Broadening of droplet size distributions from entrainment and mixing in a cumulus cloud

Lasher-Trapp, Sonia; Cooper, William A.; Blyth, Alan

doi:10.1256/qj.03.199

Cited by 174 publications

(250 citation statements)

References 29 publications

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“…Warner, 1969b;Pinsky and Khain, 2002) or entrainment (Warner, 1969a;Paluch and Knight, 1984;Brenguier and Grabowski, 1993;Su et al, 1998;Lasher-Trapp et al, 2005). As discussed in Brenguier and Grabowski (1993, section 3b and Figures 6-8 in particular), entrainment-related activation above the cloud base occurs in vortical structures associated with cloud-environment interface instabilities that bring environmental cloud-free air into the cloud.…”

Section: Activation and Growth Of Cloud Dropletsmentioning

confidence: 86%

“…For 100% inhomogeneous mixing the dominant mode occurs at a value of a several microns smaller than both the homogeneous and adiabatic cases. Lasher-Trapp et al (2005) attribute this to the dominance of entrained CCN rather than droplets formed at cloud base. The importance of entrainment in broadening the droplet size distribution can be appreciated by comparing the droplet size distribution at a central point in the cloud early in its evolution, which is close to its adiabatic form, with a droplet size distribution near the cloud top (which exhibits significant broadening).…”

Section: Homogeneous and Inhomogeneous Mixingmentioning

confidence: 95%

“…The idea is to combine an Eulerian fluid-flow model with an ensemble of Lagrangian fluid trajectories. Such methods have been quite successful in explaining some aspects of the impact of turbulence on cloud microphysics (see, for example, the discussion in Stevens et al, 1996;Feingold et al, 1998;Lasher-Trapp et al, 2005;Pinsky et al, 2008b;Magaritz et al, 2009). However, the main problem with such approaches is that, in a turbulent environment, a fluid parcel loses its identity relatively quickly.…”

Section: Effect Of Entrainment On Droplet Size Distributionmentioning

confidence: 99%

“…Lasher- Trapp et al (2005) considered the effect of entrainment on the droplet size distribution in LES of a cumulus cloud. They used a Lagrangian stochastic model (which introduces subgrid variability) to calculate the trajectories of fluid particles along which they calculated the supersaturation, which in turn was used to calculate the growth of cloud droplets.…”

Section: Homogeneous and Inhomogeneous Mixingmentioning

confidence: 99%

“…Droplet size distributions at a given location in the cloud were calculated from an ensemble of trajectories reaching that point. Their model allowed for explicit variation of the degree to which mixing was assumed to be homogeneous or inhomogeneous (see equations (7) and (9) in Lasher-Trapp et al (2005) and the attendant discussion for details). They found that for purely homogeneous mixing (see Figure 6 of LasherTrapp et al, 2005) entrainment led to a broadening of the droplet size distribution near the cloud top, with a peak occurring at a value of a several microns larger than for a control simulation with adiabatic ascent alone (no entrainment).…”

Section: Homogeneous and Inhomogeneous Mixingmentioning

confidence: 99%

See 4 more Smart Citations

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: Activation and Growth Of Cloud Dropletsmentioning

confidence: 86%

Section: Homogeneous and Inhomogeneous Mixingmentioning

confidence: 95%

Section: Effect Of Entrainment On Droplet Size Distributionmentioning

confidence: 99%

Section: Homogeneous and Inhomogeneous Mixingmentioning

confidence: 99%

Section: Homogeneous and Inhomogeneous Mixingmentioning

confidence: 99%

See 3 more Smart Citations

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

Effects of entrainment and mixing on droplet size distributions in warm cumulus clouds

Tölle

Krueger

2014

J Adv Model Earth Syst

103

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A long-standing problem in cloud physics is the broadening of the cloud droplet spectrum in warm cumulus clouds. To isolate the changes of the droplet size distribution (DSD) due to entrainment and turbulent mixing, we used the Explicit Mixing Parcel Model (EMPM). The EMPM explicitly represents spatial variability due to entrainment and turbulent mixing down to the smallest turbulence scales in a onedimensional domain. Several thousand individual droplets evolve by condensation or evaporation according to their local environments. We used EMPM results to characterize the evolution of the DSD due to entrainment and isobaric mixing for a wide range of conditions in a 20 m domain, including variations in entrained environmental air fraction, the turbulence dissipation rate, the size of the entrained blobs, and the relative humidity of the entrained air. We found that the broadening of the DSD due to entrainment and isobaric mixing for a specific value of the entrained air relative humidity depends only on the eddy mixing time scale and the LWC after mixing. Broadening increases substantially as the evaporation time scale decreases due to decreasing relative humidity of the entrained air. Our results also show that it is possible to parameterize the effects of entrainment and mixing on the droplet number concentration. The comprehensive results obtained for one set of values of entrained air relative humidity, droplet size, and droplet concentration should be extended to other values.

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Scale dependence of entrainment‐mixing mechanisms in cumulus clouds

Liu

Niu

et al. 2014

JGR Atmospheres

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This work empirically examines the dependence of entrainment-mixing mechanisms on the averaging scale in cumulus clouds using in situ aircraft observations during the Routine Atmospheric Radiation Measurement Aerial Facility Clouds with Low Optical Water Depths Optical Radiative Observations (RACORO) field campaign. A new measure of homogeneous mixing degree is defined that can encompass all types of mixing mechanisms. Analysis of the dependence of the homogenous mixing degree on the averaging scale shows that, on average, the homogenous mixing degree decreases with increasing averaging scales, suggesting that apparent mixing mechanisms gradually approach from homogeneous mixing to extreme inhomogeneous mixing with increasing scales. The scale dependence can be well quantified by an exponential function, providing first attempt at developing a scale-dependent parameterization for the entrainment-mixing mechanism. The influences of three factors on the scale dependence are further examined: droplet-free filament properties (size and fraction), microphysical properties (mean volume radius and liquid water content of cloud droplet size distributions adjacent to droplet-free filaments), and relative humidity of entrained dry air. It is found that the decreasing rate of homogeneous mixing degree with increasing averaging scales becomes larger with larger droplet-free filament size and fraction, larger mean volume radius and liquid water content, or higher relative humidity. The results underscore the necessity and possibility of considering averaging scale in representation of entrainment-mixing processes in atmospheric models.

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Broadening of droplet size distributions from entrainment and mixing in a cumulus cloud

Cited by 174 publications

References 29 publications

Droplet growth in warm turbulent clouds

Droplet growth in warm turbulent clouds

Effects of entrainment and mixing on droplet size distributions in warm cumulus clouds

Scale dependence of entrainment‐mixing mechanisms in cumulus clouds

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