Analysis of CCCma Radiative Transfer Calculations for Single‐Layer Overcast Ice Clouds

Abstract Understanding interactions between incoming shortwave (SW) solar radiation and clouds is essential for quantifying and modeling Earth's Radiation Budget (ERB). Ice clouds are particularly problematic due to their wide variability in crystal habits, sizes, and shapes. In this study, data from NASA's Cloud and Earth Radiative Energy System (CERES) are used to identify single‐layer overcast ice clouds and calculate surface and top‐of‐atmosphere (TOA) SW fluxes using the Canadian Centre for Climate Modeling and Analysis (CCCma) Radiative Transfer Model (RTM). A total of 361 SW flux observations from 11 surface sites spanning different climatic regions, together with CERES SYN1deg satellite observations at the TOA, are used to evaluate the CCCma RTM's performance. The CCCma RTM exhibits mean bias errors (MBEs) of +3.7 W m −2 at the surface and +4.1 W m −2 at the TOA, with root mean square errors (RMSEs) of 72.7 and 33.2 W m −2 , respectively. Correspondingly, the CERES SYN1deg Fu‐Liou RTM shows MBEs of −12.1 and +18.5 W m −2 and RMSEs of 75.0 and 34.5 W m −2 for surface and TOA, respectively. MBE differences between the two RTMs are due to differing treatments of model physics, while their larger RMSEs at the surface result from both imprecise inputs and spatial variabilities of both inputs and surface observed flux. Plain Language Summary Understanding how clouds interact with incoming sunlight is crucial for studying how energy enters and leaves the climate system. Ice clouds, in particular, are challenging to study because their ice crystals vary widely in shape and size. In this work, ice clouds are identified using NASA's Clouds and the Earth's Radiant Energy System (CERES) data over 11 ground sites that measure sunlight intensity. Atmospheric radiative transfer models are used to calculate sunlight reaching the surface and how much is reflected back to space (top‐of‐atmosphere, TOA). To test the models' performances, their estimates are compared with ground site observations as well as satellite measurements. Given myriad uncertainties in both model input data and point measurements made at the surface, both models predict surface and TOA radiant fluxes satisfactorily well. Key Points 361 single‐layer overcast ice clouds were selected over 11 surface sites using NASA CERES SYN1deg cloud product Compared to the observations, the CCCma has mean biases of +3.7 W m −2 at the surface and +4.1 W m −2 at the TOA The large RTMs' RMSEs at the surface are due to input inaccuracies and spatial variability rather than model physics