- In a study, researchers attempt to demystify the definition of extreme rainfall events in India.
- The study finds post-1980s, the increase in the extremes over large parts of India is due to an expansion in the average area covered by extreme rainfall but not the number of incidents.
- This means that after the 1980s, the likelihood of neighbouring areas getting extreme rainfall (collocated extremes) at the same time has also gone up.
Increasingly images of torrential rains triggering devastating floods and killing humans and animals are slowly replacing recollections of romance associated with Indian summer monsoons.
While extreme weather events are not new to the monsoon, India is seeing consistent intense events with climate change and man-made activities enhancing flood risk. In 2018, the Kerala floods claimed over 483 lives and displaced more than a million people, causing a loss of Rs. 310 billion (31,000 crores).
Read more: As floods repeat this year in Kerala, experts point to climate change.
This year, in 2019, in Kerala, June and July were months of inadequate rains, followed by a burst of intense rains in the early weeks of August, causing floods and landslides in the northern districts. Kodagu and Chikkamaguluru districts of Karnataka, which are in line with Kerala, had the same pattern. Avalanche, a river valley in the Nilgiris mountains in Tamil Nadu, received more than 900 mm of rain in one day. Vast swathes of land in the northeast, in Assam, were rendered infertile by the recent floods that claimed over 60 lives.
There has been a threefold increase in widespread extreme rain events over large parts of India, found a 2017 study, calling for exploiting the “predictability of these widespread extremes and in improving their forecasts”, given the massive economic losses due to extreme weather events related disasters.
The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate released on September 25, 2019, warned that with any degree of additional warming, events that occurred once per century in the past will occur every year by 2050 in many
regions, increasing risks for many low-lying coastal cities and small islands.
“The very definition of rainfall extreme affects the conclusions drawn and the way we forecast and manage floods,” said Arindam Chakraborty at Indian Institute of Science, Bengaluru, who with colleagues set out to demystify the definition of rainfall extreme.
In their recently released study, they find that, in the last six decades, large parts of India witnessed an increase in severe rainfall events since 1980. This increase is essentially an expansion in their size, which means an increase in the area receiving these intense bouts of rainfall, but not necessarily an increase in the number of incidents.
“We showed that post-1980s the increase in the extremes over central India is due to an increase in the average area covered by extreme rainfall but not numbers. Which means that after 1980s, the likelihood of neighbouring areas getting extreme rainfall (collocated extremes) at the same time, has gone up,” Chakraborty told Mongabay-India.
“If we add up, in square kilometres, the areas experiencing extreme rainfall, then we can see that area coverage has expanded,” Chakraborty said.
And the observation that large events over central India are mainly observed post–1980 coincides with the weakening of large-scale monsoon circulation and rapid warming of the Indian Ocean, they said.
Climate variability researcher Vimal Mishra, who was not associated with the study, said the findings of the present work are in agreement with the previous studies that show the increase in wide-spread extreme precipitation events in India.
“However, it is surprising that the frequency of these events did not change while the spatial extent (area covered) has been increasing. We expect a rise in the frequency of the extreme precipitation events with the warming climate,” Mishra of IIT-Gandhinagar’s Water and Climate Lab, told Mongabay-India.
Mishra said the duration of the events also needs to be factored in. “While there might not be a significant increase in the single day extreme precipitation events, multi-day extreme precipitation events are also increasing in the warming climate,” Mishra added.
Studying area over numbers to measure extreme rainfall events
Chakraborty and team analysed India Meteorological Department’s daily gridded rainfall data from 1952 to 2015 to revisit the definition of rainfall extremes. A rainfall event in a grid is counted as extreme if it occurs when the daily rainfall exceeds a threshold.
Using an algorithm they identified severe rainfall events that occur in adjacent grids (collocated).
“We showed that when ‘extreme’ is defined based on collocated grids, the number of extremes did not change during the past couple of decades. However, the increase in the area (reported as ‘number of extremes’ by previous studies) is due to the increase in the size of those ‘collocated extreme events,” explained Chakraborty.
As analysed in the study, out of the 20 large extreme rainfall events (ERE) that occurred during the study period, 90 percent of them are post–1980 and 65 percent are post–2000. The medium events show a rise in number mainly after 1990.
“The small events show a peak between the mid-1970s and the early 1990s. Over the study period, medium and large EREs show a significant rising trend. However, small EREs do not show any significant trend,” the authors write in the study.
Chakraborty said their study opens up a new angle to brainstorm on why these extremes occur.
“Although several studies have confirmed an increase in extreme rainfall, especially that over Indian summer monsoon domain, a convincing answer to the mechanism behind remains elusive,” he said.
For instance, in a 2019 study, researchers at IIT-Bhubaneswar pointed to a confluence of factors that powered the extreme rainfall in Kerala in 2018, triggering the worst flood in a century in the country. They pointed to a surge in moisture packet helped by the Arabian Sea monsoon flow and depression over the Bay of Bengal and features of the Western Ghats mountains triggered the torrential rains that battered Kerala in August 2018. They said a similar set of actors could be at play in creating chaos in the 2019 rainfall event in the coastal state.
Read more: Kerala floods: Unpacking the reasons for heavy, sustained rainfall.
Understanding the spread of extreme rainfall for flood management
Chakraborty said the impact of extremes also differs if several extremes are experienced simultaneously in neighbouring areas compared to scattered events. We wanted to understand these in our study, said the researcher.
This has major implication in flood forecasting and management. Acting together, severe rainfall over adjacent areas (a large collocated extreme event) should have a more intense impact on flood than when they are scattered.
“The result is directly relevant to disaster mitigation and management. In general, their policy should be tuned to such information. In terms of forecasting, we should now test whether weather models can capture the collocated extremes,” noted Chakraborty.
Disaster management expert Anshu Sharma agrees that forecasting systems are crucial to disaster management: short term predictions for preparedness and relief and long term for planning.
“If people know what’s coming, and when, and how much, they can take better preemptive action. Importantly, it can boost confidence in warnings and avoid anxiety levels that are hard for us to imagine. The warnings need to be hyper-local and precise though,” said Sharma, co-founder of SEEDS, a not-for profit organisation that enables community resilience through practical solutions in the areas of disaster readiness, response and rehabilitation.
Further, the analysis says that all severe rainfall events in the study area coincided with the development of low-pressure system in that region. “And global weather patterns influence extreme rainfall in India,” added Chakraborty.