Terminal Velocity and Shape of Cloud and Precipitation Drops Aloft

Beard, Kenneth V.

doi:10.1175/1520-0469(1976)033<0851:tvasoc>2.0.co;2

Cited by 580 publications

(383 citation statements)

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“…While Stokes drag is appropriate for droplets less than 30 µm in radius, for larger droplets a nonlinear drag must be assumed since the droplet Reynolds number is O(1). A parametrization due to Beard (1976) includes Reynolds number effects and has been successfully used in cloud simulations. The effect of a nonlinear drag on droplet collisions and growth is discussed in section 4.2.2.…”

Section: Small-scale Turbulence and Cloud Microphysicsmentioning

confidence: 99%

Droplet growth in warm turbulent clouds

Devenish

Bartello

Brenguier

et al. 2012

Quart J Royal Meteoro Soc

245

281

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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 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 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 different processes 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 important a role in the evolution of the droplet spectrum. We consider the role of turbulence in the mixing of dry and cloudy air, methods of quantifying this mixing and the effect that it has on the droplet spectrum. Copyright

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Section: Small-scale Turbulence and Cloud Microphysicsmentioning

confidence: 99%

Droplet growth in warm turbulent clouds

Devenish

Bartello

Brenguier

et al. 2012

Quart J Royal Meteoro Soc

245

281

View full text Add to dashboard Cite

show abstract

“…Table 1 lists some commonly used empirical formulas for the terminal velocity of falling raindrops. Physically-based formulas usually divide the population of raindrops into several size ranges that correspond to different, physically-distinct flow regimes (e.g., Beard, 1976; Seinfeld and Pandis, 2006;Jacobson, 2005). Different formulas are employed in different ranges.…”

Section: Raindrop Terminal Velocity V (D P )mentioning

confidence: 99%

“…Different formulas are employed in different ranges. In the present study, we follow the theoretical formula of Beard (1976) …”

Section: Raindrop Terminal Velocity V (D P )mentioning

confidence: 99%

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Uncertainty assessment of current size-resolved parameterizations for below-cloud particle scavenging by rain

Wang¹,

2010

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Abstract. Current theoretical and empirical size-resolved parameterizations of the scavenging coefficient ( ), a parameter commonly used in aerosol transport models to describe below-cloud particle scavenging by rain, have been reviewed in detail and compared with available field and laboratory measurements. Use of different formulations for raindropparticle collection efficiency can cause uncertainties in sizeresolved values of one to two orders of magnitude for particles in the 0.01-3 µm diameter range. Use of different formulations of raindrop number size distribution can cause values to vary by a factor of 3 to 5 for all particle sizes. The uncertainty in caused by the use of different droplet terminal velocity formulations is generally small than a factor of 2. The combined uncertainty due to the use of different formulations of raindrop-particle collection efficiency, raindrop size spectrum, and raindrop terminal velocity in the current theoretical framework is not sufficient to explain the one to two order of magnitude under-prediction of for the theoretical calculations relative to the majority of field measurements. These large discrepancies are likely caused by additional known physical processes (i.e, turbulent transport and mixing, cloud and aerosol microphysics) that influence field data but that are not considered in current theoretical parameterizations. The predicted size-resolved particle concentrations using different theoretical parameterization can differ by up to a factor of 2 for particles smaller than 0.01 µm and by a factor of >10 for particles larger than 3 µm after 2-5 mm of rain. The predicted bulk mass and number concentrations (integrated over the particle size distribution) can differ by a factor of 2 between theoretical and empirical parameterizations after 2-5 mm of moderate intensity rainfall.

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“…The terminal velocity of cloud drops is estimated following Beard (1976) in three ranges of the particle diameter (0.5 μm-19 μm, 19 μm-1.07 mm, 1.07 mm-7 mm). Another approximation by Best (1950) This table was created based on simulations of hydrodynamic droplet interactions over a broad range of droplet radii (1-300 μm), including collisions among small cloud droplets as well as between small cloud droplets and small raindrops.…”

Section: Condensation Growth With Entrainmentmentioning

confidence: 99%

Understanding aerosol-cloud interactions in the development of orographic cumulus congestus during IPHEx

Duan

Petters

Barros

2017

Preprint

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Abstract.A new cloud parcel model (CPM) including activation, condensation, collision-coalescence, and lateral entrainment processes is presented here to investigate aerosol-cloud interactions (ACI) in cumulus development prior to rainfall onset. The CPM was employed along with ground based radar and surface aerosol measurements to predict the vertical structure of cloud formation at early stages and evaluated against airborne observations of cloud microphysics and 10 thermodynamic conditions during the Integrated Precipitation and Hydrology Experiment (IPHEx) over the Southern Appalachian Mountains. Further, the CPM was applied to explore the space of ACI physical parameters controlling cumulus congestus growth not available from measurements, and to examine how variations in aerosol properties and microphysical processes influence the evolution and thermodynamic state of clouds over complex terrain via sensitivity analysis. Modelling results indicate that aerosol-cloud droplet number concentration (CDNC) closure is achieved optimally to ~ 1.3% of the 15 observations for condensation coefficient ( ) = 0.01 and within 5% for 0.01< < 0.015, and the corresponding spectra in the predictions are in good agreement with IPHEx aircraft observations around the same altitude. This is in contrast with larger closure errors and high values reported in previous studies assuming adiabatic conditions. Entrainment is shown to govern the vertical development of clouds and the change of droplet numbers with height, and the sensitivity analysis suggests that entrainment strength and condensation process are mutually compensating to attain aerosol-CDNC closure. 20Simulated CDNC also exhibits high sensitivity to variations in initial aerosol concentration at cloud base, but weak sensitivity to aerosol hygroscopicity. Exploratory multiple-parcel simulations capture realistic time-scales of vertical development of cumulus congestus (deeper clouds and faster droplet growth). These findings provide new insights into determinant factors of mid-day cumulus congestus formation that can explain a large fraction of warm season rainfall in mountainous regions. 25

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Terminal Velocity and Shape of Cloud and Precipitation Drops Aloft

Cited by 580 publications

References 0 publications

Droplet growth in warm turbulent clouds

Droplet growth in warm turbulent clouds

Uncertainty assessment of current size-resolved parameterizations for below-cloud particle scavenging by rain

Understanding aerosol-cloud interactions in the development of orographic cumulus congestus during IPHEx

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