Clouds hold the key to predicting monsoon and climate change, but there is very little we can say with certainty about them
UNDERSTANDING CLOUDS, that cover 60 per cent of the planet, is essential to understanding weather. For one, they produce rain, snow, hail and lightning and, therefore, hold clues to the microphysical processes in the atmosphere; two, they modulate the planetary energy budget (the amount of solar energy circulating in the planet) and the hydrological cycle; and three, they explain cloud-radiation feedback, or the dynamics of clouds’ interaction with solar radiation, which is necessary to predict climate change.
Additionally, understanding them could help enhance artificial precipitation through cloud seeding. Our capacity to predict lightning, which kills thousands every year, too would improve if we understood clouds better.
These floating entities can be studied well only by using remote sensing methods like satellites, radars and aircraft. So far, our understanding of clouds is very rudimentary.
We know that they are a mass of very tiny droplets of water, one-hundredth of a millimetre in diameter, or ice crystals floating in the atmosphere, and are formed when water vapour condenses on a condensation nuclei, or an aerosol. As the droplets collide, some grow larger and start to fall. If the cloud is dense enough to form droplets greater than one tenth of millimetre in diameter, the droplets reach the ground as rain.
All clouds do not precipitate, and even in the ones that do, the precipitation efficiency varies. Some clouds give a drizzle, others torrential rain. In numerical weather prediction models (these are mathematical models which take into consideration dynamics and physics of the atmosphere) to predict rain, representation of clouds and the precipitation processes is inadequate. We are not certain about the number of aerosols in a cloud or what a change in one variable might do to others. Most of the errors in predicting rainfall come from this inadequate representation.
Even in coupled climate models, which take into account factors relating to oceanic processes as well as atmospheric circulations, improvements are needed to simulate monsoon conditions. This type of model requires a simulation of three dimensional cloud distribution and its effect on the atmospheric radiative budget.
Clouds modulate planetary energy budget as they interact with both solar radiation coming from the sun and infrared radiation emitted by the earth and its atmosphere. The largest uncertainties associated with climate change projections are related to clouds.
Clouds normally scatter solar radiation, but absorb infrared radiation. Low clouds scatter more solar radiation than they absorb infrared radiation, and thus have a cooling effect. However, high clouds have a warming effect because of high absorption of infrared radiation. With global warming, some prediction models say we are likely to have more low clouds, while others predict more high clouds. If we witness an increase in low clouds, we should expect reduction in global warming.
Clouds, thus, have an important role in long-term climatic changes either directly through their impact on the radiative fluxes, or
indirectly through their interaction with other variables such as atmospheric temperature, pressure, surface temperature and humidity. Understanding the cloud-radiation interaction is, therefore, essential to accurately simulate climate change patterns. A difference in cloud distribution can affect the simulation of surface energy budget and, thus, surface temperatures. However, quantifying the radiative impacts of clouds is quite difficult. For instance, water droplet distribution decides how reflective a cloud will be, but this distribution is difficult to measure.
Initiatives to decode cloud
In 2009, the Union Ministry of Earth Sciences launched a programme to understand clouds and to improve their representation in weather and climate models. The Cloud-Aerosol Interaction and Enhancement of Precipitation is a national programme targeted to understand the interaction between aerosols and clouds and the microphysical properties of clouds. Under this programme, many observational exercises have been conducted, including aircraft observations. In the next three years, more exhaustive observational campaigns will be conducted to understand the precipitation enhancement processes (artificial rainmaking).
The establishment of a high-altitude cloud observatory at Mahabaleshwar by the Indian Institute of Tropical Meteorology, Pune, in 2014 was another milestone in weather studies in India. Since the observatory is at a height of about 1.2 km, it works as a natural laboratory. Monsoon clouds forming in the valley pass through the various instruments kept at the observatory, allowing microphysical observations.
These initiatives have added to our understanding of clouds, but there is still a long way before we can claim to have decoded the internal mechanisms of clouds.
Clouds can be studied well only by using remote sensing methods like satellites, radars and aircraft. So far, our understanding of clouds is very rudimentary
MRAJEEVAN Weather scientist and secretary, Ministry of Earth Sciences, Government of India