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) 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 het and hom. 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) of cirrus cloud ice particle number concentration, effective diameter (D_e), ice water content, shortwave extinction coefficient (α_ext), optical depth (τ), and cloud radiative temperature based on Imaging Infrared Radiometer (IIR) and CALIOP (Cloud and Aerosol Lidar with Orthogonal Polarization) co-located observations onboard CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation). Transition from het only to hom affected regimes are identified using α_ext and D_e. 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 liquid origin cirrus. Using τ distributions to establish a proxy for cloud net 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. Moreover, the climate intervention method known as cirrus cloud thinning could be an effective cooling method at high latitudes based on this τ-weighted hom fraction. A conceptual model of cirrus cloud characterization is proposed from these retrievals.