Climate models overestimate the radiative effect of thin Arctic ice clouds

Liquid-containing clouds have an important impact on Arctic winter climate because they suppress radiative cooling of the surface. Pure ice clouds have a much weaker effect on longwave radiation, and often permit substantial surface radiative cooling. Here, we show that climate models typically underestimate the difference in surface radiation under low-level liquid and ice clouds in the Arctic. The analysed models consistently overestimate the longwave radiative effect of thin ice clouds compared to ground-based observations from the MOSAiC expedition. This mismatch occurs despite realistic ice cloud effective radii in models, and thus cannot be due to errors in cloud properties. The model behaviour reflects the relationship between ice water path and cloud optical thickness that is at the core of commonly used ice optics parametrizations, but this relationship is inconsistent with MOSAiC observations. Ice optics parametrizations have been developed for cirrus clouds, and low-level Arctic ice clouds may be more heterogeneous, or have different ice habits, reducing their radiative impact compared to a cirrus cloud of the same ice water path and effective radius. Our results suggest that climate models understimate the surface warming effect of an increasing liquid fraction of cloud condensate in a warming Arctic.