IITM study tracks inter-seasonal rainfall variations over Mumbai

Mumbai records over 2200 mm during monsoon.

Every year during monsoon, residents of Mumbai brace for the many struggles they will face due to spells of heavy and extremely heavy rainfall. With an increase in urban flooding due to rising intense spells within a short span, it has become crucial to understand the rainfall characteristics over India’s financial capital in order to come up with improved mitigation measures for the maximum city.

A latest study, led by researchers from city-based Indian Institute of Tropical Meteorology (IITM), has tried to understand the morphology of clouds, rainfall dynamics and rainfall microphysics. It has traced spatial and temporal variation of clouds and rainfall during pre-monsoon and monsoon seasons over Mumbai.

Lead author Kaustav Chakravarty of the study, published in the journal Natural Hazards of Springer, said such scientific understanding is crucial for enhancing the city’s future urban flood forecasts.

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According to the India Meteorological Department (IMD), Mumbai records about 2200mm of rain between June to September. But in recent years, the city has been recording its annual rainfall quota within these four months.

Such a trend led to several meteorologists carrying out focused studies aimed at improving understanding of rainfall over Mumbai. Alongside strengthening of rain gauge network, a dedicated Integrated Flood Warning System (I-FLOWS) for real-time monitoring was installed during the 2020 monsoon season.

One of the findings suggests that the rainfall droplet size during pre-monsoon rain was larger than those received during the monsoons. Rainfall during the pre-monsoon season, between March and May, is often associated with convective activities.

“During pre-monsoon months, land heating is significantly high, resulting in a higher rate of evaporation. With strong updraft available along with weather conditions of a coastal location, rainfall droplet size during pre-monsoon was found to be larger than 1.5mm in diameter,” said Chakravarty, a scientist from IITM.

Rainfall and lightning observations during 2018 recorded at IMD’s Santacruz observatory and the Chhatrapati Shivaji Maharaj International Airport were primarily used in this collaborative research, done with experts from IMD, Cochin University of Science and Technology and University of Texas, USA.

The year 2018 was chosen as a maiden campaign-study initiated after the installation of Disdrometer, an instrument which reveals rainfall microphysics over a locality. This data was combined with observations from the S-band Doppler Weather Radar and Maharashtra Lightning Detector Network.

The most convective activity is triggered during evening hours, which also can be useful for city planners, administrations and people to avoid inconveniences. Associated lightning during these months were also recorded.

“Half of Mumbai’s rainfall recorded during pre-monsoon is due to easterly winds, even though it accounts for only 9 per cent of other factors, triggering rainfall over the city,” he said.

Normally, monsoon onset over Mumbai is around June 10. June generally records light to moderate intensity rain varying between 0 to 40m per day. Whereas in July, heavy spells, with rainfall intensity ranging between 40 and 180 mm per day, are common.

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