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Discussion papers
https://doi.org/10.5194/acp-2018-1333
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1333
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Jan 2019

Research article | 10 Jan 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Observation of absorbing aerosols above clouds over the South- East Atlantic Ocean from the geostationary satellite SEVIRI – Part 1: Method description and sensitivity

Fanny Peers1, Peter Francis2, Cathryn Fox2, Steven J. Abel2, Kate Szpek2, Michael I. Cotterell1,2, Nicholas W. Davies1,2, Justin M. Langridge2, Kerry G. Meyer3, Steven E. Platnick3, and Jim M. Haywood1,2 Fanny Peers et al.
  • 1College of Engineering, Mathematics, and Physical Sciences, University of Exeter, Exeter, UK
  • 2Met Office, Exeter, UK
  • 3NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

Abstract. High temporal resolution observations from satellites have a great potential for studying the impact of biomass burning aerosols and clouds over the South East Atlantic Ocean (SEAO). This paper presents a method developed to retrieve simultaneously aerosol and cloud properties in aerosol above cloud conditions from the geostationary instrument Meteosat Second Generation/Spinning Enhanced Visible and Infrared Imager (MSG/SEVIRI). The above-cloud Aerosol Optical Thickness (AOT), the Cloud Optical Thickness (COT) and the Cloud droplet Effective Radius (CER) are derived from the spectral contrast and the magnitude of the signal measured in three channels in the visible to shortwave infrared region. The impact of the absorption from atmospheric gases on the satellite signal is corrected by applying transmittances calculated using the water vapour profiles from a Met Office forecast model. The sensitivity analysis shows that a 10 % error on the humidity profile leads to an 18.5 % bias on the above-cloud AOT, which highlights the importance of an accurate atmospheric correction scheme. In situ measurements from the CLARIFY-2017 airborne field campaign are used to constrain the aerosol size distribution and refractive index that is assumed for the aforementioned retrieval algorithm. The sensitivities in the retrieved AOT, COT and CER to the aerosol model assumptions are assessed. Although an uncertainty of 31.2 % is observed on the above-cloud AOT, the retrieval of the absorption AOT and both cloud properties is weakly sensitive to the aerosol model assumptions, with biases lower than 7 % and 3 % respectively. The stability of the retrieval over time is analysed. For observations outside of the backscattering glory region, the time-series of the aerosol and cloud properties are physically consistent, which confirms the ability of the retrieval to monitor the temporal evolution of aerosol above cloud events over the SEAO.

Fanny Peers et al.
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Short summary
The measurements from the geostationary satellite MSG/SEVIRI are used to retrieve the cloud and above-cloud aerosol properties over the South Atlantic. The technic relies on the spectral contrast and the magnitude of the signal in the visible to shortwave infrared region as well as on the atmospheric correction based on forecasted water vapour profiles. The sensitivity analysis and the stability of the retrieval over time show the great potential of the high temporal resolution observations.
The measurements from the geostationary satellite MSG/SEVIRI are used to retrieve the cloud and...
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