) by Kostas Tsigaridis

AR by Hannah Marie Horowitz on behalf of the Authors (04 Oct 2017)

Short summary

Africa is a major source of particles (or aerosols) from dust and fires, which impact climate. Models used to predict impacts of future climate change have not been well tested for aerosols over Africa. In this study we evaluate aerosols in the CCAM climate model against observations across Africa and surrounding regions. We find the model generally captures observed variability but overestimates dust in northern Africa, which has implications for its representation of climate feedbacks.