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Discussion papers
https://doi.org/10.5194/acp-2019-1176
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-1176
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 05 Mar 2020

Submitted as: research article | 05 Mar 2020

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This preprint is currently under review for the journal ACP.

Observation of absorbing aerosols above clouds over the South-East Atlantic Ocean from the geostationary satellite SEVIRI – Part 2: Comparison with MODIS and aircraft measurements from the CLARIFY-2017 field campaign

Fanny Peers1, Peter Francis2, Steven J. Abel2, Paul A. Barrett2, Keith N. Bower3, Michael I. Cotterell1,2,a, Ian Crawford3, Nicholas W. Davies1,2, Cathryn Fox2, Stuart Fox2, Justin M. Langridge2, Kerry G. Meyer4, Steven E. Platnick4, Kate Szpek2, and Jim M. Haywood1,2 Fanny Peers et al.
  • 1College of Engineering, Mathematics, and Physical Sciences, University of Exeter, Exeter, UK
  • 2Met Office, Fitzroy Road, Exeter, UK
  • 3Centre for Atmospheric Science, School of Earth and Environmental Science, University of Manchester, Manchester, UK
  • 4NASA GSFC, Maryland, USA
  • anow at: School of Chemistry,University of Bristol, Bristol, UK

Abstract. To evaluate the SEVIRI retrieval for aerosols above clouds presented in Part 1 of the companion paper, the algorithm is applied over the South East Atlantic Ocean during the CLARIFY-2017 field campaign period. The first step of our analysis compares the retrieved aerosol and cloud properties against equivalent products from the MODIS MOD06ACAERO retrieval (Meyer et al., 2015). While the correlation between the two satellite retrievals of the above-cloud Aerosol Optical Thickness (AOT) is good (R = 0.75), the AOT retrieved by SEVIRI is 16.5 % smaller than that obtained from the MODIS retrieval. This difference in AOT is attributed mainly to the more absorbing aerosol model assumed for the SEVIRI retrieval compared to MODIS. The underlying Cloud Optical Thickness (COT) derived from the two satellites are in good agreement (R = 0.90). The Cloud droplet Effective Radius (CER) retrieved by SEVIRI is consistently smaller than MODIS by ~2 µm, which is mainly caused by the use of different spectral bands of the satellite instruments. In the second part of our analysis, we compare the forecast water vapour profiles used for the SEVIRI atmospheric correction as well as the aforementioned aerosol and cloud products with in situ measurements made from the Facility for Airborne Atmospheric Measurements (FAAM) aircraft platform during the CLARIFY-2017 campaign. Around Ascension Island, the column water vapour used to correct the SEVIRI signal is overestimated by 3.1 mm in the forecast compared to that measured by dropsondes. However, the evidence suggests that the accuracy of the atmospheric correction improves closer to the African coast. Consistency is observed between the SEVIRI above-cloud AOT and in situ measurements (from cavity ring-down spectroscopy instruments) when the measured single scattering albedo is close to that assumed in the retrieval algorithm. On the other hand, the satellite retrieval overestimates the AOT when the assumed aerosol model is not absorbing enough. Consistency is also found between the cloud properties retrieved by SEVIRI and the CER measured by a cloud droplet probe and the liquid water path derived from a microwave radiometer. Despite the instrumental limitations of the geostationary satellite, the consistency obtained between SEVIRI, MODIS and the aircraft measurements demonstrates the ability of the retrieval in providing additional information on the temporal evolution of the aerosol properties above clouds.

Fanny Peers et al.

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Latest update: 01 Apr 2020
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Short summary
Satellite observations at high temporal resolution are a valuable asset to monitor the transport of biomass burning plumes and the cloud diurnal cycle in the South Atlantic, but they need to be validated. Cloud and above-cloud aerosol properties retrieved from SEVIRI are compared against MODIS and measurements from the CLARIFY-2017 campaign. While some systematic differences are observed between SEVIRI and MODIS, the overall agreement in the cloud and aerosol properties is very satisfactory.
Satellite observations at high temporal resolution are a valuable asset to monitor the transport...
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