Joint Seminar: The Surface Longwave Cloud Radiative Effect from Space Lidar Observations

Clouds warm the surface in the longwave (LW) by emitting LW radiations to it. This warming effect can be quantified through the surface LW cloud radiative effect (CRE). The surface LW CRE is estimated using a space radiometer CERES completed with radiometer observations on global long time scale (2005-2021) but has some bias over continents and icy surfaces (Kato et al., 2018). It is also estimated globally using the combination of radar, lidar and spaceborne radiometer but this 5 years dataset stops in 2011 when CloudSat radar experienced a battery anomaly that limited observations to daytime scenes (L’Ecuyer et al., 2019). To get a long time series (2006-2020) of the surface LW CRE more reliable over continent and icy surface than the existing passive remote sensing record, we propose in this study new estimates of the global surface LW CRE from space-based lidar observations only. Indeed, we show from 1D atmospheric column radiative transfer calculations, that surface LW CRE linearly decreases with the cloud altitude. These computations allow to establish simple relationships between the surface LW CRE, and five cloud properties well observed by the CALIPSO space lidar over all types of surfaces: the opaque cloud cover and altitude, and the thin cloud cover, altitude, and emissivity. We use these relationships to retrieve the LW CRE at global scale over the 2008-2020 period. We validate this new CRE product by comparing it to existing satellite data on both global averages and on instantaneous collocated data, and we also compared it to ground based observations. The global mean CALIPSO-based retrieval (27 W m-2) is 0.1 W m-2 larger than the one derived from CERES space radiometer. Over the 2008–2011 time period, CALIPSO-based retrieval (27.7 W m-2) is 0.7 W m-2 larger than the one derived from combined space radar, lidar, and radiometer observations. Our estimates show that globally, opaque clouds drive the surface LW CRE and warm the surface by 23.1 W m-2 where thin clouds contribute only by 4 W m-2. The simplicity of this new CALIPSO-derived retrieval  opens the possibility to directly attribute the time variation of the surface LW CRE to specific variations of a cloud property in future works.




13:30 h


Virtual Seminar


Assia Arouf


Diego Jiménez-de-la-Cuesta Otero

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