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Longwave radiative effect of the cloud twilight zone

Abstract

Clouds play a key role in Earth’s radiation budget, covering more than 50% of the planet. However, the binary delineation of cloudy and clear sky is not clearly defined due to the presence of a transitionary zone, known as the cloud twilight zone, consisting of liquid droplets and humidified to dry aerosols. The twilight zone is an inherent component of cloud fields, yet its influence on longwave-infrared radiation remains unknown. Here we analyse spectral data from global satellite observations of shallow cloud fields over the ocean to estimate a lower bound on the twilight zone’s effect on longwave radiation. We find that the average longwave radiative effect of the twilight zone is ~0.75 W m–2, which is equivalent to the radiative forcing from increasing atmospheric CO2 by 75 ppm. We also find that the twilight zone in the longwave occupies over 60% of the apparent clear sky within the analysed low-level cloud fields. As low-level clouds are relatively warm, the overall longwave radiative contribution from the twilight zone is likely to be higher. We suggest that the twilight zone needs to be accounted for to accurately quantify cloud radiative effects and close the global energy budget.

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Fig. 1: Clouds’ TLW.
Fig. 2: Twilight zone signal as a function of DFNC.
Fig. 3: Global distribution of samples.
Fig. 4: The TLW RE.
Fig. 5: Global analysis results.

Data availability

The MODIS level 2 products—cloud mask, cloud properties and level 1B raw data—are available from the Atmosphere Archive and Distribution System (LAADS) Distributed Active Archive Center (DAAC), https://ladsweb.modaps.eosdis.nasa.gov/. The MODIS sea surface temperature products of levels 2 and 3 are available from Ocean Color Web, https://oceancolor.gsfc.nasa.gov/Source data are provided with this paper.

Code availability

The radiation transfer codes are open access; SHDOM is available at http://coloradolinux.com/shdom/; SBDART is available at https://github.com/paulricchiazzi/SBDART.

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Acknowledgements

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (CloudCT, grant agreement No. 810370). A.B.K. is supported in part by NSF AGS-1639868.

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Contributions

E.E. and I.K. jointly conceived the principal idea. E.E. carried out the analysis. E.E., I.K., O.A., A.B.K. and A.R. discussed results and wrote the paper.

Corresponding author

Correspondence to Ilan Koren.

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Peer review information Primary Handling Editors: Tamara Goldin; Heike Langenberg; Tom Richardson.

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Supplementary information

Supplementary Information

Supplementary Figs 1–7, discussion and references.

Supplementary Data 1

Imaging Source Data for Supplementary Fig. 3.

Supplementary Data 2

Statistical Source Data for Supplementary Fig. 7.

Source data

Source Data Fig. 1

Imaging Source Data

Source Data Fig. 2

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Source Data Fig. 4

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Source Data Fig. 5

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Eytan, E., Koren, I., Altaratz, O. et al. Longwave radiative effect of the cloud twilight zone. Nat. Geosci. 13, 669–673 (2020). https://doi.org/10.1038/s41561-020-0636-8

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