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Thunderstorm dumps 235mm of rain as Mumbai sees back-to-back extreme weather events

A thunderstorm, described by meteorologists as massive and monstrous, deposited a whopping 235mm of rain in the city between 12am and 4am on Sunday, triggering two landslides, leaving several dead, and throwing civic infrastructure out of gear
By Prayag Arora-Desai, Mumbai
PUBLISHED ON JUL 19, 2021 12:15 AM IST

A thunderstorm, described by meteorologists as massive and monstrous, deposited a whopping 235mm of rain in the city between 12am and 4am on Sunday, triggering two landslides, leaving several dead, and throwing civic infrastructure out of gear.

This happened less than 48 hours after a similar high-impact weather event event resulted in the season’s highest daily rainfall (253mm) early Friday. But Sunday’s downpour proved to be more widespread, with 19 Brihanmumbai Municipal Corporation’s (BMC) monitoring locations (across 14 wards) recording extremely heavy rain — over 204.5mm — within just a few hours. While Colaba recorded only 13mm of rain on July 16, both Santacruz and Colaba monitoring stations saw rains over 195mm on Sunday.

Most of the rainfall was received between 12am and 2am, ward-level data shows. Environment minister Aaditya Thackeray even labelled the event as a ‘mini cloudburst,’ though the India Meteorological Department (IMD) did not term it as one.

Experts on the other hand said it was a thunderstorm event, but one whose impact is comparable to a cloudburst — which is described by the IMD as “any event where 100 millimetres of rainfall have fallen in an hour over a region that is 20-30 square kilometres in area.”

Meteorologists were once again stumped by the scale of the downpour, which is distinct from typical monsoon showers. Like Friday’s event, the one that took place in the wee hours of Sunday was not triggered by large-scale weather systems typically associated with monsoon rains, such as a deep layer of winds from the west over Mumbai, a monsoon low-pressure system over east or central India, or a vigorous offshore trough.

“Such rainfall, known as stratiform precipitation, is relatively steady and continuous on a day,” said Akshay Deoras, an independent meteorologist and PhD researcher at the University Of Reading, England.

Rather, the events observed in Mumbai were due to “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. “This is ideal for convective instability, particularly given the daytime heat,” said Deoras.

This analysis was seconded by Dr JR Kulkarni, a retired meteorologist from the Indian Institute of Tropical Meteorology (IIMT), who said, “There is a wind trough persisting from the Arabian Sea to the Bay of Bengal passing through Mumbai in which easterly and westerly winds are flowing, causing favourable condition for continuous convection at the low level over Mumbai and even parts in the southern Konkan belt. Because there is no low-pressure area over the Bay of Bengal, and because the monsoon trough is also not in its normal position, such events are difficult to predict.”

In the case of Sunday’s thunderstorm, the cloud cover seems to have started forming late Saturday over Raigad district, where the hilly topography would have driven the formation of thunder clouds, meteorologists said. The cloud cover — which amassed in size to touch a cloud-top height of 18kms (twice the size of Mt. Everest, and about 25,000 feet higher than the cruising altitude of commercial airlines) — then moved from the southeast to the northwest of Mumbai, causing much wider rainfall than the downpour on early Friday.

“The cloud-top height is a good indicator of the intensity of a storm. Furthermore, the vertical wind shear is significant, which is prolonging the lifespan of thunderstorms,” Deoras said, adding, “Such thunderstorms are uncommon for Mumbai or the west coast during an active phase of the monsoon, and in a month like July. The cloud top height of this monster thunderstorm is comparable to the one that produced rains on 26 July 2005.”

Despite the scale of the downpour, civic authorities said they received no warning of extreme weather from the IMD. “Our models did not see such an intense event, but an impact based forecast was put out at 1am warning people,” said Dr Jayanta Sarkar, head of the IMD’s regional meteorological centre in Mumbai.

Experts, however, once again criticised the IMD for the poor functioning of its doppler radar in Colaba, which could have been used to provide actionable information to civic authorities. However, the radar is currently producing images at intervals of between one to two hours (as against the optimum 15 minutes), which is too late for real-time disaster management.

At 10:30pm on Saturday, for example, the IMD put out a thunderstorm warning with chances of “moderate to intense spells of rain” for the Konkan coast. However, the Nowcast was based on a doppler image produced at 9:30pm. “It does not help stakeholders since they can’t figure out anything from such radars, except for looking into the past,” said Deoras.

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