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Rueful rainfall linked to aerosols

Read time: 4 mins
Photo : Arati Halbe, Gubbi Labs, through flickr

Nearly two-thirds of the Indian population depends on agriculture for their livelihood. Disruptions in rainfall pattern affect them all as most of the agricultural land is rain-fed. Over the second half of the 20th century, levels of grain production have fallen due to deficiencies in rainfall.  Are the errant monsoon rains linked to the rising levels of pollution? A study from the Indian Institute of Technology Bombay says so. The researchers, Mr Prashant Dave, Prof. Mani Bhushan and Prof. Chandra Venkatraman have discovered that aerosols cause aggravated breaks in the monsoon, which could significantly influence drought and impact agriculture.

Aerosols are fine solid particles, liquid droplets or a mixture of solid-liquid particles suspended in the air. Dust, sea-salt, particles emitted from burning fossil-fuels, vehicular exhaust—all contribute to the rise in aerosol levels in the atmosphere. Black carbon or soot, which absorbs sunlight, and compounds such as sulfates and nitrates, which scatter light, are the significant components of atmospheric aerosols.

Although previous studies have attributed increases or decreases in monsoon rainfall to changes in atmospheric aerosols, the relationship has not been examined, specifically in terms of causes and effects, with observational data. “The question we asked was, can we go beyond correlation and establish causation in a system as complex as the atmosphere to understand linkages between aerosols and monsoon rainfall changes using mainly observational data?” said Prof. Mani Bhushan.

In this study, published in Scientific Reports (from the Nature group), the researchers have used  aerosol levels and cloud properties derived from satellite data, and rainfall measured using ground-based instruments, during the years 2000 to 2009. They analysed this data to study the regional differences between the presence of aerosols and the corresponding precipitation.

Changes in weather patterns, though seemingly chaotic, are the result of a few visible and invisible fundamental processes repeating themselves. Meteorologists divide the study of the atmosphere into four groupings---micro-, meso-, synoptic and global---depending on the scale and the duration of these processes. Microscale events such as cloud puffs or gusts occur in an area within the radius of 1 km or less, while global scale events span over 1000 km and last at least a month.

This study examines processes at the mesoscale, spanning 10 to 1000 km, in regions over India experiencing high levels of aerosol particles and low levels of rainfall. On this scale, the interplay between moving air and water is the driving force behind precipitation. The sun’s radiation heats-up the earth's surface, which then heats the air just above it. Warm air rises, carrying evaporated water with it, and cools to form cloud droplets. These droplets conglomerate over time and fall to the ground under their weight.

However, the increase in the aerosol levels in the atmosphere disturbs the delicate equilibrium. Since aerosols absorb or scatter sunlight, the amount of sunlight reaching the ground decreases and the ground surface remains cool, while the layer in which absorbing aerosols are present gets heated up. As a consequence, instead of rising to form clouds, the water vapour diverges horizontally and gets dispersed. Along with this, the air stagnates leading to a loss of vertical motion of air and water; thus suppressing the rainfall.

The researchers of the study observed frequent rainfall breaks, lasting longer than seven days several times in a season, in areas with high levels of aerosols. If the pattern continues, the pauses could merge into a drought and prolong an existing drought condition. At the micro scale, a single drop of rain is formed by vapours around a nucleus such as a speck of dust. The researchers also studied if an increase in the levels of aerosols changed the size of individual raindrops in the cloud, leading to subsequent suppression of rainfall. However, they did not find a strong correlation between the two.

Thus, in regions of high levels of aerosols, the more likely reason for the repeated occurrence of low rainfall, according to the study, is the absorption of solar radiation by aerosols that prevents the upward movement of water vapour and air and increased horizontal divergence of moisture. Within a day of increase in aerosol levels, these effects manifest and last for two or more days.

“The study makes important linkages between air pollution and climate change, in regard to regional monsoon rainfall. It is important to understand how such disruptions would evolve, with expected future increases in aerosol emissions driven by human activities” concluded Prof. Chandra Venkataraman.

This study was supported by the Indian Institute of Technology Bombay, Centre of Excellence in Climate Studies (IITB-CECS) project of the Department of Science and Technology (DST), New Delhi, India