Articles | Volume 16, issue 14
Atmos. Chem. Phys., 16, 9421–9433, 2016
Atmos. Chem. Phys., 16, 9421–9433, 2016

Research article 29 Jul 2016

Research article | 29 Jul 2016

Conditions for super-adiabatic droplet growth after entrainment mixing

Fan Yang1, Raymond Shaw1, and Huiwen Xue2 Fan Yang et al.
  • 1Atmospheric Sciences Program and Department of Physics, Michigan Technological University, Houghton, Michigan, USA
  • 2Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China

Abstract. Cloud droplet response to entrainment and mixing between a cloud and its environment is considered, accounting for subsequent droplet growth during adiabatic ascent following a mixing event. The vertical profile for liquid water mixing ratio after a mixing event is derived analytically, allowing the reduction to be predicted from the mixing fraction and from the temperature and humidity for both the cloud and environment. It is derived for the limit of homogeneous mixing. The expression leads to a critical height above the mixing level: at the critical height the cloud droplet radius is the same for both mixed and unmixed parcels, and the critical height is independent of the updraft velocity and mixing fraction. Cloud droplets in a mixed parcel are larger than in an unmixed parcel above the critical height, which we refer to as the “super-adiabatic” growth region. Analytical results are confirmed with a bin microphysics cloud model. Using the model, we explore the effects of updraft velocity, aerosol source in the environmental air, and polydisperse cloud droplets. Results show that the mixed parcel is more likely to reach the super-adiabatic growth region when the environmental air is humid and clean. It is also confirmed that the analytical predictions are matched by the volume-mean cloud droplet radius for polydisperse size distributions. The findings have implications for the origin of large cloud droplets that may contribute to onset of collision–coalescence in warm clouds.

Short summary
When dry air is mixed into a cloud, droplets evaporate. If the diluted cloud mixture continues to rise, the remaining droplets will grow. In this work we show theoretically and computationally that a critical height exists, above which the droplets in a mixed, diluted cloud volume become larger than those in an undiluted volume. An environment that is humid and aerosol free is most favorable for producing such large droplets, which may contribute to the onset of precipitation formation.
Final-revised paper