Additional global climate cooling by clouds due to ice crystal complexity

Emma Järvinen, Olivier Jourdan, David Neubauer, Bin Yao, Chao Liu, Meinrat O. Andreae, Ulrike Lohmann, Manfred Wendisch, Greg M. McFarquhar, Thomas Leisner, Martin Schnaiter
2018 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> Ice crystal submicron structures have a large impact on the optical properties of cirrus clouds and consequently on their radiative effect. Although there is growing evidence that atmospheric ice crystals are rarely pristine, direct in situ observations of the degree of ice crystal complexity are largely missing. Here we show a comprehensive in situ data set of ice crystal complexity coupled with measurements of the cloud angular scattering functions collected
more » ... ions collected during a number of observational airborne campaigns at diverse geographical locations. Our results demonstrate that an overwhelming fraction (between 61<span class="thinspace"></span>% and 81<span class="thinspace"></span>%) of atmospheric ice crystals sampled in the different regions contain mesoscopic deformations and, as a consequence, a similar flat and featureless angular scattering function is observed. A comparison between the measurements and a database of optical particle properties showed that severely roughened hexagonal aggregates optimally represent the measurements in the observed angular range. Based on this optical model, a new parameterization of the cloud bulk asymmetry factor was introduced and its effects were tested in a global climate model. The modelling results suggest that, due to ice crystal complexity, ice-containing clouds can induce an additional short-wave cooling effect of <span class="inline-formula">−1.12</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>2</sup></span> on the top-of-the-atmosphere radiative budget that has not yet been considered.</p>
doi:10.5194/acp-18-15767-2018 fatcat:kl7etvhx5zew7pkgpkkxn3hjee