Multiple monsoonal weather systems behind Delhi's record rainfall: IMD

Multiple large-scale monsoonal weather systems created conditions for mesoscale convective activity over Delhi NCR, resulting in intense thunderstorms and heavy rainfall during the early hours of June 28, the India Meteorological Department said.

This activity was supported by thermodynamic instability in the atmosphere, which is favourable for thunderstorms, it said in a statement explaining the reason for the record rainfall that brought India's national capital to its knees.

The India Meteorological Department (IMD) said the monsoon was stalled over eastern India for an extended period due to weak monsoon currents over the Bay of Bengal. From June 25 onwards, the monsoon currents strengthened with the development of an upper air cyclonic circulation over the central and northern Bay of Bengal at middle atmospheric levels.

At the same time, the Southwest Monsoon currents over the Arabian Sea intensified, with increased wind speeds and the strengthening of the offshore west coast trough. An east-west shear zone formed across northern peninsular India at middle atmospheric levels.

On June 26 and 27, strong southeasterly winds from the Bay of Bengal began blowing towards northwest India, including Delhi. A low-pressure area developed over the northwest Bay of Bengal and the adjacent coasts of north Odisha and Gangetic West Bengal around midnight on June 27. This further enhanced the flow of warm and moist air from the Bay of Bengal towards northwest India, including Delhi, the IMD said.

It said an upper air cyclonic circulation was present over northwest Madhya Pradesh at lower levels, and another one over Haryana and the neighbouring regions at middle levels, leading to the convergence of warm moist winds up to middle levels.

An anticyclone centered to the northeast of Delhi at 9-12 km above sea level provided strong upper-level divergence, supporting lower-level wind convergence.

"Under such large scale monsoonal synoptic weather systems prevailing over the region, mesoscale (10-100 km) convective activity occurred over Delhi NCR leading to intense thunderstorm and heavy rainfall activity during early hours of June 28. This mesoscale activity was supported by thermodynamic instability in the atmosphere favourable for thunderstorm," the statement said.

The Safdarjung Observatory, the city's primary weather station, recorded 228.1 mm of rainfall in the 24 hours ending at 8.30 am on Friday, more than three times the June rainfall average of 74.1 mm and the highest for the month in 88 years -- since 1936.

According to the IMD, the weather station at Lodhi Road logged 192.8 mm of rainfall in the 24 hours ending at 8.30 am, Ridge 150.4 mm, Palam 106.6 mm, Delhi University 139 mm, Pitampura 138 mm, Pusa 89 mm, Mayur Vihar 75 mm and Tughlakabad 70.5 mm.

The IMD, which defines very heavy rain as rainfall amounting to between 124.5 and 244.4 mm in a day, said later in the morning that the monsoon has arrived.