Conditions for super-adiabatic droplet growth after entrainment mixing
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...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-07-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/9421/2016/acp-16-9421-2016.pdf |
| Summary: | 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. |
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| ISSN: | 1680-7316 1680-7324 |