Impacts of Ice Cloud Particle Size Uncertainty on Radiation, and Precipitation and Climate Sensitivity and the Significance of Future Satellite-Based Constraints

Ice cloud particle size is important to determining ice cloud radiative effect and precipitating rate. However, there is a lack of accurate ice particle effective radius (R_ei) observation on the global scale and the parameterization of R_ei in climate models is poorly constrained. We conduct a modeling study to assess the sensitivity of climate simulations to R_ei. Perturbations to R_ei are represented in ice fall speed parameterization and radiation scheme, respectively, in NCAR CESM1 model with a slab ocean configuration. We show that an increase in ice fall speed due to a larger R_ei results in a longwave cooling dominating over a shortwave warming, a global mean surface temperature decrease, and precipitation suppression. Similar longwave and shortwave cloud radiative effect changes occur when R_ei is perturbed in the radiation scheme. Perturbing falling snow particle size (R_es) results in much smaller changes in the climate responses. We further show that varying R_ei and R_es by 50% to 200% relative to the control experiment can cause climate sensitivity to differ by +12.3% to −6.2%. A future mission under design with combined multi-frequency microwave radiometers and cloud radar can reduce the uncertainty ranges of R_ei and R_es from a factor of 2 to ±25%, which would help reducing the climate sensitivity uncertainty pertaining to ice cloud particle size by approximately 60%.