scholarly journals Progress on Predicting the Breadth of Droplet Size Distributions Observed in Small Cumuli

2011 ◽  
Vol 68 (12) ◽  
pp. 2921-2929 ◽  
Author(s):  
Jennifer L. Bewley ◽  
Sonia Lasher-Trapp
Keyword(s):  
Droplet Size ◽  
Cloud Condensation Nuclei ◽  
Droplet Diameter ◽  
Droplet Size Distribution ◽  
Trade Wind ◽  
Droplet Growth ◽  
Size Distributions ◽  
Model Resolution ◽  
Modeling Framework ◽  
Droplet Size Distributions

Abstract A modeling framework representing variations in droplet growth by condensation, resulting from different saturation histories experienced as a result of entrainment and mixing, is used to predict the breadth of droplet size distributions observed at different altitudes within trade wind cumuli observed on 10 December 2004 during the Rain in Cumulus over the Ocean (RICO) field campaign. The predicted droplet size distributions are as broad as those observed, contain similar numbers of droplets, and are generally in better agreement with the observations when some degree of inhomogeneous droplet evaporation is considered, allowing activation of newly entrained cloud condensation nuclei. The variability of the droplet growth histories, resulting primarily from entrainment, appears to explain the magnitude of the observed droplet size distribution widths, without representation of other broadening mechanisms. Additional work is needed, however, as the predicted mean droplet diameter is too large relative to the observations and likely results from the model resolution limiting dilution of the simulated cloud.

Time Evolution of the Droplet Size Distribution in Dropwise Condensation

2020 ◽  
Author(s):  
Maofei Mei ◽  
Feng Hu ◽  
Chong Han ◽  
Yan Sun ◽  
Dongdong Liu
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Power Law ◽  
Droplet Size Distribution ◽  
Droplet Growth ◽  
Size Distributions ◽  
Dropwise Condensation ◽  
Dominant Feature ◽  
Droplet Size Distributions ◽  
Instantaneous Distribution

Abstract Droplet growth processes during dropwise condensation are simulated with a help of computer. We focus on instantaneous and time-averaged characteristics of droplet size distributions. Based on simulation results, shift of a single peak from small to large size is a significant characteristic for the instantaneous distribution before the first departure. Once condensing surface was refreshed time and again by shedding droplets, then coexistence, shift and combination of multiple peaks is the dominant feature. This indicates that the instantaneous droplet size distribution highly depends on growth time and target area. The findings can explain why different distribution characteristics were reported in experiments. Different from the instantaneous distribution, time-averaged size distributions for coalesced droplets follow a power-law style due to a collaboration of coalescence events and re-nucleation behaviors. However, the size range for the power-law distributions were affected by nucleation density. This requires an appropriate usage of the empirical or fractal model to predict theoretically heat transfer rate of dropwise condensation. The present work provides a comprehensive understanding of the instantaneous and time-averaged characteristics of droplet size distributions.

Lagrangian stochastic microphysics at unresolved scales in turbulent cloud simulations

2020 ◽  
Author(s):  
Gustavo Abade ◽  
Marta Waclawczyk ◽  
Wojciech W. Grabowski ◽  
Hanna Pawlowska
Keyword(s):  
Droplet Size ◽  
Cloud Condensation Nuclei ◽  
Droplet Size Distribution ◽  
Cloud Microphysics ◽  
Cloud Droplets ◽  
Adiabatic Cooling ◽  
Subgrid Scales ◽  
The Mean ◽  
Microphysical Processes ◽  
Droplet Size Distributions

<p>Turbulent clouds are challenging to model and simulate due to uncertainties in microphysical processes occurring at unresolved subgrid scales (SGS). These processes include the transport of cloud particles, supersaturation fluctuations, turbulent mixing, and the resulting stochastic droplet activation and growth by condensation. In this work, we apply two different Lagrangian stochastic schemes to model SGS cloud microphysics. Collision and coalescence of droplets are not considered. Cloud droplets and unactivated cloud condensation nuclei (CCN) are described by Lagrangian particles (superdroplets). The first microphysical scheme directly models the supersaturation fluctuations experienced by each Lagrangian superdroplet as it moves with the air flow. Supersaturation fluctuations are driven by turbulent fluctuations of the droplet vertical velocity through the adiabatic cooling/warming effect. The second, more elaborate scheme uses both temperature and vapor mixing ratio as stochastic attributes attached to each superdroplet. It is based on the probability density function formalism that provides a consistent Eulerian-Lagrangian formulation of scalar transport in a turbulent flow. Both stochastic microphysical schemes are tested in a synthetic turbulent-like cloud flow that mimics a stratocumulus topped boundary layer. It is shown that SGS turbulence plays a key role in broadening the droplet-size distribution towards larger sizes. Also, the feedback on water vapor of stochastically activated droplets buffers the variations of the mean supersaturation driven the resolved transport. This extends the distance over which entrained CNN are activated inside the cloud layer and produces multimodal droplet-size distributions.</p>

Dynamics of Intermittent Sprays

1999 ◽  
Author(s):  
Badih A. Jawad
Keyword(s):  
Droplet Size ◽  
Droplet Size Distribution ◽  
Sampling Technique ◽  
Size Distributions ◽  
Ambient Conditions ◽  
Spray Formation ◽  
Fuel Droplets ◽  
Liquid Immersion ◽  
Particle Sizer ◽  
Droplet Size Distributions

Abstract It is considered that droplet size distribution changes with time and space, since diesel fuel sprays are found to be transient and intermittent due to variations in ambient pressures. Therefore the obscuration signal (extinction of light due to particle field) obtained from a particle sizer for a single injection of fuel over the whole region of spray is necessary to determine the spray characteristics. Previous studies dealing with sprays have observed fuel droplets by use of the sedimentation tower method or liquid immersion sampling technique. However, in these technique droplets are usually sampled after spray formation is complete. The completion time of spray formation appears to vary with ambient conditions, thus making spray measurements under transient conditions during injection difficult. It is the objective of this paper to shine some light on the dynamics of spray motion, leading to a better understanding of the droplet size distributions.

Impact of entrainment-mixing and turbulent fluctuations on droplet size distributions in a cumulus cloud: An investigation using Lagrangian microphysics with a sub-grid-scale model

2021 ◽  
Author(s):  
Kamal Kant Chandrakar ◽  
Wojciech W. Grabowski ◽  
Hugh Morrison ◽  
George H. Bryan
Keyword(s):  
Droplet Size ◽  
Cloud Droplet ◽  
Relative Dispersion ◽  
Size Distributions ◽  
Modeling Framework ◽  
Turbulent Fluctuations ◽  
Droplet Activation ◽  
Droplet Size Distributions ◽  
Entrainment Mixing ◽  
Grid Length

AbstractEntrainment-mixing and turbulent fluctuations critically impact cloud droplet size distributions (DSDs) in cumulus clouds. This problem is investigated via a new sophisticated modeling framework using the CM1 LES model and a Lagrangian cloud microphysics scheme – the “super-droplet method” (SDM) – coupled with sub-grid-scale (SGS) schemes for particle transport and supersaturation fluctuations. This modeling framework is used to simulate a cumulus congestus cloud. Average DSDs in different cloud regions show broadening from entrainment and secondary cloud droplet activation (activation above the cloud base). DSD width increases with increasing entrainment-induced dilution as expected from past work, except in the most diluted cloud regions. The new modeling framework with SGS transport and supersaturation fluctuations allows a more sophisticated treatment of secondary activation compared to previous studies. In these simulations, it contributes about 25%of the cloud droplet population and impacts DSDs in two contrastingways: narrowing in extremely diluted regions and broadening in relatively less diluted. SGS supersaturation fluctuations contribute significantly to an increase in DSD width via condensation growth and evaporation. Mixing of super-droplets from SGS velocity fluctuations also broadens DSDs. The relative dispersion (ratio of DSD dispersion and mean radius) negatively correlates with grid-scale vertical velocity in updrafts, but is positively correlated in downdrafts. The latter is from droplet activation driven by positive SGS supersaturation fluctuations in grid-mean subsaturated conditions. Finally, the sensitivity to model grid length is evaluated. The SGS schemes have greater influence as the grid length is increased, and they partially compensate for the reduced model resolution.

Spatial distributions of cloud droplet size distributions from cloudbow observations measured with specMACS

2021 ◽  
Author(s):  
Veronika Pörtge ◽  
Tobias Kölling ◽  
Tobias Zinner ◽  
Linda Forster ◽  
Claudia Emde ◽  
...  
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Cloud Droplet ◽  
Droplet Size Distribution ◽  
Wide Field ◽  
Size Distributions ◽  
Vertical Profiles ◽  
Camera System ◽  
Cloud Droplet Size Distribution ◽  
Droplet Size Distributions

<p>The evolution of clouds and their impact on weather and climate is closely related to the cloud droplet size distribution, which is often represented by two parameters: the cloud droplet effective radius (reff) and the effective variance (veff). The droplet radius (reff) determines the radiative effect of clouds on climate. The effective variance is a measure of the width of the size distribution which is, for instance, important to understand the formation of precipitation or entrainment and mixing processes. We present an airborne remote-sensing technique to determine reff and veff from high-resolution polarimetric imaging observations of the LMU cloud camera system specMACS.</p><p>Recently the spectral camera system has been upgraded by a wide-field polarization resolving RGB camera which was operated for the first time on the HALO aircraft during the EUREC<sup>4</sup>A campaign. The new polarimeter is ideally suited for observing the cloudbow - an optical phenomenon which forms by scattering of sunlight by liquid water cloud droplets at cloud top. The cloudbow is dominated by single scattering which has two implications: Its visibility is significantly enhanced in polarized measurements and its structure is sensitive to the cloud droplet size distribution at cloud top. This allows the retrieval of reff and veff by fitting the observed polarized cloudbow reflectances against a look-up table of pre-computed scattering phase functions.</p><p>The characteristics of the polarimeter are optimized for the measurement of the cloudbow. The wide field-of-view is key for observing the cloudbow (scattering angle 135° -165°) for a wide range of solar positions. Another advantage is the high spatial and temporal resolution which allows the study of small-scale variability of cloud microphysics at cloud top with a horizontal resolution of up to 20 m. Combining the polarimetric cloudbow technique with an existing stereographic retrieval of cloud geometry allows to derive vertical profiles of the droplet size distribution at cloud top. Observations of different EUREC<sup>4</sup>A cloud fields are used to demonstrate the retrieval technique and to present first spatial distributions and vertical profiles of cloud droplet size distributions.</p>

scholarly journals Technical note: Equilibrium droplet size distributions in a turbulent cloud chamber with uniform supersaturation

2020 ◽  
Vol 20 (13) ◽  
pp. 7895-7909
Author(s):  
Steven K. Krueger
Keyword(s):  
Droplet Size ◽  
Cloud Condensation Nuclei ◽  
Lower Boundary ◽  
Droplet Size Distribution ◽  
Terminal Velocity ◽  
Technical Note ◽  
Cloud Chamber ◽  
Droplet Radius ◽  
Droplet Growth ◽  
The Mean

Abstract. In a laboratory cloud chamber that is undergoing Rayleigh–Bénard convection, supersaturation is produced by isobaric mixing. When aerosols (cloud condensation nuclei) are injected into the chamber at a constant rate, and the rate of droplet activation is balanced by the rate of droplet loss, an equilibrium droplet size distribution (DSD) can be achieved. We derived analytic equilibrium DSDs and probability density functions (PDFs) of droplet radius and squared radius for conditions that could occur in such a turbulent cloud chamber when there is uniform supersaturation. We neglected the effects of droplet curvature and solute on the droplet growth rate. The loss rate due to fallout that we used assumes that (1) the droplets are well-mixed by turbulence, (2) when a droplet becomes sufficiently close to the lower boundary, the droplet's terminal velocity determines its probability of fallout per unit time, and (3) a droplet's terminal velocity follows Stokes' law (so it is proportional to its radius squared). Given the chamber height, the analytic PDF is determined by the mean supersaturation alone. From the expression for the PDF of the radius, we obtained analytic expressions for the first five moments of the radius, including moments for truncated DSDs. We used statistics from a set of measured DSDs to check for consistency with the analytic PDF. We found consistency between the theoretical and measured moments, but only when the truncation radius of the measured DSDs was taken into account. This consistency allows us to infer the mean supersaturations that would produce the measured PDFs in the absence of supersaturation fluctuations. We found that accounting for the truncation radius of the measured DSDs is particularly important when comparing the theoretical and measured relative dispersions of the droplet radius. We also included some additional quantities derived from the analytic DSD: droplet sedimentation flux, precipitation flux, and condensation rate.

scholarly journals Fog Classification by Their Droplet Size Distributions: Application to the Characterization of Cerema’s Platform

Atmosphere ◽  
2020 ◽  
Vol 11 (6) ◽  
pp. 596 ◽  
Author(s):  
Pierre Duthon ◽  
Michèle Colomb ◽  
Frédéric Bernardin
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Droplet Size Distribution ◽  
Transportation Systems ◽  
Distribution Range ◽  
Size Distributions ◽  
Atmospheric Conditions ◽  
Different Types ◽  
Radiation Fogs ◽  
Droplet Size Distributions

Fog is one of major challenges for transportation systems. The automation of the latter is based on perception sensors that can be disrupted by atmospheric conditions. As fog conditions are random and non-reproducible in nature, Cerema has designed a platform to generate fog and rain on demand. Two types of artificial fog with different droplet size distributions are generated: they correspond to radiation fogs with small and medium droplets. This study presents an original method for classifying these different types of fog in a descriptive and quantitative way. It uses a new fog classification coefficient based on a principal component analysis, which measures the ability of a pair of droplet size distribution descriptors to differentiate between the two different types of fog. This method is applied to a database containing more than 12,000 droplet size distributions collected within the platform. It makes it possible to show: (1) that the two types of fog proposed by Cerema have significantly different droplet size distributions, for meteorological visibility values from 10 m to 1000 m; (2) that the proposed droplet size distribution range is included in the natural droplet size distribution range; (3) that the proposed droplet size distribution range should be extended in particular with larger droplets. Finally, the proposed method makes it possible to compare the different fog droplet size distribution descriptors proposed in the literature.

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