The role of air turbulence in warm rain initiation

Wang, Lian‐Ping; Grabowski, Wojciech W.

doi:10.1002/asl.210

Cited by 64 publications

(70 citation statements)

References 33 publications

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“…The collection kernel K is calculated from the traditional collision efficiencies as given by Hall (1980) and includes turbulence effects by an enhancement factor for the collision efficiency by Wang and Grabowski (2009) as well as parameterizations for particle relative velocities and changes in the particle radial distribution based on Ayala et al (2008). These turbulence effects are steered by the kinetic energy dissipation rate calculated in the LES subgrid-scale model (Riechelmann et al, 2012).…”

Section: Summary and Discussionmentioning

confidence: 99%

“…These turbulence effects are steered by the kinetic energy dissipation rate calculated in the LES subgrid-scale model (Riechelmann et al, 2012). The parameterizations by Ayala et al (2008) are a direct function of ; the tabulated values of the enhancement factor for the collision efficiency by Wang and Grabowski (2009) are interpolated to the present value of . The Eqs.…”

Section: Summary and Discussionmentioning

confidence: 99%

See 1 more Smart Citation

On the limits of Köhler activation theory: how do collision and coalescence affect the activation of aerosols?

Hoffmann

2017

Atmos. Chem. Phys.

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Abstract. Activation is necessary to form a cloud droplet from an aerosol, and it is widely accepted that it occurs as soon as a wetted aerosol grows beyond its critical radius. Traditional Köhler theory assumes that this growth is driven by the diffusion of water vapor. However, if the wetted aerosols are large enough, the coalescence of two or more particles is an additional process for accumulating sufficient water for activation. This transition from diffusional to collectional growth marks the limit of traditional Köhler theory and it is studied using a Lagrangian cloud model in which aerosols and cloud droplets are represented by individually simulated particles within large-eddy simulations of shallow cumuli. It is shown that the activation of aerosols larger than 0.1 µm in dry radius can be affected by collision and coalescence, and its contribution increases with a power-law relation toward larger radii and becomes the only process for the activation of aerosols larger than 0.4-0.8 µm depending on aerosol concentration. Due to the natural scarcity of the affected aerosols, the amount of aerosols that are activated by collection is small, with a maximum of 1 in 10 000 activations. The fraction increases as the aerosol concentration increases, but decreases again as the number of aerosols becomes too high and the particles too small to cause collections. Moreover, activation by collection is found to affect primarily aerosols that have been entrained above the cloud base.

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Section: Summary and Discussionmentioning

confidence: 99%

Section: Summary and Discussionmentioning

confidence: 99%

On the limits of Köhler activation theory: how do collision and coalescence affect the activation of aerosols?

Hoffmann

2017

Atmos. Chem. Phys.

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“…In section 4.1 we consider its effect on condensational growth. Droplet growth by gravitational collision-coalescence only becomes efficient once a certain fraction of droplets reaches 30-50 µm in radius (Pruppacher and Klett, 1997, sections 14.5, 15.1-15.3;Xue et al, 2008;Wang and Grabowski, 2009;Grabowski and Wang, 2009). The rapid growth of droplets from 15 to 50 µm remains difficult to explain.…”

Section: Small-scale Turbulence and Cloud Microphysicsmentioning

confidence: 99%

“…When the effect of turbulence on collision efficiency is also considered, Wang and Grabowski (2009) concluded that the warm rain initiation time could be reduced up to a factor of two. Using the same turbulent collision kernel and a rising adiabatic parcel model that combines droplet activation, diffusional growth and turbulent collision-coalescence, Grabowski and Wang (2009) showed that air turbulence could reduce the warm rain initiation time by 25-40%.…”

Section: Impact Of Turbulent Collision-coalescence On Droplet Growthmentioning

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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“…The collection kernel developed by Ayala et al (2008b) and Wang and Grabowski (2009) is used (hereafter the AW kernel), in which the effects of turbulence from relative velocity, preferential concentration, and the enhanced collision efficiency are parameterized as a function of the dissipation rate e and the Taylor-microscale Reynolds number. The Hall kernel (Hall 1980), which considers only gravitational collision, is also used for comparison.…”

Section: Model Descriptionmentioning

confidence: 99%

Investigation of droplet dynamics in a convective cloud using a Lagrangian cloud model

Lee

Noh

Raasch

et al. 2014

Meteorol Atmos Phys

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A precipitating convective cloud is simulated successfully using the Lagrangian cloud model, in which the flow field is simulated by large eddy simulation and the droplets are treated as Lagrangian particles, and the results are analyzed to investigate precipitation initiation and to examine the parameterization of cloud microphysics. It is found that raindrops appear initially near the cloud top, in which strong turbulence and broadened droplet spectrum are induced by the entrainment of dry air, but high liquidwater mixing ratio is maintained within cloud parts because of insufficient mixing. Statistical analysis of the downward vertical velocity of a droplet W reveals that the transition from cloud droplets to raindrops occurs in the range 20 lm \ r \ 100 lm, while the variation of W depends on turbulence as well as the droplet radius r. The general pattern of the raindrop size distribution is found to be consistent with the Marshall-Palmer distribution. The precipitation flux can be underestimated substantially, if the terminal velocity w s is used instead of W, but it is not sensitive to the choice of the critical droplet radius dividing cloud drops and raindrops. It is also found that precipitation starts earlier and becomes stronger if the effect of turbulence is included in the collection kernel.

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The role of air turbulence in warm rain initiation

Cited by 64 publications

References 33 publications

On the limits of Köhler activation theory: how do collision and coalescence affect the activation of aerosols?

On the limits of Köhler activation theory: how do collision and coalescence affect the activation of aerosols?

Droplet growth in warm turbulent clouds

Investigation of droplet dynamics in a convective cloud using a Lagrangian cloud model

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