Traditionally, the diffusional growth of a cloud droplet population is calculated using values of the environmental conditions that represent averages over large volumes, the so called macroscopic conditions (Srivastava 1989). However, it is apparent that the growth rate of an individual droplet is a function of the temperature and the vapor pressure in its immediate environment. These quantities vary from droplet to droplet and with time in a turbulent medium such as a cumulus cloud. In most theoretical and numerical studies of clouds, the hypothesis is made that these variations are unimportant when calculating the growth of an ensemble of droplets. The objective of this work is to determine the validity of this hypothesis. In order to do so we use a 3D turbulence model coupled with a cloud droplet growth model which solves for the trajectories and growth of several tens of thousands of individual droplets as a function of their local conditions (microscopic approach). / A series of experiments with various initial size distributions were conducted using no turbulent flow conditions or one of three turbulent flows with increasing eddy dissipation rate. The results show that in the absence of any turbulent flow or sedimentation of droplets, the non-uniform distribution of cloud droplets in space results in significant variance of the distribution of the supersaturation perturbation over all droplets (DSP) and the distribution of the degree of growth (DDG), defined as the Lagrangian integral of the supersaturation perturbation along each droplet's trajectory. The variance of the DDG is directly responsible for the broadening of the microscopic size distribution relative to the macroscopic size distribution. However, in the presence of turbulence and sedimentation of droplets, the variance of the DSP is significantly reduced. Furthermore, the average, over all droplets, of the decorrelation time of the supersaturation perturbation decreases as a function of increasing level of turbulence. Consequently, the variance of the DDG is significantly reduced compared to the no turbulence and no sedimentation experiments and furthermore, it decreases as a function of increasing level of turbulence. / We have found that for the typical levels of turbulence found in adiabatic cloud cores, the spatial distribution of the larger cloud droplets can significantly deviate from a Poisson distribution. The increasing preferential concentration as a function of increasing level of turbulence does contribute to an increase in the DSP as a function of increasing level of turbulence. However, the DDG decreases as a function of increasing level of turbulence. / These results are at odds with those in the idealized studies of Pinsky et al. (1996) and Shaw et al. (1998). These authors specified and maintained very significant preferential concentration artificially rather than obtaining it by solving explicitly the trajectories of the droplets. / Comparison of our results with the observations of Brenguier and Chaumat (096) made in adiabatic cloud cores lead us to the conclusion that the microscopic approach, even under the most favorable condition of no turbulence, produces too little broadening to explain the observations.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.35638 |
| Date | January 1998 |
| Creators | Vaillancourt, Paul. |
| Contributors | Yau, M. K. (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Atmospheric and Oceanic Sciences.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001610668, proquestno: NQ44614, Theses scanned by UMI/ProQuest. |
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