Improving the Representation of Cloud Entrainment Mixing in Models
Accurate representation of cloud processes is critical for understanding and simulating climate and cloud-climate feedbacks. One process that appears to play a critical role in cloud evolution, but which is not well understood or simulated in models, involves the entrainment of dry surrounding air into a cloud. It is not well known to what extent entrainment has a uniform effect on the cloud droplets (homogeneous mixing), so that all droplets evaporate at a similar rate, or whether some drops shrink much more than others (inhomogeneous mixing). The different behaviors would have a significant influence on subsequent cloud microphysics (such as a cloud’s ability to rain) and on radiative effects. Recent work examining these cloud behaviors is reported by researchers at Brookhaven National Laboratory who used the Atmospheric Radiation Measurement Climate Research Facility Southern Great Plains site during the March 2000 Cloud Intensive Observation Period. Data were analyzed from 16 non-drizzling flight legs in five warm continental stratocumulus clouds. The data indicated that inhomogeneous entrainment-mixing processes occurred more often than the homogeneous entrainment-mixing mechanism. The researchers derived a more robust characterization of entrainment-mixing processes, including a probabilistic description using a dimensionless number that indicates the degree of homogeneous versus inhomogeneous mixing. The authors argue that the common wisdom of classifying entrainment-mixing processes into several distinct types appears oversimplified. Rather, the derivation of a mechanism continuum over these types is desirable but challenging. This new study provides an important first step in that direction.
Lu, C., Y. Liu, and S. Niu. 2011. “Examination of Turbulent Entrainment-Mixing Mechanisms Using a Combined Approach,” Journal of Geophysical Research: Atmospheres 116, D20207, DOI: 10.1029/2011JD015944.