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

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

doi:10.5194/acp-18-7251-2018

Abstract

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.

Highlights

Theoretical studies indicate that for droplets in the size range of 15–30 μm in radius, referred to as the condensation– collision size gap, neither condensational growth nor collisional growth is effective (Pruppacher and Klett, 1997) in producing precipitation
Wide droplet size distribution (DSD) and large droplets are frequently observed in cumulus and even stratocumulus clouds (e.g., Brenguier and Chaumat, 2001; Pawlowska et al, 2006; Prabha et al, 2012)
This study focuses on the effect of smallscale turbulence-containing eddies in the inertial and dissipation range with length scales 10 m as shown in Fig. 1 of Grabowski and Wang (2013), which can be resolved by the technique of direct numerical simulation (DNS)

Summary

Introduction

Theoretical studies indicate that for droplets in the size range of 15–30 μm in radius, referred to as the condensation– collision size gap, neither condensational growth nor collisional growth is effective (Pruppacher and Klett, 1997) in producing precipitation. Sardina et al (2015) used a similar model to Lanotte et al (2009) but extended the simulation time to 20 min to be comparable to the formation time of rain revealed in real observations They found that the variance of the droplet size distribution was mainly determined by the large-scale flow, i.e., the large-hopping effect suggested by Grabowski and Wang (2013) and studied by Grabowski and Abade (2017). This assumption may be justifiable in a parcel model due to the nonoverlapping droplet size regimes of the two growth processes in still air This assumption is questionable in DNS studies which reveal substantial turbulent enhancement of collisions among droplets in the condensation–collision size gap.

Model description and experimental setup

Results and discussion

Conclusions

Microscopic equations