‘Mini cloudbursts’ increasing by 10% every year, particularly on western coast: Researchers

The morning after Mumbai got battered by record-setting rain — which flooded low-lying areas, triggering landslides, house collapses and electrocutions that killed at least 31 people — state environment minister Aaditya Thackeray attributed the deluge to “mini cloudbursts in a short span with full fury of nature”

The morning after Mumbai got battered by record-setting rain — which flooded low-lying areas, triggering landslides, house collapses and electrocutions that killed at least 31 people — state environment minister Aaditya Thackeray attributed the deluge to “mini cloudbursts in a short span with full fury of nature”. Ongoing research by meteorologists indicate that mini cloudbursts — sudden, heavy rain spells of the kind that occurred over Mumbai early on Sunday — are increasing by around 10% every year.

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The scale of the weekend disaster can be better understood by looking at the civic body’s ward-wise hourly rainfall data for Saturday-Sunday. When almost all monitoring locations received barely any rain in the hour between 11pm and midnight, the subsequent two hours saw torrential downpours clocking between 100 and 150mm on average. Several locations saw between 90 and 110mm of rain in just a single hour, causing damage to live and property that would be akin to a cloudburst, even though the India Meteorological Department (IMD) didn’t declare one.

The term “mini cloudburst” (MCB) was first used in 1999 by scientists CS Ramage and Thomas A Schroeder while studying high rainfall events at Mount Waialeale in Hawaii (one of the world’s wettest regions). However, Ramage and Schroeder did not actually quantify the amount of rainfall required for an event to be classified as such.

This happened in 2018, when a four-member team from Pune’s Indian Institute of Tropical Meteorology (IITM) and Texas A&M University-Corpus Christi, defined MCBs as short-term, but intense rainfall events characterised by 50mm (or more) precipitation in two consecutive hours.

The paper, led by principal investigator Nayana Deshpande, was published in 2018 in the International Journal of Climatology, a peer-reviewed journal of the Royal Meteorological Society. In light of the downpour in Mumbai recently, their findings and continued research hold warning signs for the city’s climate future. The same researchers, who spoke about the yearly 10% rise in MCBs, told the Hindustan Times on Monday that the frequency of such high-impact rainfall events seems to be increasing particularly over peninsular India, including the western coastline.

Earlier analysis of hourly rainfall data between 1969 and 2015 across 126 rain gauges revealed that India records 200 MCB events a year on average, and that their frequency along the west coast seems to be increasing. Between 2005 and 2015, India saw roughly 1,100 MCBs, while Mumbai on average saw above five such events in a year.

While Deshpande (and others) do not present similar data for subsequent years, ongoing work by her team indicates that the frequency of these events has been increasing even after 2015. This year alone, there have been at least three such events in Mumbai, on June 9, July 16 and July 18.

“The specific number, location and intensity of MCB events in more recent years will be published in a paper sometime next year. However, we can say even after 2015, the number of mini cloudbursts in India seems to be increasing by an average of 10% year on year. This is only a marginal increase in absolute terms, taking into account inter-annual variability of rainfall and the fact that we are only considering four months of the monsoon. But it is nevertheless worrisome. We have seen two back-to-back events in Mumbai last week. As MCBs become more frequent in the long-term, it means more flash floods, more landslides, more problems for disaster relief and so on,” said Dr JR Kulkarni, one of researchers.

The mini cloud bursts that lashed several parts of Mumbai on Sunday, Hindustan Times had reported, were the result of “convective precipitation”, triggered by a rapid “in situ” convergence of moisture-laden clouds that were formed as easterly winds in the mid-atmospheric levels transported dry air over warm and moist air in the lower levels, brought by westerly winds. The cloud structure responsible for the deluge touched a height of almost 60,000 feet -- more than twice the cruising altitude for most commercial airlines.

While rainfall exceeding 100mm in an hour — a phenomena largely associated with cloud conditions in the Himalayan region and which leads to flash floods — has already been categorised as cloudburst events, according to the IMD, so far there have been no categories to define intense, but short bursts of rain.

M Rajeevan Nair, secretary, ministry of earth sciences, said, “This is certainly a plausible observation. We have also noticed that such kinds of extreme rainfall events, of 50mm or more in two hours, are increasing in multiple meteorological subdivisions. At the moment there is no need to introduce another official term, and when required it will be done globally by the World Meteorological Observation. But the important thing is that we are aware of the impact caused by such accidents. The IMD is working at various levels to fine tune flood warning systems to mitigate the damage.”