The Hindu Business Line : Scientists discover barrier to raindrops reaching drizzle size

SCIENTISTS with the US Department of Energy have discovered what they describe as a statistical barrier to the number of raindrops that achieve drizzle size, a barrier that paradoxically speeds up drizzle formation.

A physical limit on the number of cloud droplets that grow big enough to form drizzle grow faster because those few droplets that cross the drizzle barrier readily collect enough surrounding droplets to fall - instead of staying stuck in the clouds competing for a limited water supply and never getting quite big enough.

Under the new theory, the rate of drizzle drop formation is dependent on the cloud liquid water content, the droplet concentration and turbulence.

Atmospheric turbulence causes fluctuations in the rates of droplet growth and evaporation in clouds.

Once a drop grows large enough to fall under gravity, it begins to collect the smaller cloud droplets that surround it.

The research findings could lead to more accurate weather forecasts and climate models, scientists said.

This process is called collection and it refers to the volumetric gain of a specified drop large enough to have a significant gravitational fall velocity so as to accrete the smaller, slower falling droplets that typify the main population of the cloud.

Collection is, thus, an additional growth mechanism that becomes available to those relatively few droplets that, through chance fluctuations, reach fall velocity size.

Older theories of drizzle formation left scientists with a puzzle: none explained how drizzle could form within the typical cloud lifetime.

In these earlier models, droplets simply coagulated with each other to reach larger size.

In this process, many droplets are free to begin growing, but they all end up competing for the available cloud water at the same time.

This competition prevents any of them from quickly reaching a size large enough to begin falling as drizzle.

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