Relationship Between Entrainment Rate and Microphysics in Cumulus Clouds
Entrainment of dry air into clouds significantly affects cloud properties. Although entrainment rate is one of the strongest controls on the climate sensitivity of climate models, the relationship between entrainment rate and cloud 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 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. They found that an increase in entrainment rate leads to an increase in relative dispersion but decreases in liquid water content, droplet concentration, and mean droplet radius. This relationship between entrainment rate and microphysics suggests spectra broadening toward small droplet sizes and the dominance of homogeneous entrainment mixing, a possible reason why none of these cumulus clouds were drizzling. This result also sheds light on developing a parameterization that links entrainment, mixing, and cloud microphysics.
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, 2333–38. DOI: 10.1002/grl.50445.