Growth of Cloud Droplets in a Turbulent Environment

Abstract

Motivated by the need to resolve the condensation-coalescence bottleneck in warm rain formation, a significant number of studies have emerged in the past 15 years concerning the growth of cloud droplets by water-vapor diffusion and by collision-coalescence in a turbulent environment. With regard to condensation, recent studies suggest that small-scale turbulence alone does not produce a significant broadening of the cloud-droplet spectrum because of the smearing of droplet-scale fluctuations by rapid turbulent and gravitational mixing. However, different diffusional-growth histories associated with large-eddy hopping could lead to a significant spectral broadening. In contrast, small-scale turbulence in cumulus clouds makes a significant contribution to the collision-coalescence of droplets, enhancing the collection kernel up to a factor of 5, especially for droplet pairs with a low gravitational collision rate. This moderate level of enhancement has a significant impact on warm rain initiation. The multiscale nature of turbulent cloud microphysical processes and open research issues are delineated throughout. 293Annu. Rev. Fluid Mech. 2013.45:293-324. Downloaded from www.annualreviews.org by University of Delaware on 02/13/13. For personal use only.

The effects of turbulent collision–coalescence on precipitation formation and precipitation-dynamical feedbacks in simulations of stratocumulus and shallow cumulus convection

“…Observations of droplet growth tend to show a faster evolution and broader drop size distribution compared to the theoretically calculated drop spectra, where the equations are applied to a randomly distributed population of drops whose motion is governed by gravitational forcing (see review by Grabowski and Wang, 2013). Several physical effects have been suggested to play an important role in the reduction of the growth times, including entrainment and mixing of dry air, turbulence and the role of giant cloud condensation nuclei (e.g.…”

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

“…An interesting question is to explain why the CDSD is wider than predicted and the presence of the large droplet sizes in the tail of the distribution (e.g., Siebert and Shaw, 2017), which might be related to the fast-rain process in the atmosphere (e.g., Göke et al, 2007). Several pos- 15 sible mechanisms have been proposed, including the existence of giant cloud condensational nuclei (GCCN, usually defined as aerosols with dry diameter larger than few µm) (e.g., Feingold et al, 1999;Yin et al, 2000;Jensen and Lee, 2008;Cheng et al, 2009;Jensen and Nugent, 2017), lucky cloud droplets (e.g., Kostinski and Shaw, 2005;Naumann and Seifert, 2015;Lozar and Muessle, 2016), mixing with environmental air (e.g., Lasher-Trapp et al, 2005;Cooper et al, 2013;Korolev et al, 2013;Yang et al, 2016), supersaturation fluctuations (e.g., Chandrakar et al, 2016;Siebert and Shaw, 2017), and enhancement of collision 20 efficiency due to turbulence or charge (e.g., Paluch, 1970;Grabowski and Wang, 2013;Falkovich and Pumir, 2015;Lu and Shaw, 2015). Recently, Jensen and Nugent (2017) investigated the effect of GCCN on droplet growth and rain formation using a cloud parcel model.…”

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

“…Turbulence can also modulate the condensational growth of cloud droplets through mixing and entrainment (e.g., Lasher-Trapp et al, 2005;Cooper et al, 2013;Korolev et al, 2013;Yang et al, 2016). In addition, turbulence can enhance the collision efficiency between droplets and produce "lucky" cloud droplets through stochastic collisions, which has been confirmed by direct numerical simulations and Lagrangian drop models (e.g., Paluch, 1970;Kostinski and Shaw, 2005;Falkovich and Pumir, 2007;Grabowski and Wang, 2013;Naumann and Seifert, 2015;de Lozar and Muessle, 2016).…”