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Acceleration of rain initiation by cloud turbulence
G Falkovich1, A Fouxon, M G Stepanov
1Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel. fnfal@wicc.weizmann.ac.il
Nature
|September 13, 2002
Summary
Air turbulence accelerates cloud droplet collisions, leading to faster rain formation. This study quantifies droplet collision rates in turbulent flows, revealing mechanisms that enhance droplet growth and impact rainfall prediction.
Area of Science:
- Atmospheric science
- Fluid dynamics
- Cloud physics
Background:
- Cloud droplets grow by collision and coalescence to form raindrops.
- Air turbulence is hypothesized to be a primary driver of droplet collisions, especially for similar-sized droplets.
- Quantitative models of droplet collision in turbulence are crucial for accurate rain prediction.
Purpose of the Study:
- To derive a formula for the collision rate of small heavy particles in turbulent flow.
- To investigate the influence of turbulence intermittency and gravity on droplet collision rates.
- To introduce and explain the 'sling effect' mechanism for droplet collisions.
Main Methods:
- Utilized a recently developed formalism for tracing random trajectories of particles.
- Derived a formula for collision rates based on particle dynamics in turbulent flow.
- Analyzed the interplay between turbulence, gravity, and particle inertia.
Main Results:
- Turbulent vortices enhance droplet collision rates by creating concentration inhomogeneities and droplet jets.
- Turbulence intermittency amplifies inertial effects, further increasing collision probabilities.
- The 'sling effect' mechanism, driven by detached droplet jets, contributes to accelerated collisions.
Conclusions:
- Air turbulence significantly accelerates the growth of cloud droplets to raindrop size.
- The derived formula and identified mechanisms provide a quantitative basis for rain prediction models.
- Understanding these turbulent effects is key to improving our comprehension of precipitation processes.
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