Improved Understanding of Relationship Between Cloud Mixing and Cloud Droplets


Mixing or “entrainment” of dry air into clouds significantly affects cloud properties. Entrainment rate is one of the strongest controls on how clouds respond to climate change, yet the relationship between entrainment rate and cloud droplet characteristics or “microphysics” remains largely unexplored. To fill this knowledge gap, U.S. Department of Energy scientists at Brookhaven National Laboratory (BNL) examined the relationships between entrainment rate and key microphysical quantities in shallow cumuli clouds collected during the Routine AAF [Atmospheric Radiation Measurement (ARM) Aerial Facility] Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign. The entrainment rate is derived from a new approach also developed at BNL. The team found that an increase in mixing or entrainment rate leads to an increase in the range of droplet sizes, with overall decreases in liquid water content, droplet concentration, and mean droplet size, as well as a tendency to not drizzle. The results provide new understanding of the relationships between entrainment, mixing, and cloud microphysics and will be important for deriving physical representation of these properties in climate models.


Lu, C., Y. Liu, S. Niu, and A. M. Vogelmann. 2013. “Empirical Relationship Between Entrainment Rate and Microphysics in Cumulus Clouds,” Geophysical Research Letters 40(10), 2333–38. DOI: 10.1002/grl.50445.