Particle-laden turbulence and rain initiation

6 problems

• Rate of sling events and caustics for inertial particles

  Partially resolved

  Heavy particles in turbulence are sometimes flung out of intense whirls so that particles at the same place move with very different speeds, which raises collision rates. The frequency of these events at small particle inertia is predicted but not measured accurately.

• Turbulent enhancement of droplet collision rates at cloud Reynolds numbers

  Open

  Turbulence can make cloud droplets collide more often by clustering them and giving them different velocities. How large this effect is at the turbulence levels of real clouds, far beyond what computers can simulate, is unknown.

• When do finite-size particles enhance or damp carrier turbulence?

  Open

  Particles larger than the smallest eddies, carried by a turbulent flow, can either calm the turbulence or make it stronger through the wakes they shed. No reliable rule predicts which happens.

• Sign of turbulence modulation by heavy particles smaller than eddies

  Open

  Tiny heavy particles, smaller than the smallest eddies, can either drain energy from the turbulence that carries them or feed it, depending on their inertia and how many there are. No theory predicts which happens.

• How do cloud droplets cross the size gap to form warm rain?

  Contested

  Cloud droplets grow quickly by condensation while tiny and by sweeping up others once large, but in between, at a few tens of micrometers, both processes are slow. Yet rain appears within about 20 minutes, and which process bridges the gap is disputed.

• Does turbulence speed up or slow down settling heavy particles?

  Contested

  Turbulence can make heavy particles fall faster by steering them into downward-moving fluid, or slower through drag effects and lingering in upward flow. Which effect wins under given conditions is disputed.