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Abstract:
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 dataset of ice crystal complexity coupled with measurements of the cloud asymmetry factor collected at diverse geographical locations. Our results demonstrate that an overwhelming fraction (between 61 and 81%) of atmospheric ice crystals in the different regions sampled contain submicron deformations and, as a consequence, a low asymmetry factor of 0.75 is observed. The measured cloud angular light scattering functions were parameterized in terms of the cloud bulk asymmetry factor and tested in a global climate model. The modelling results suggest that due to ice crystal complexity, ice clouds can induce an additional cooling effect of −1.12Wm−2 on the radiative budget that has not yet been considered.
