Experimental Study on the Influence of Turbulence on Droplet Size Growth in Relation to Understanding the Onset of Rain in Warm Clouds

Understanding the microphysics related to droplet size growth under turbulence is crucial to comprehend the rapid onset of rain in warm clouds. This lack of understanding represents a great uncertainty in our prediction of the occurrence of precipitation and its intensity. Cloud droplets are formed when water vapor condenses on aerosol particles. These activated aerosol particles are known as cloud condensation nuclei (CCN) and are typically of size less than 1 μm. After activation, CCN grows into cloud droplets via condensation and can reach a size of O(10 μm). The final cloud droplet size distribution after growth by condensation depends upon the CCN concentration and the available liquid water content (LWC).

Further growth of cloud droplets leading to precipitation is through collision and coalescence. When a significant fraction of droplets in the cloud droplet size distribution grow to a size O(100 μm), gravity can induce collision and coalescence by differential settling leading to precipitation. However, the processes involved during the growth of droplets from O(10 μm) to a size distribution where gravity can significantly enhance collisions still need to be better understood. Turbulence in clouds is believed to play a vital role in enhancing the rate of collisions between droplets. However, the parameters and the mechanisms associated with this bottleneck growth remain poorly understood.

In this study, we create cloud-like conditions in the laboratory and experimentally investigate the influence of turbulence on droplet size distributions. We designed and fabricated a turbulence chamber where we can generate homogeneous and isotropic turbulence of various intensities with U_rms ranging from 0.4 m/sec to 1.34 m/sec. We seed the chamber with droplet size distributions and volume fractions similar to that observed in clouds and measure the changes in droplet size distributions for different turbulence intensities using Phase Doppler particle analyzer (PDPA). We observed that wider droplet size distributions, mimicking clouds originating in marine ambiance, grow significantly under the influence of turbulence when compared with a slender droplet size distribution, mimicking clouds observed in polluted ambient conditions. Furthermore, we observed that the growth of the droplet size distribution was further enhanced with an addition of a small fraction of bigger droplets to both distributions. We believe the addition of a small fraction of bigger droplets to the distribution induces large relative velocities between droplets, thereby enhancing collisions and coalescence. To understand the mechanisms associated with the droplet size growth, we analyzed their clustering characteristics in turbulence using planar Mie-scattering images. We observed an increase in clustering with turbulence intensity for the droplet size distributions where we observed a growth. Thus, we believe droplet clustering due to turbulence plays a significant role in enhancing collisions and coalescence. These results provide a direct validation of the lucky droplet size growth model, where a few bigger droplets trigger the runaway growth in droplet size distribution. Also, the experimental observations made in this study provide direct insight into the mechanisms associated with the growth of droplet size distributions in turbulence.