Stratocumulus cloud-top entrainment has a significant effect on cloud properties, but there are few observations quantifying its impact. Using explicit 0D parcel model simulations, initialized with below-cloud in-situ measurements, and validated with in-situ measurements of cloud properties, the shortwave cloud radiative forcing (SWCF) was reduced by up to 100 W m^-2 by cloud-top entrainment in the Southern Ocean. The impact of entrainment-corrected SWCF is between 2 and 20 times that of changes in the aerosol particle concentration or updraft at cloud base. The variability in entrainment-corrected SWCF accounts for up to 50 W m^-2 uncertainty in estimating cloud forcing. Measurements necessary for estimating the impact of entrainment on cloud properties can be constrained from existing airborne platforms and provide a first-order approximation for cloud radiative properties of non-precipitating stratocumulus clouds. These measurement-derived estimates of entrainment can be used to validate and improve parameterizations of entrainment in Global Climate Models.