Advances in CALIPSO (IIR) cirrus cloud property retrievals – Part 2: Global estimates of the fraction of cirrus clouds affected by homogeneous ice nucleation

Abstract. Cirrus clouds can form through two ice nucleation pathways (homo- and heterogeneous ice nucleation; henceforth hom and het, respectively) that result in very different cloud physical and radiative properties. While important to the climate system, they are poorly understood due to lack of knowledge on the relative roles of hom and het. This study differs from earlier relevant studies by estimating the relative radiative contribution of hom-affected cirrus clouds. Here, we employ new global retrievals (described in Part 1: Mitchell et al., 2025; henceforth M2025) of cirrus cloud ice particle number concentration, effective diameter (De), ice water content (IWC), shortwave extinction coefficient (αext), optical depth (τ) and cloud radiative temperature based on Imaging Infrared Radiometer (IIR) and Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP) co-located observations onboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). Transition from het dominated to hom-affected regimes are identified using αext and De. Over oceans outside the tropics in winter, the zonal fraction of hom-affected cirrus generally ranges between 20 % and 35 %, with comparable contributions from in situ and warm base cirrus. Using τ distributions to establish a proxy for net cloud radiative effect (CRE), the τ-weighted fraction for hom-affected cirrus over oceans outside the tropics during winter was > 50 %, indicating that hom cirrus play an important role in climate. Using these retrievals (including those relating to the cloud geometric thickness), a conceptual model of cirrus cloud characterization is proposed.