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Droplet growth in warm turbulent clouds
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Dr Benjamin Devenish
Dr Rutgerus Ijzermans
Emeritus Professor Mike Reeks
Devenish BJ, Bartello P, Brenguier J-L, Collins LR, Grabowski WW, Ijzermans RHA, Malinowski SP, Reeks MW, Vassilicos JC, Wang L-P, Warhaft Z
Quarterly Journal of the Royal Meteorological Society
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In this survey we consider the impact of turbulence on cloud formation from the cloud scale to the droplet scale. We assess progress in understanding the effect of turbulence on the condensational and collisional growth of droplets and the effect of entrainment and mixing on the droplet spectrum. The increasing power of computers and better experimental and observational techniques allow for amuch more detailed study of these processes than was hitherto possible. However, much of the research necessarily remains idealized and we argue that it is those studies which include such fundamental characteristics of clouds such as droplet sedimentation and latent heating that are most relevant to clouds. Nevertheless, the large body of research over the last decade is beginning to allowtentative conclusions to be made. For example, it is unlikely that small-scale turbulent eddies (i.e. notthe energy-containing eddies) alone are responsible for broadening the droplet size spectrum during the initial stage of droplet growth due to condensation. It is likely, though, that small-scale turbulence plays a signiﬁcant role in the growth of droplets through collisions and coalescence. Moreover, it has been possible through detailed numerical simulations to assess the relative importance of differentprocesses to the turbulent collision kernel and how this varies in the parameter space that is important to clouds. The focus of research on the role of turbulence in condensational and collisional growth has tended to ignore the effect of entrainment and mixing and it is arguable that they play at least as an important a role in the evolution of the droplet spectrum.We consider the role of turbulence in the mixingof dry and cloudy air,methods of quantifying thismixing and the effect that it has on the droplet spectrum.
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