Nothing clouds our understanding of climate change more than clouds themselves. With clouds continuing to be an enigma, modellers who attempt to predict which way global weather changes are heading are just shooting in the dark. The Intergovernmental Panel on Climate Change (IPCC), a UN-backed scientific body, in its report released two years ago termed clouds the most urgent scientific problem that needed attention. Now, it seems, scientists have found a silver lining.
A team of researchers in Bangalore, led by Roddam Narasimha, has reproduced clouds in the lab, a feat many others have attempted in the past but failed. Scientists at the Jawaharlal Nehru Centre for Advanced Scientific Research and the Indian Institute of Science (IISc) — both based in Bangalore — designed and built a bench-scale apparatus that can create different types of clouds by simply manipulating the flow, temperature and stratification of fluids at different densities. The two metre-high device is basically a water tank with a small hole in the bottom, through which coloured fluids are injected in order to create cloud-like formations. The tank is also equipped with heating elements that simulate energy transfers caused by condensation and freezing, and establish atmosphere-like temperature profiles. Using the apparatus, the scientists tracked the life cycles of several types of cumulus clouds, which contribute the maximum to atmospheric heating and thus influence global atmospheric circulations.
“The simulations are the first ever to capture cloud forms and evolution in the laboratory,” says Narasimha, who initiated research on cloud flows some 20 years ago when he was at the IISc. The present work, reported in the Proceedings of National Academy of Sciences, builds on his earlier work as well as that done in a few other labs around the world since then.
“The apparatus and the measurement techniques we employ have improved greatly. So we now have the capability to control and manage cloud floes, which have proved crucial in gaining new understanding,” Narasimha told KnowHow.
“Currently, there is no lab in the world that can completely reproduce a cloud — from condensation of water vapour to the dynamics of the moist convection that occurs within the cloud,” he claims. Moreover, cloud formation and dynamics are to this day not well understood, he adds.
What has been puzzling scientists for long is how clouds interact with the atmosphere. This chiefly takes place through entrainment — the process of clouds mixing with the surrounding air. “What we know is that classical fluid-dynamical theories and models do not work here,” observes Narasimha. This is a process that will be easy to study by using coloured fluids to form clouds.
|WEATHER GODS: The research team with the cloud machine|
Clouds are difficult to study for a variety of reasons. First, an individual cloud has a short life span — ranging from 5-10 minutes to about an hour. Second, field experiments are expensive. The task is made even more difficult by the fact that no two clouds are identical.
Aircraft can fly across clouds at various levels collecting data, but often a cloud has changed by the time it takes an aircraft to traverse it from bottom to top. Similarly, satellites can provide data on cloud cover, long wave radiation reflected back from earth and atmospheric parameters like humidity. But they cannot provide wind data at different altitudes or detailed information on the structure of a cloud.
Clouds, especially cumulus clouds, can have many forms — from dense, sharp edged domes that look like a cauliflower to lean and tall towers that can rise to heights of 15 km or more. “These clouds are common in India and the tropics, and are major agents in transporting energy from tropical land and ocean surfaces to greater altitudes in the atmosphere: they act like chimneys,” explains Narasimha.
A better understanding of clouds would be a boon for monsoon forecasters. One of the outstanding issues in predicting cloud behaviour is the link between cloud microphysics (condensation, droplet growth, etc) and dynamics (buoyancy, velocity, turbulence, etc). Currently they are treated separately, says Ganapati Shankar Bhat, co-author and professor, Centre for Atmospheric and Oceanic Sciences, IISc. The present study, however, suggests that there is a crucial link between the two and energy transfers owing to water state changes play a vital role in deciding how clouds form and evolve.
“Cloud heating by condensation of vapour drives the wind systems in the tropics, At present, many assumptions made in the cloud models are ad hoc owing to lack of data. If cloud physics and dynamics are not realistically reflected, how can we expect models to simulate cloud systems realistically,” asks Bhat.
Madhavan Nair Rajeevan, senior scientist at the National Atmospheric Research Laboratory at Tirupati, thinks that this is an important piece of research. Clouds are critically important for monsoon prediction as well as for climate change projections. But there is a lot of uncertainty about how cloud behaviour affects weather and climate, says Rajeevan.