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Abstract

Over the last 1.5 years a CALIPSO satellite retrieval for cirrus clouds has been developed for number concentration Ni, effective diameter De and ice water content IWC. In general, these retrieved properties agree favorably with corresponding aircraft measurements from Krämer et al. (2020, ACP), but also show evidence that cirrus clouds form in two ways; (1) primarily through heterogeneous ice nucleation (het) only (henceforth het cirrus) and (2) through het and homogeneous ice nucleation (hom) combined (henceforth hom cirrus). Het cirrus represent 55% to 80% of all sampled cirrus (depending on 30° latitude band and appear to be most sampled during field campaigns) and have visible extinction coefficients αext < 0.3 km-1. Hom cirrus have αext > 0.3 km-1, and outside the tropics, are responsible for 72% to 81% of the estimated net cloud radiative effect for all cirrus clouds (depending on latitude band). They are often associated with warm fronts and orographic gravity waves. Using a simple model based on hom theory, the model-predicted Ni, De, and IWC are found to agree well with CALIPSO retrievals of these cloud properties that, based on retrieved Ni, correspond to hom, thus corroborating the role of hom. These results indicate that the climate intervention method known as cirrus cloud thinning (CCT) could be effective during Polar Night (i.e., winter, when Arctic Amplification is strongest). Moreover, hom cirrus in climate models may be underestimated due to the uniform treatment of pre-existing ice over prescribed atmospheric layers (i.e., ice nucleation is near cloud top).