Articles | Volume 18, issue 10
Research article
25 May 2018
Research article |  | 25 May 2018

Bridging the condensation–collision size gap: a direct numerical simulation of continuous droplet growth in turbulent clouds

Sisi Chen, Man-Kong Yau, Peter Bartello, and Lulin Xue

Abstract. In most previous direct numerical simulation (DNS) studies on droplet growth in turbulence, condensational growth and collisional growth were treated separately. Studies in recent decades have postulated that small-scale turbulence may accelerate droplet collisions when droplets are still small when condensational growth is effective. This implies that both processes should be considered simultaneously to unveil the full history of droplet growth and rain formation. This paper introduces the first direct numerical simulation approach to explicitly study the continuous droplet growth by condensation and collisions inside an adiabatic ascending cloud parcel. Results from the condensation-only, collision-only, and condensation–collision experiments are compared to examine the contribution to the broadening of droplet size distribution (DSD) by the individual process and by the combined processes. Simulations of different turbulent intensities are conducted to investigate the impact of turbulence on each process and on the condensation-induced collisions. The results show that the condensational process promotes the collisions in a turbulent environment and reduces the collisions when in still air, indicating a positive impact of condensation on turbulent collisions. This work suggests the necessity of including both processes simultaneously when studying droplet–turbulence interaction to quantify the turbulence effect on the evolution of cloud droplet spectrum and rain formation.

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Short summary
This paper introduces a sophisticated approach to incorporate the droplet hydrodynamic collision and condensation processes into a single DNS modeling framework. Arguably, this model provides a sophisticated approach to study the warm-rain initiation problem that has puzzled the cloud physics community for decades. The results show the increased condensation-mediated collisions when turbulence intensifies, indicating a positive impact of turbulence on droplet condensational–collisional growth.
Final-revised paper