Droplet growth in warm turbulent clouds

Abstract

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

Inhomogeneous distribution of water droplets in cloud turbulence

“…Thus formation of inhomogeneities of particles by turbulence is small-scale phenomenon, often disregarded due to finite resolution of instruments. Since it is at those scales that droplets collide then the study of the inhomogeneities is necessary to predict rain formation.Much progress was obtained in the study of nonuniform distributions of inertial particles in the flow when gravity is negligible [1][2][3][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. In the regime where the inertia is not too large, which is where turbulence is most relevant in the rain formation process [3], the paircorrelation function of concentration was demonstrated to obey a power-law with negative exponent signifying singular (fractal) structure formed by particles in space.…”

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

“…This might be due to other mechanisms, such as supersaturation fluctuations in a turbulent environment or the collision coalescence process. It should be mentioned that the CDSD observed in previous studies might have the problem of instrumental broadening due to low instrument resolution or long-distance averaging of the sampling volume (Brenguier et al, 2011;Devenish et al, 2012). A broad CDSD is also observed by recent holographic measurements, which limit the effect of instrument broadening and have much higher temporal and spatial resolution than other instruments, such as particle-counting probes (Beals et al, 2015;Glienke et al, 2017;Desai et al, 2018).…”

“…Turbulence can result in not only upward and downward oscillations but also in entrainment and mixing (Shaw, 2003;Devenish et al, 2012). The latter can cause cloud droplet evaporation, deactivation and reactivation (Korolev et al, 2013;Yang et al, 2016).…”

“…Turbulence is ubiquitous in the clouds and can cause CDSD broadening in both condensation and collision processes (e.g., Shaw, 2003;Devenish et al, 2012). Turbulence induces vertical oscillations of air parcels and causes fluctuations in temperature, water vapor concentration and supersaturation (e.g., Ditas et al, 2012;Hammer et al, 2015).…”

“…First, turbulenceinduced mixing and entrainment can trigger in-cloud activation of haze particles, which can broaden the left branch of the size distribution (e.g., Khain et al, 2000;Devenish et al, 2012;Yang et al, 2016;Grabowski et al, 2018). Secondly, giant cloud condensational nuclei (GCCN, usually defined as aerosols with a dry diameter larger than a few µm) provides an embryo for large droplets, which can broaden the right branch of the size distribution and can be important for warm rain initiation (e.g., Johnson, 1982;Feingold et al, 1999;Yin et al, 2000;Jensen and Lee, 2008;Cheng et al, 2009).…”