Constraining a Radiative Transfer Model with Satellite Retrievals: Implications for Cirrus Cloud Thinning

Abstract. The complex mechanisms governing the formation of cirrus clouds pose significant challenges in the accurate simulation of cirrus clouds within climate models, leading to uncertainties in predicting the cirrus cloud response to aerosols and efficacy of cirrus cloud thinning (CCT), a climate intervention method. One issue is related to the relative contributions of homogeneous and heterogeneous ice nucleation. Recent satellite observations from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) suggest that cirrus clouds strongly affected by homogeneous ice nucleation (i.e., homogeneous cirrus) play a more important role than previously assumed. We employ a radiative transfer model to quantify the cloud radiative effect for homogeneous and heterogeneous cirrus clouds at the top of atmosphere (TOA), Earth's surface, and within the atmosphere. The experiments are conducted using cirrus ice water content and effective diameter vertical profiles from CALIPSO retrievals for homogeneous and heterogeneous cirrus clouds across different regions (Arctic, Antarctic, and midlatitude) and surface types (ocean and land). Results indicate that homogeneous cirrus clouds exhibit stronger radiative effects than heterogeneous cirrus, implying that transitioning from homogeneous to heterogeneous cirrus, as an indicator of CCT efficacy, could induce substantial surface cooling, particularly in polar regions during winter. Estimated instantaneous surface cooling effects range from -0.7 to -1.0 W m^-2, with the TOA cooling reaching up to -1.6 W m^⁻2. This study highlights the need for improved representation of homogeneous cirrus in models to better predict the climatic impacts of cirrus clouds and to assess the CCT viability.