The Influence of Entrainment and Mixing on the Initial Formation of Rain in a Warm Cumulus Cloud

The objective of this study is to address the problem of the production of rain in warm cumulus clouds that has been observed to occur within about 20 min. A hybrid model approach is used where a microphysical parcel model is run along trajectories produced by a 3D cloud model, with sufficiently hig...

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Bibliographic Details
Published in:Journal of the atmospheric sciences 2013-06, Vol.70 (6), p.1727-1743
Main Authors: COOPER, William A, LASHER-TRAPP, Sonia G, BLYTH, Alan M
Format: Article
Language:English
Subjects:
Clouds
Coalescence
Cumulus clouds
Droplets
Earth, ocean, space
Embryos
Entrainment
Exact sciences and technology
External geophysics
Floods
Meteorology
Moisture content
Physics of the high neutral atmosphere
Precipitation
Rain
Raindrops
Studies
Water content
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Summary:The objective of this study is to address the problem of the production of rain in warm cumulus clouds that has been observed to occur within about 20 min. A hybrid model approach is used where a microphysical parcel model is run along trajectories produced by a 3D cloud model, with sufficiently high resolution to allow explicit representation of the effects of entrainment and mixing. The model calculations take the next step from the previous study, which showed that entrainment and mixing can accelerate the diffusional growth of cloud droplets to the production of raindrops by collision and coalescence. The mechanism depends on the variability in droplet trajectories arriving at a given location and time in a cumulus cloud. The resulting broadening favors collisions among droplets in the main peak of the droplet size distribution, which leads to the production of raindrop embryos. However, this production and the subsequent growth of the embryos to become raindrops only occur in regions of relatively high cloud water content. The modeling framework allows an objective test of this sequence of events that explain the seemingly contradictory notions of the enhancement of cloud droplet growth as a result of entrainment and mixing and the need for substantial cloud water content for collision and coalescence growth. The results show that raindrops can be produced within 20 min in warm cumulus clouds. The rain produced is sensitive to giant aerosols, but modification of the modeling framework is required to conduct a more robust test of their relative importance.
ISSN:0022-4928
1520-0469
DOI:10.1175/JAS-D-12-0128.1