Fine Structure of Cloud Droplet Concentration as Seen from the Fast-FSSP Measurements. Part I: Method of Analysis and Preliminary Results (original) (raw)
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Evidence for inertial droplet clustering in weakly turbulent clouds
Tellus B, 2007
A B S T R A C T Simultaneous observations of cloud droplet spatial statistics, cloud droplet size distribution and cloud turbulence were made during several cloud passages, including cumulus clouds and a stratus cloud. They provide evidence that inertial droplet clustering occurs even in weakly turbulent clouds. The measurements were made from the Airborne Cloud Turbulence Observation System suspended from a tethered balloon. For a profile through a stratus cloud with gradually changing droplet Stokes number, droplet clustering, quantified by the pair correlation function, is observed to be positively correlated with the droplet Stokes number. This implies that the droplet collision rate, which is relevant to drizzle formation via droplet coalescence, depends not only on the droplet size distribution, but also on the cloud turbulence. For cumulus clouds, the relation between droplet clustering and Stokes number seems more complicated. Stokes number is determined by measuring droplet size and local energy dissipation rate, the latter requiring highresolution air velocity measurements not possible on fast-flying aircraft.
Effects of in-cloud nucleation and turbulence on droplet spectrum formation in cumulus clouds
Quarterly Journal of the Royal Meteorological Society, 2002
Drop spectrum evolution is investigated using a moving mass grid microphysical cloud parcel model containing 2000 mass bins and allowing turbulent effects on droplet collisions. Utilization of precise methods of diffusion and collision drop growth eliminates any arti cial droplet spectrum broadening. Simulation of continental, intermediate and maritime clouds is conducted using different concentrations of cloud condensation nuclei and different vertical velocities at the cloud base.
Turbulence effects on droplet growth and size distribution in clouds—A review
Journal of Aerosol Science, 1997
The paper is focused on inertia effects among drops moving within a turbulent cloud on size distribution evolution and formation of rain. Two related mechanisms are discussed: (1) the occurrence of inertia-induced relative velocities between drops falling within a turbulent flow, and (2) the tendency of inertial drops to concentrate within certain areas of turbulent flow with a corresponding concentration decrease elsewhere. It is shown that these turbulence-induced mechanisms lead to a broadening of the droplet spectrum during the early stages of cloud formation. Turbulence also decreases the minimum size of droplets are able to collide with the smaller ones. Thus, turbulence seems to bridge drop growth by condensation and coagulation. Due to the non-linear nature of the kinetic equation of coalescence, effects of positive fluctuations of drop concentration dominate and lead to a faster droplet spectrum broadening. Turbulence effects for ice particles, especially ice crystals and snowflakes, are expected to be much more pronounced than those demonstrated for water drops, because of a smaller terminal fall velocity of ice particles and a comparably large mass. Large turbulent eddies can concentrate ice crystals within certain areas, where ice crystals concentration will be substantially greater than mean ice crystal concentration. Thus, turbulence can contribute to the formation of high ice crystal concentration observed in mixed-phase clouds. The investigation of turbulence effects in clouds drop evolution is a field currently undergoing very rapid development. Some unsolved problems are discussed.
Atmospheric Research, 2010
The role of turbulence in droplet growth in clouds is controversial, in part because of the difficulty of studying underlying processes in the cloud environment, and in part because of the difficulty of achieving real cloud conditions in controlled laboratory or computational studies. This paper is a synthesis of research on turbulence effects on cloud droplets that includes field and laboratory studies. Results from cloud measurements show that the turbulence exhibits similar internal intermittency to that observed in the laboratory, and in direct numerical simulations. We explore the consequences of this by relating measurements of droplet accelerations in the laboratory, to conditions observed in the clouds. We show that there is a strong likelihood of droplet accelerations in clouds exceeding the acceleration due to gravity. We discuss these observations in terms of the dynamics of droplets, including velocity statistics and clustering, and their influence on droplet growth.
Quarterly Journal of the Royal Meteorological Society, 1999
It is shown that the inertia of small droplets leads to the formation of drop velocity flux divergence, with the maximum of the spatial spectrum at scales from 1 cm to 2.5 cm for different values of the dissipation rate. The spacelength of correlation of the divergence field, calculated using the Batchelor model of isotropic and homogeneous turbulence, is also of centimetre scale. These results are interpreted in such a way that the drop inertia leads to the formation of the centimetre-scale structure of a cloud with 'spots' of enhanced and decreased drop concentration. Because of temporal and spatial changes of the turbulent flow structure, droplets appear alternately within the areas of positive or negative drop-flux velocity divergence and tend to leave the areas of the positive and enter the areas of negative drop-flux velocity divergence. Drop motions through the interface between two cloudy, or cloudy and clear-air, volumes lead to drop exchange between these volumes (inertial drop mixing). The characteristic time of this process was shown to be of the same order as the characteristic time-scales of molecular diffusion, droplet evaporation and gravitational sedimentation. Possible effects of the inertial mixing are illustrated using a simple model, in which a comparatively large air volume consists of many centimetre-scale volumes of different drop-flux divergence and ascent velocity within a cloud updraught. Drop exchange between the centimetre-scale volumes, together with the generation of supersaturation in the ascending volumes, leads to the evolution of droplet size spectra. The inertial mixing 'itself' leads to a weak droplet spectrum broadening in the case of uniform initial droplet concentration. The droplet spectrum broadening appears to be much more pronounced in cases of cloudy and clear-air volumes mixing, especially when fresh nucleation is assumed. It is shown that inertial mixing leads to the homogenization of the drop spectrum, i.e. to the formation of the same drop spectrum shape in all centimetrescale volumes. Thus, inertial mixing may be important for the formation of local droplet spectra. At the same time, the inertial mixing may lead to significant fluctuations of drop concentration in these volumes.
Droplet growth in warm turbulent clouds
Quarterly Journal of the Royal Meteorological Society, 2012
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 a much 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 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 allow tentative conclusions to be made. For example, it is unlikely that small‐scale turbulent eddies (i.e. not the energy‐containing eddies) alone are responsible for broadening the droplet size spectrum during the i...
Growth of Cloud Droplets in a Turbulent Environment
Annual Review of Fluid Mechanics, 2013
Motivated by the need to resolve the condensation-coalescence bottleneck in warm rain formation, a significant number of studies have emerged in the past 15 years concerning the growth of cloud droplets by water-vapor diffusion and by collision-coalescence in a turbulent environment. With regard to condensation, recent studies suggest that small-scale turbulence alone does not produce a significant broadening of the cloud-droplet spectrum because of the smearing of droplet-scale fluctuations by rapid turbulent and gravitational mixing. However, different diffusional-growth histories associated with large-eddy hopping could lead to a significant spectral broadening. In contrast, small-scale turbulence in cumulus clouds makes a significant contribution to the collision-coalescence of droplets, enhancing the collection kernel up to a factor of 5, especially for droplet pairs with a low gravitational collision rate. This moderate level of enhancement has a significant impact on warm rain initiation. The multiscale nature of turbulent cloud microphysical processes and open research issues are delineated throughout.
Effect of the droplet activation process on microphysical properties of warm clouds
Environmental Research Letters, 2010
This study investigates the effect of the droplet activation process on microphysical characteristics of warm clouds represented by correlation statistics between cloud droplet effective radius r e and cloud optical thickness τ c. The conceptual adiabatic model is first employed to interpret satellite-observed r e-τ c correlation statistics over two different regions and to reveal distinctively different increasing patterns of droplet number concentration between these regions. This difference is attributed to a different behavior of the droplet activation process induced by differing microphysical conditions of aerosols. Numerical experiments of changing aerosol size spectrum are then performed with a spectral bin microphysics cloud model. The results show that the slope of the size spectrum controls the r e-τ c relationship through its effect on increasing pattern of droplet number concentration due to the activation process. The simulated results are also found to reproduce the r e-τ c correlation statistics closely resembling those observed when the slope parameter and the aerosol amount are appropriately chosen. These results suggest that the r e-τ c correlation statistics observed by remote sensing studies contains a signature of how the droplet activation process takes place in the real clouds.
Scale-dependent droplet clustering in turbulent clouds
Journal of Fluid Mechanics, 2001
The current understanding of fundamental processes in atmospheric clouds, such as nucleation, droplet growth, and the onset of precipitation (collision–coalescence), is based on the assumption that droplets in undiluted clouds are distributed in space in a perfectly random manner, i.e. droplet positions are independently distributed with uniform probability. We have analysed data from a homogeneous cloud core to test this assumption and gain an understanding of the nature of droplet transport. This is done by examining one-dimensional cuts through clouds, using a theory originally developed for x-ray scattering by liquids, and obtaining statistics of droplet spacing. The data reveal droplet clustering even in cumulus cloud cores free of entrained ambient air. By relating the variance of droplet counts to the integral of the pair correlation function, we detect a systematic, scale-dependent clustering signature. The extracted signal evolves from sub- to super-Poissonian as the length...
Dispersion of droplet clouds in turbulence
Physical Review Letters, 2016
We measure the absolute dispersion of clouds of monodisperse, phosphorescent droplets in turbulent air by means of high-speed image-intensified video recordings. Laser excitation allows the initial preparation of well-defined, pencil-shaped luminous droplet clouds in a completely nonintrusive way. We find that the dispersion of the clouds is faster than the dispersion of fluid elements. We speculate that preferential concentration of inertial droplet clouds is responsible for the enhanced dispersion.