Preprints
https://doi.org/10.5194/acp-2021-844
https://doi.org/10.5194/acp-2021-844

  20 Oct 2021

20 Oct 2021

Review status: this preprint is currently under review for the journal ACP.

Nontrivial Relations Among Particle Collision, Relative Motion and Clustering in Turbulent Clouds: Computational Observation and Theory

Ewe-Wei Saw1,2 and Xiaohui Meng1 Ewe-Wei Saw and Xiaohui Meng
  • 1School of Atmospheric Sciences and Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, Sun Yat-Sen University, Zhuhai, China
  • 2Ministry of Education Key Laboratory of Tropical Atmosphere-Ocean System, Zhuhai, China

Abstract. Considering turbulent clouds containing small heavy particles, we investigate the reverse effect of particle collision, in particular collision-&-coagulation, on particle clustering and relative motion. We perform various cases of direct numerical simulation (DNS) of coagulating particles in isotropic turbulent flow and find that, due to collision-coagulation, the radial distribution functions (RDF) fall-off dramatically at scales r ∼ d (where d is the particle diameter) to small but finite values; while the mean radial-component of particle relative velocities (MRV) increase sharply in magnitudes. Based on a previously proposed Fokker-Planck (drift-diffusion) framework, we derive a theoretical account of the relationship among particle collision-coagulation rate, RDF and MRV. The theory includes contribution from turbulent-fluctuations absent in earlier mean-field theories. We show numerically that the theory accurately account for the DNS results. We also proposed a phenomenological model for the MRV which is accurate when calibrated using 4th moments of the fluid velocities. We uncover a paradox: the unjustified accuracy of the differential version of the theory. Our result demonstrate strong coupling between RDF and MRV and implies that earlier isolated studies on either RDF or MRV have limited relevance for predicting particle collision rate.

Ewe-Wei Saw and Xiaohui Meng

Status: open (until 03 Dec 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-844', Anonymous Referee #1, 05 Nov 2021 reply
  • RC2: 'Comment on acp-2021-844', Anonymous Referee #2, 28 Nov 2021 reply

Ewe-Wei Saw and Xiaohui Meng

Ewe-Wei Saw and Xiaohui Meng

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Short summary
Collision-coagulation of small droplets in turbulent clouds leads to the production of rain. Turbulence causes droplet clustering and higher relative droplet velocities and these should enhance collision-coagulation rate. We find surprisingly that coagulation starkly diminishes clustering and strongly alters relative velocities. We provide a theory that explains the preceding result. Our results call for a new perspective in how we understand particle/droplet collision in clouds.
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