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Caustic activation of rain showers
Michael Wilkinson1, Bernhard Mehlig, Vlad Bezuglyy
1Faculty of Mathematics and Computing, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom.
Physical Review Letters
|August 16, 2006
Summary
Turbulent air causes moisture droplet collisions to rapidly increase past a certain turbulence intensity threshold. This phenomenon, explained by fold caustics and the Stokes number, is key to understanding rainfall formation in clouds.
Area of Science:
- Fluid Dynamics
- Cloud Physics
- Turbulence Research
Background:
- Explaining the rapid onset of rainfall in convecting clouds remains a significant challenge in atmospheric science.
- Previous theories have not fully accounted for the abrupt increase in droplet collision rates observed under certain conditions.
Purpose of the Study:
- To quantitatively explain the abrupt increase in collision rate of visible moisture droplets in turbulent air.
- To investigate the role of fold caustics in droplet velocity fields and their impact on collision dynamics.
Main Methods:
- Developed a theoretical model analyzing droplet collision rates in turbulent air.
- Introduced the Stokes number (St) as a measure of turbulence intensity, analogous to temperature in chemical kinetics.
- Quantitatively demonstrated the formation of fold caustics in droplet velocity fields.
Main Results:
- Collision rates increase abruptly once turbulence intensity surpasses a specific threshold.
- The collision rate follows an exponential relationship with the Stokes number: exp(-C/St).
- The theory does not rely on spatial clustering of particles to explain increased collisions.
Conclusions:
- Fold caustics in the velocity field are the primary mechanism driving the sudden increase in droplet collision rates.
- The Stokes number effectively quantifies the turbulence intensity's role in droplet collisions.
- This research provides a novel, non-clustering explanation for the rapid initiation of rainfall in convective clouds.
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