How do aerosols affect the Indian monsoon?

Published: February 11, 2019
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With rising pollution levels, small particles suspended in the air, such as dust, soot, and organic matter are also increasing. These particles, called aerosols, are known to affect the formation of clouds that bring rain, besides affecting our health. Clouds influence the climate on our planet as they reflect some of the Sun’s radiation, thus regulating how much of it reaches the Earth. However, the exact extent to which aerosols affect the climate in a region is uncertain as there are variations in aerosols and the clouds at different times and places.

In a recent study, researchers from the Indian Institute of Technology Kanpur, Pacific Northwest National Laboratory, USA, University of Hyderabad, and Indian Institute of Technology Delhi have tried to understand how aerosols affect the Indian monsoon season. The results of the study were published in the journal Nature Communications. The study was partially supported by the University Grants Commission, Department of Science and Technology, and the Ministry of Earth Sciences, Government of India.

The researchers of the study used satellite data that contains information about cloud properties, such as cloud fraction, cloud optical thickness and cloud top pressure. It also has data on the amount of solar radiation that gets reflected from the top of the atmosphere and the surface of the Earth. This data was collected over a period of 15 years, from 2002 to 2016. In particular, the researchers studied convective clouds that bring monsoon rains, and are formed by the warm, humid air that rises from the ground. The researchers identified the central region of India, including the states of Madhya Pradesh, Bihar, Jharkhand, and Uttar Pradesh, for the study as these states lie in ‘extreme’ water scarcity zones as the region lacks the infrastructure for regular water supply.

When moisture condenses around particles, such as aerosols, cloud droplets are formed which join together to form a big conglomeration that is traditionally called the cloud. Too many aerosols result in numerous such cloud droplets, which take a long time to grow into rain. Hence, clouds hang in the sky for longer, before they rain, and reflect a higher amount of the Sun’s radiation into the atmosphere, resulting in a cooling effect on the surface of the Earth directly below the cloud.

During the monsoon months, the warm, humid air that exists above the polluted Earth’s surface rises and adds more cloud droplets to the existing cloud, which grows taller into the upper, colder layers of the atmosphere. At freezing temperatures, ice starts forming within the cloud, and the latent heat of freezing is released, which further promotes the humid air to rise, and this cycle continues. The result is a large convective tower cloud that brings heavy rainfall. Thus, the presence of high amounts of aerosols during the monsoon months lead to the formation of taller clouds with a large number of ice particles and, eventually, heavier rainfall.

The researchers of the current study, in their analysis of the satellite data, found that such a phenomenon occurred over central India. They observed that with the increasing population of aerosols in the atmosphere in these regions, there was an increase in stratiform clouds and the amount of solar radiation reflected from them. The researchers validated their observations with another dataset in which thin clouds were unlikely to be mistaken for aerosol. The results reaffirmed the correlation between aerosol density and reflection of solar radiation from clouds observed in the initial analysis.

“Meteorology can also affect aerosol-cloud data analysis. For example, high wind flow can cause an increase in both clouds and aerosols during monsoon. We re-did the correlation analysis between aerosol and cloud properties within narrow meteorology ranges for different variables, and found that the associations persisted”, say the researchers, speaking about ruling out other factors that could lead to the observed correlation between aerosol and clouds.

The researchers corroborated their findings by running two numerical simulations. The simulations recreated the realistic cloud population occurrence spanning 5 days in August 2011, where one simulation had a high amount of aerosol particles imitating the real scenario and the other had a lower amount of aerosol particles. Larger quantities of water droplets and ice particles in the clouds were observed when the aerosol concentration was high. Many thunderstorm clouds also formed frequently with an increase in aerosol levels.

Stratiform anvil clouds associated with convective thunderstorm clouds that are flattened at the top as the warmer air in the higher layers of the atmosphere prevents them from rising further, are increasingly formed when the levels of aerosols are higher. These clouds are thicker and broader than during low aerosol conditions. The many long-lasting stratiform anvils reflect most of the solar radiation into space, having an enhanced cooling effect on the surface of the Earth. “Location of clouds is important because if the cloud is present at a higher altitude, it will have more ice particles and fewer water droplets in it. It affects the frequency, intensity and spatial pattern of rainfall”, explain the researchers.

In highly polluted areas, this phenomenon results in a reduction in the maximum temperature during the day. Ironically, this is an indication of global warming, say the researchers. “Our finding shows that aerosol-cloud interactions are strong enough to affect this range. This interaction can further affect the predicted warming trends in the future. It will also influence the surface energy of the Earth since the clouds trap some of the solar radiation reflected from the Earth’s surface for longer. All these taken together, when accumulated over large time scales, say decades, can have a large impact on the clouds, rainfall, and radiation interactions over India”, they warn.


‘Two’ good of a time? A case for two time zones in India

Written for Research Matters (Publication link)

It’s no fun stepping out of your office into the dark at 5 o’clock in the evening, nor is it pleasant to have sunlight on your face disturbing your early morning slumber at 4 o’clock. However, if you are in any of the states in the North East, this is the reality! The current Indian Standard Time (IST) affects the lives of people here as daylight dawns and ends way earlier than the typical 9-5 working hours. Although these states have long demanded a separate time zone, there has been no implementable solution yet, and in 2017, the Guwahati High Court dismissed a public interest litigation requesting it.

In a recent study, researchers from the CSIR-National Physical Laboratory have once again looked at the possibility of having two time zones for India. They propose to introduce a new time zone, called IST-II, for states on the east of West Bengal and also have suggested a feasible way to implement it. The study has been published in the journal Current Science and funded by the Department of Science and Technology.

“Implementation of IST-II in these states would not only enhance the efficiency of the populace as their daily working hours get synchronous to the circadian clocks but would also save a significant amount of energy”, argue the researchers.

The world is divided into twenty-four time zones, each with a longitudinal width of 15 degrees. As one moves eastwards, one hour gets added per zone. Many countries span across time zones, like the USA, which has nine time zones. However, although India is almost 29⁰ longitudinally wide, it has a single time zone. This fact impacts the north-eastern states where the day starts as early as 4 AM in the summer and ends by 4 PM in the winter. Since our body is tuned to sleep during the dark and be active when there’s light, the fact that the daylight time and the clock are out of sync affects the productivity of those living in the northeastern states.

In the current study, the researchers compared the sunrise and sunset timings of ten regions across India, starting from the easternmost Dong in Arunachal Pradesh to the westernmost Ghuar Mota in Gujarat. “The Sun rises earlier in the east and, for many months, the sunrise is almost two hours earlier than the western regions,” remark the researchers. If a person in such regions followed the Indian Standard Time, he/she would have already spent about five hours of daylight before beginning work and would step out when it’s already dark.

The researchers propose to address this issue by creating IST-II, a new time zone that adds one hour to the existing IST. They suggest that the current IST (UTC + 5:30 h), represented by the longitude passing through 82°33′E, cover the regions falling between longitude 68°7′E and 89°52′E. The second time zone, IST-II, represented by the longitude passing through 97°30′E, should encompass the regions between 89°52′E and 97°25′E. The researchers suggest the north-eastern states of Assam, Meghalaya, Nagaland, Arunachal Pradesh, Manipur, Mizoram, and Tripura, as well as the Andaman and Nicobar Islands, to follow this time zone.

The study also puts forth a demarcation line between the two time zones. “The demarcation line has to be the border between West Bengal and Assam, where the country has the narrowest spatial neck,” say the researchers.

This division will also help reduce the logistical challenges for transportation systems like the railways. In India, the train timings and signals are not fully automated yet. Since the proposed IST-II region has only two train stations, it would be easy to manually change the times on the trains passing through and avoid any mishaps, argue the researchers.

Adding a new time zone not only could help improve the lifestyle of people in these states but also has added advantages. For example, the researchers state that since most of the work would then be done in daylight hours, there will be a significant reduction in electric consumption in these states. “The estimated annual energy saving comes out to be 2×107 kWh, which is significant considering the fact that these states are already short on electric power”, they say.

The study revisits the idea of having two time zones in India, akin to the ‘Bombay Time’ and ‘Calcutta Time’, established by the British in 1884. The researchers state that the current timekeeper of India, the National Physical Laboratory located in New Delhi, is well equipped to maintain two time zones if another such lab is established in the region that falls under IST-II.

“Since NPL-New Delhi already has the required technical expertise, duplicating this facility is just a matter of availability of appropriate funds and space”, they point out.