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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/amt-2020-246
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-246
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  21 Jul 2020

21 Jul 2020

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A revised version of this preprint is currently under review for the journal AMT.

A new measurement approach for validating satellite-based above cloud aerosol optical depth

Charles K. Gatebe1,2, Hiren Jethva1,2, Ritesh Gautam3, Rajesh Poudyal2,4, and Tamas Várnai2,5 Charles K. Gatebe et al.
  • 1Universities Space Research Association (USRA), Columbia, MD, 21046, USA
  • 2NASA Goddard Space Flight Center, Greenbelt, Maryland, 20771, USA
  • 3Environmental Defense Fund, Washington, DC 20009, USA
  • 4Science Systems and Applications, Inc. (SSAI), Lanham, MD 20706, USA
  • 5University of Maryland, Baltimore County, Baltimore, MD 21250, USA

Abstract. The retrieval of aerosol parameters from passive satellite instruments in cloudy scenes is challenging partly because clouds and cloud-related processes may significantly modify aerosol optical depth (AOD) and particle size, a problem that is further compounded by the 3D radiative processes. Recent advances in retrieval algorithms such as the “color ratio” method which utilizes the measurements at a shorter (470 nm) and a longer (860 nm) wavelength have demonstrated the simultaneous derivation of AOD and cloud optical depth (COD) for scenes where absorbing aerosols are found to overlay low-level cloud decks. This study shows simultaneous retrievals of above-cloud aerosol optical depth (ACAOD) and aerosol-corrected cloud optical depth (COD) from airborne measurements of cloud-reflected and sky radiances using the color ratio method. These airborne measurements were taken over marine stratocumulus clouds with NASA's Cloud Absorption Radiometer (CAR) during SAFARI 2000 field campaign offshore of Namibia. The ACAOD is partitioned between the AOD below aircraft (AOD_cloudtop) and above aircraft AOD (AOD_sky). The results show good agreement between AOD_sky and sunphotometer measurements of the above aircraft AOD. The results also show that the use of aircraft-based sunphotometer measurements to validate satellite retrievals of the ACAOD is complicated by the lack of information on AOD below aircraft. Specifically, the CAR-retrieved AOD_cloudtop captures this “missing” aerosol layer caught between the aircraft and cloud top, which is required to quantify above cloud aerosol loading and effectively validate satellite retrievals. In addition, the study finds a strong anticorrelation between the AOD_cloudtop and COD for cases where COD < 10 and a weaker anticorrelation for COD > 10, which may be associated with the uncertainties in the color ratio method at lower AODs and CODs. The influence of 3D radiative effects on the retrievals is examined and the results show that at cloud troughs, 3D effects increase retrieved ACAOD by about 3–10 % and retrieved COD by about 25 %. The results show that the color ratio method has little sensitivity to 3D effects at overcast stratocumulus cloud decks. These results demonstrate a novel airborne measurement approach for assessing satellite retrievals of aerosols above clouds, thereby filling a major gap that exists in the global aerosol observations.

Charles K. Gatebe et al.

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

Charles K. Gatebe et al.

Data sets

CAR Safari BRDF Measurements L1 V2 Charles Gatebe, Michael King, and Rajesh Poudyal https://doi.org/10.5067/RAQCJ0SV90IE

Charles K. Gatebe et al.

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Short summary
The retrieval of aerosol parameters from passive satellite instruments in cloudy scenes is challenging, partly because clouds and cloud-related processes may significantly modify the aerosol properties and partly because of the 3D radiative effects. This study shows demonstrate a novel airborne measurement approach for assessing satellite retrievals of aerosols above clouds using SAFARI 2000 field data over ocean, thereby filling a major gap that exists in the global aerosol observations.
The retrieval of aerosol parameters from passive satellite instruments in cloudy scenes is...
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