"Acceleration of rain initiation by cloud turbulence"
Prof. Gregory Falkovich
of Physics of Complex Systems, The Weizmann Institute
Vapour condensation in cloud cores produces small droplets that are close
to one another in size. In warm clouds, droplets grow to raindrop size by
coalescence due to collisions. Air turbulence is thought to be the main cause
for collisions of close in size droplets exceeding radii of a few microns,
and therefore rain prediction requires a quantitative description of droplet
collision in turbulence. Turbulent vortices act as small centrifuges that
spin heavy droplets out, creating concentration inhomogeneities and jets
of droplets, both of which increase the mean collision rate. A formula for
the collision rate of small heavy particles in a turbulent flow is to be
decribed. Its derivation uses a formalism for tracing random trajectories
recently developed in particularly by Russian-Israeli collaboration. I describe
an enhancement of inertial effects by turbulence intermittency and an interplay
between turbulence and gravity that determines the collision rate. I present
a new mechanism, the `sling effect', for collisions due to jets of droplets
detached from the air flow. Preliminary quantitative analysis shows that
even moderate turbulence can substantially accelerate the transition from
condensation to coalescence stage in the interval of few tens of microns.
One concludes that air turbulence can substantially accelerate the appearance
of large droplets that trigger rain.
Dr. Ron Lifshitz, x5145