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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 10, issue 12 | Copyright
Atmos. Meas. Tech., 10, 4659-4685, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 04 Dec 2017

Research article | 04 Dec 2017

The link between outgoing longwave radiation and the altitude at which a spaceborne lidar beam is fully attenuated

Thibault Vaillant de Guélis1, Hélène Chepfer1, Vincent Noel2, Rodrigo Guzman3, Philippe Dubuisson4, David M. Winker5, and Seiji Kato5 Thibault Vaillant de Guélis et al.
  • 1LMD/IPSL, Université Pierre et Marie Curie, Paris, France
  • 2Laboratoire d'Aérologie, CNRS, Toulouse, France
  • 3LMD/IPSL, CNRS, École polytechnique, Palaiseau, France
  • 4Laboratoire d'Optique Atmosphérique, Université Lille, Lille, France
  • 5NASA Langley Research Center, Hampton, Virginia, USA

Abstract. According to climate model simulations, the changing altitude of middle and high clouds is the dominant contributor to the positive global mean longwave cloud feedback. Nevertheless, the mechanisms of this longwave cloud altitude feedback and its magnitude have not yet been verified by observations. Accurate, stable, and long-term observations of a metric-characterizing cloud vertical distribution that are related to the longwave cloud radiative effect are needed to achieve a better understanding of the mechanism of longwave cloud altitude feedback. This study shows that the direct measurement of the altitude of atmospheric lidar opacity is a good candidate for the necessary observational metric. The opacity altitude is the level at which a spaceborne lidar beam is fully attenuated when probing an opaque cloud. By combining this altitude with the direct lidar measurement of the cloud-top altitude, we derive the effective radiative temperature of opaque clouds which linearly drives (as we will show) the outgoing longwave radiation. We find that, for an opaque cloud, a cloud temperature change of 1K modifies its cloud radiative effect by 2Wm−2. Similarly, the longwave cloud radiative effect of optically thin clouds can be derived from their top and base altitudes and an estimate of their emissivity. We show with radiative transfer simulations that these relationships hold true at single atmospheric column scale, on the scale of the Clouds and the Earth's Radiant Energy System (CERES) instantaneous footprint, and at monthly mean 2°× 2° scale. Opaque clouds cover 35% of the ice-free ocean and contribute to 73% of the global mean cloud radiative effect. Thin-cloud coverage is 36% and contributes 27% of the global mean cloud radiative effect. The link between outgoing longwave radiation and the altitude at which a spaceborne lidar beam is fully attenuated provides a simple formulation of the cloud radiative effect in the longwave domain and so helps us to understand the longwave cloud altitude feedback mechanism.

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