AbstractIce particle size is pivotal to determining ice cloud radiative effect and precipitating rate. However, there is a lack of accurate ice particle effective radius (Rei) observation on the global scale to constrain its representation in climate models. In support of future mission design, here we present a modeling assessment of the sensitivity of climate simulations to Rei and quantify the impact of the proposed mission concept on reducing the uncertainty in climate sensitivity. We perturb the parameters pertaining to ice fall speed parameter and Rei in radiation scheme, respectively, in National Center for Atmospheric Research CESM1 model with a slab ocean configuration. The model sensitivity experiments show that a settling velocity increase due to a larger Rei results in a longwave cooling dominating over a shortwave warming, a global mean surface temperature decrease, and precipitation suppression. A similar competition between longwave and shortwave cloud forcing changes also exists when perturbing Rei in the radiation scheme. Linearity generally holds for the climate response for Rei related parameters. When perturbing falling snow particle size (Res) in a similar way, we find much less sensitivity of climate responses. Our quadrupling CO2 experiments with different parameter settings reveal that Rei and Res can account for changes in climate sensitivity significantly from +12.3% to −6.2%. By reducing the uncertainty ranges of Rei and Res from a factor of 2 to ±25%, a future satellite mission under design is expected to improve the climate state simulations and reduce the climate sensitivity uncertainty pertaining to ice particle size by approximately 60%. Our results highlight the importance of better observational constraints on Rei by satellite missions.