A cloud-by-cloud approach for studying aerosol–cloud interaction in satellite observations

Alexandri, Fani; Müller, Felix; Choudhury, Goutam; Achtert, Peggy; Seelig, Torsten; Tesche, Matthias

The effective radiative forcing (ERF) due to aerosol–cloud interactions (ACIs) and rapid adjustments (ERFaci) still causes the largest uncertainty in the assessment of climate change. It is understood only with medium confidence and is studied primarily for warm clouds. Here, we present a novel cloud-by-cloud (Cinline-formula×C) approach for studying ACI in satellite observations that combines the concentration of cloud condensation nuclei (inline-formulanCCN) and ice nucleating particles (inline-formulanINP) from polar-orbiting lidar measurements with the development of the properties of individual clouds by tracking them in geostationary observations. We present a step-by-step description for obtaining matched aerosol–cloud cases. The application to satellite observations over central Europe and northern Africa during 2014, together with rigorous quality assurance, leads to 399 liquid-only clouds and 95 ice-containing clouds that can be matched to surrounding inline-formulanCCN and inline-formulanINP respectively at cloud level. We use this initial data set for assessing the impact of changes in cloud-relevant aerosol concentrations on the cloud droplet number concentration (inline-formulaNd) and effective radius (inline-formulareff) of liquid clouds and the phase of clouds in the regime of heterogeneous ice formation. We find a inline-formula M9inlinescrollmathml normal Δ ln⁡ N d / normal Δ ln⁡ n CCN 83pt14ptsvg-formulamathimge61e700e0197fd86dd9db654b2c3fa30 amt-17-1739-2024-ie00001.svg83pt14ptamt-17-1739-2024-ie00001.png of 0.13 to 0.30, which is at the lower end of commonly inferred values of 0.3 to 0.8. The inline-formula M10inlinescrollmathml normal Δ ln⁡ r eff / normal Δ ln⁡ n CCN 83pt14ptsvg-formulamathimgab65cdac2a3a80af6fd45251483c82d2 amt-17-1739-2024-ie00002.svg83pt14ptamt-17-1739-2024-ie00002.png between inline-formula−0.09 and inline-formula−0.21 suggests that inline-formulareff decreases by inline-formula−0.81 to inline-formula−3.78 nm per increase in inline-formulanCCN of 1 cminline-formula−3. We also find a tendency towards more cloud ice and more fully glaciated clouds with increasing inline-formulanINP that cannot be explained by the increasingly lower cloud top temperature of supercooled-liquid, mixed-phase, and fully glaciated clouds alone. Applied to a larger number of observations, the Cinline-formula×C approach has the potential to enable the systematic investigation of warm and cold clouds. This marks a step change in the quantification of ERFaci from space.

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Alexandri, Fani / Müller, Felix / Choudhury, Goutam / et al: A cloud-by-cloud approach for studying aerosol–cloud interaction in satellite observations. 2024. Copernicus Publications.

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