scholarly journals On the sensitivity of droplet size relative dispersion to warm cumulus cloud evolution

2012 ◽  
Vol 39 (13) ◽  
pp. n/a-n/a ◽  
Author(s):  
E. Tas ◽  
I. Koren ◽  
O. Altaratz
Keyword(s):  
Droplet Size ◽  
Relative Dispersion ◽  
Cumulus Cloud ◽  
Cloud Evolution

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.

scholarly journals Initiation of coalescence in a cumulus cloud: a beneficial influence of entrainment and mixing

2011 ◽  
Vol 11 (4) ◽  
pp. 10557-10613 ◽  
Author(s):  
W. A. Cooper ◽  
S. G. Lasher-Trapp ◽  
A. M. Blyth
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Cloud Condensation Nuclei ◽  
Droplet Size Distribution ◽  
Main Peak ◽  
Cumulus Cloud ◽  
Cumulus Clouds ◽  
Warm Rain ◽  
Short Time ◽  
Giant Nuclei

Abstract. Although rain has been observed to form in warm cumulus clouds within about twenty minutes, calculations that represent condensation and coalescence accurately in such clouds have had difficulty producing rainfall in such a short time except via processes involving giant cloud condensation nuclei (with diameters larger than 2 μm). This model-based study explores a different possible mechanism for accelerating the production of warm rain, one that depends on the variability in droplet trajectories arriving at a given location and time in a cumulus cloud. In the presence of entrainment such droplets experience different growth histories, and the result is broadening of the droplet size distribution. That broadening favours coalescence, leading to embryos that grow to raindrops. These calculations do lead to production of rain that is within the lower range of observations for clouds of Florida, USA, the location on which the input conditions were based. The process emphasized in this study, the formation of drizzle via collisions among droplets in the main peak of the droplet size distribution, complements the growth of precipitation on giant nuclei, which is also an important source of the first rain in the case studied. The results indicate that the mechanism developed here should be considered an important influence on the formation of rain in warm clouds.

scholarly journals The relative dispersion of cloud droplets: its robustness with respect to key cloud properties

2014 ◽  
Vol 14 (8) ◽  
pp. 11153-11176
Author(s):  
E. Tas ◽  
A. Teller ◽  
O. Altaratz ◽  
D. Axisa ◽  
R. Bruintjes ◽  
...  
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Thermodynamic Characteristics ◽  
Cloud Droplet ◽  
Droplet Size Distribution ◽  
Liquid Water Content ◽  
Relative Dispersion ◽  
Distribution Width ◽  
Airborne Measurements ◽  
Cloud Droplet Size Distribution

Abstract. The relative dispersion (ε) of cloud droplet size distribution, defined as the ratio between cloud droplet size distribution width (σ) and cloud droplet average radius (⟨r⟩), is investigated using airborne measurements of warm cumulus clouds. The data is used to study the relation of ε with microphysical and thermodynamic characteristics of the clouds. The results show that ε is constrained with average values in the range of ~0.25–0.35. It is shown that ε is not correlated with the cloud droplet concentration or with the Liquid Water Content (LWC). However, the relative dispersion variance (related to the third moment of the droplets distribution) shows sensitivity to the droplets' concentration and LWC, suggesting smaller ε variability in more adiabatic regions in the clouds. A clear criterion for the usage of the in situ airborne measurements data for statistical moments' calculations is suggested.

scholarly journals Influence of Turbulent Fluctuations on Cloud Droplet Size Dispersion and Aerosol Indirect Effects

2018 ◽  
Vol 75 (9) ◽  
pp. 3191-3209 ◽  
Author(s):  
K. K. Chandrakar ◽  
W. Cantrell ◽  
R. A. Shaw
Keyword(s):  
Droplet Size ◽  
Indirect Effects ◽  
Stochastic Theory ◽  
Cloud Droplet ◽  
Droplet Size Distribution ◽  
Cloud Chamber ◽  
Relative Dispersion ◽  
Number Density ◽  
Aerosol Indirect Effects ◽  
Removal Time

Abstract Cloud droplet relative dispersion, defined as the standard deviation over the mean cloud droplet size, is of central importance in determining and understanding aerosol indirect effects. In recent work, it was found that cloud droplet size distributions become broader as a result of supersaturation variability and that the sensitivity of this effect is inversely related to cloud droplet number density. The subject is investigated in further detail using an extensive dataset from a laboratory cloud chamber capable of producing steady-state turbulence. An extended stochastic theory is found to successfully describe properties of the droplet size distribution, including an analytical expression for the relative dispersion. The latter is found to depend on the cloud droplet removal time, which in turn increases with the cloud droplet number density. The results show that relative dispersion decreases monotonically with increasing droplet number density, consistent with some recent atmospheric observations. Experiments spanning fast to slow microphysics regimes are reported. The observed dispersion is used to estimate time scales for autoconversion, demonstrating the important role of the turbulence-induced broadening effect on precipitation development. An initial effort is made to extend the stochastic theory to an atmospheric context with a steady updraft, for which autoconversion time is the controlling factor for droplet lifetime. As in the cloud chamber, relative dispersion is found to increase with decreasing cloud droplet number density.

scholarly journals The relative dispersion of cloud droplets: its robustness with respect to key cloud properties

2015 ◽  
Vol 15 (4) ◽  
pp. 2009-2017 ◽  
Author(s):  
E. Tas ◽  
A. Teller ◽  
O. Altaratz ◽  
D. Axisa ◽  
R. Bruintjes ◽  
...  
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Measurement Data ◽  
Thermodynamic Characteristics ◽  
Cloud Droplet ◽  
Droplet Size Distribution ◽  
Liquid Water Content ◽  
Relative Dispersion ◽  
Droplet Concentration ◽  
Cloud Droplet Size Distribution

Abstract. Flight data measured in warm convective clouds near Istanbul in June 2008 were used to investigate the relative dispersion of cloud droplet size distribution. The relative dispersion (ϵ), defined as the ratio between the standard deviation (σ) of the cloud droplet size distribution and cloud droplet average radius (⟨r⟩), is a key factor in regional and global models. The relationship between ε and the clouds' microphysical and thermodynamic characteristics is examined. The results show that ε is constrained with average values in the range of ~0.25–0.35. ε is shown not to be correlated with cloud droplet concentration or liquid water content (LWC). However, ε variance is shown to be sensitive to droplet concentration and LWC, suggesting smaller variability of ϵ in the clouds' most adiabatic regions. A criterion for use of in situ airborne measurement data for calculations of statistical moments (used in bulk microphysical schemes), based on the evaluation of ϵ, is suggested.

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