Highlights What are the main findings? Ice clouds at both sites exhibit pronounced diurnal and seasonal variability, with substantial regional differences in their physical characteristics and radiative impacts, as revealed by long-term, high-resolution radar observations. Longwave radiation dominates the net radiative forcing of ice clouds at the top of the atmosphere at both sites, leading to atmospheric heating within and beneath cloud layers, while ice clouds substantially reduce surface net radiation, resulting in an overall cooling effect. What are the implications of the main findings? Ice cloud radiative effects show strong diurnal variability driven by daily changes in cloud occurrence and physical properties, highlighting a critical but previously underexplored diurnal influence on cloud-radiation interactions. This study characterizes midlatitude ice cloud physical and radiative properties across multiple timescales, providing observational constraints for improving ice cloud parameterizations and more accurately assessing cloud-radiation interactions in midlatitude climate systems.Highlights What are the main findings? Ice clouds at both sites exhibit pronounced diurnal and seasonal variability, with substantial regional differences in their physical characteristics and radiative impacts, as revealed by long-term, high-resolution radar observations. Longwave radiation dominates the net radiative forcing of ice clouds at the top of the atmosphere at both sites, leading to atmospheric heating within and beneath cloud layers, while ice clouds substantially reduce surface net radiation, resulting in an overall cooling effect. What are the implications of the main findings? Ice cloud radiative effects show strong diurnal variability driven by daily changes in cloud occurrence and physical properties, highlighting a critical but previously underexplored diurnal influence on cloud-radiation interactions. This study characterizes midlatitude ice cloud physical and radiative properties across multiple timescales, providing observational constraints for improving ice cloud parameterizations and more accurately assessing cloud-radiation interactions in midlatitude climate systems.Abstract Ice clouds play a significant role in the Earth's radiation balance due to their unique microphysical and radiative properties, which vary with formation mechanisms and regions and influence the local energy budget. In this study, six years of Ka-band Zenith Radar (KAZR) observations from the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) and the Southern Great Plains (SGP) sites, combined with the Fu-Liou radiative transfer model, were used to examine the macrophysical and microphysical properties of ice clouds, their radiative effects, and contributions to the surface energy budget. The results show that the frequency of ice cloud occurrence at SACOL is 40%, significantly higher than the 27% observed at SGP. At both sites, ice cloud altitudes exhibit an increasing trend in the context of recent warming, with a more pronounced increase at SGP. Seasonal variations are evident, with spring characterized by relatively thick and widespread ice clouds, while summer is dominated by high-altitude, optically thin clouds. Ice cloud occurrence peaks at night and decreases during the day at both sites; however, cloud diurnal variations in summer are much greater at SGP than at SACOL. Radiative analysis indicates that longwave radiation-induced warming dominates ice cloud radiative forcing. Net radiative forcing at the top of the atmosphere is 6.08 W/m2 at SACOL and 3.06 W/m2 at SGP, contributing to atmospheric heating within and beneath cloud layers. At the surface, sensible heat dominates the energy budget at SACOL (over 63%) due to its arid climate, whereas latent heat dominates at SGP (about 67%) because of abundant moisture; and ice clouds have the greatest impact in winter, reducing surface net radiation by 29% at SACOL and 26% at SGP, producing a cooling effect.