As monsoon lashes the Indian peninsula, Bengaluru scientists are trying to understand how raindrops form inside clouds.
BY DEBDUTTA PAUL
ಈ ಲೇಖನವನ್ನು ಕನ್ನಡದಲ್ಲಿ ಓದಲು ೨ನೇ ಪುಟಕ್ಕೆ ಹೋಗಿ.
Clouds are water droplets suspended in the air. They remain suspended because they continuously evaporate faster than the rate at which they fall under gravity. Rainfall occurs as the rate of falling overtakes the rate of evaporation.
A typical raindrop is about a few millimetres wide. Scientists have estimated that water droplets grow to this size inside rain clouds within half an hour.
Water droplets form around particles like dust, soot, and sea salt. The droplets start at a size of about a thousand times smaller than a millimetre and grow large by merging.
A simple estimate suggests that this process would take the droplets very long — hours to days to months. So, scientists do not understand how the droplets merge and grow to millimetres wide within minutes.
Fluid mechanics, the study of liquids and gases that flow, may offer an answer. Researchers study equations that govern the fluid’s flow, such as air and water vapour in a cloud. These fluids are turbulent, meaning the flow is irregular, and it causes different parts to mix.
Scientists think that turbulence can help water droplets merge and grow within minutes. But, studying clouds is challenging because the water droplets possess a finite mass and size. So, the individual droplets do not follow the motion of the fluid. For example, when the fluid takes a turn, the heavy droplets get carried over a little longer before the liquid drags it along with the turn.
Scientists mitigate the challenge by studying the equations governing the motion of individual droplets.
The researchers assume the droplets are small compared to the distances across which the rainclouds experience turbulence. But there are about a billion droplets every cubic metre, which means they need to study the motion of those many droplets.
On top of this, the turbulent flows of the air and water vapour affect the motion of the droplets. The fluids’ properties change rapidly across the breadth of the rain clouds, a few hundred to thousands of metres. These changes affect the droplets’ motions, and the sheer number of droplets makes the problem intractable.
Ideally, scientists would study how multiple water droplets interact with the turbulent flows. Combining this knowledge with an understanding of turbulent flows would help them predict the fluid’s properties and demonstrate how the drops become bigger. However, the number of calculations required to study this complex phenomenon is beyond the scope of modern computers.
Scientists here at ICTS-TIFR and their collaborators are attempting to solve the problem by devising ways to reduce the number of calculations. As they do that, they might have solved similar problems arising in different systems.
Take the case of marine snow. It is a combination of faecal and dead organic matter that falls from the ocean’s upper reaches to the ocean floor. They look like white flakes of snow, giving their name.
The marine snow phenomenon is essential for absorbing carbon from the atmosphere and storing it on the ocean bed.
It turns out that the equations explaining marine snow in the ocean are the same as those describing water droplets in clouds. Water gives rise to more internal friction than air. So, the organic matter falling through oceanic water meets the same mathematical conditions that water droplets meet inside clouds.
This insight lets scientists use the same mathematical methods to explore the complexities of these two distinct phenomena. The answers, they think, lie in the motion of individual particles inside the flows.
The research demonstrates the power of developing mathematical frameworks to help study diverse phenomena that are apparently different. For example, they write, “platelets in blood, dust storms,” and “planktons in oceans” can all be studied using the same mathematical equations as marine snow and water droplets.
See more about their work at The Basset–Boussinesq history force: its neglect, validity, and recent numerical developments.
The author thanks Vishal Vasan, Divya Jaganathan, and Rama Govindarajan for extensive discussions.