Measured Constraints on Cloud-Top Entrainment to Reduce Uncertainty of Non-Precipitating Stratocumulus Shortwave Radiative Forcing in the Southern Ocean

Abstract

Stratocumulus cloud top entrainment has a significant effect on cloud properties, but there are few observations quantifying its impact. Using explicit 0-D 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 nonprecipitating stratocumulus clouds. These measurement-derived estimates of entrainment can be used to validate and improve parameterizations of entrainment in Global Climate Models.Plain Language Summary Clouds over the ocean have a significant impact on climate because they reflect sunlight that would otherwise be absorbed by the ocean. Understanding and accurately modeling how much sunlight these clouds reflect is important to understand feedbacks between clouds and climate. Using a simple model and cloud measurements, mixing of moist cloud air with warm-dry air from above the cloud was shown to decrease the cloud droplet number concentration and consequently total liquid water, which significantly decreases the overall amount of sunlight reflected by the cloud. Cloud droplets form onto small particles as they enter the base of the cloud through an updraft; in the meantime, warm-dry air from above the cloud mixes downward. In this study, the warm-dry air from above the cloud was shown to be more influential on reflecting sunlight than the concentration of particles and the updraft velocity at cloud base. These results emphasize the importance of accurately accounting for the mixing in of warm-dry air from above the cloud in climate models and can be constrained by existing measurements that are readily available on weather balloons and aircraft.