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The rainfall brought in by monsoon winds to India is naturally highly variable over a particular region but now the variations are turning extreme, even over small areas such as cities. One such instance is visible over the national capital Delhi in the southwest monsoon of 2024.
The district of West Delhi has received 99 per cent less rains than the normal between June 1 and July 10, according to data from the India Meteorological Department (IMD). The adjacent district of North West Delhi experienced excess rainfall of 122 per cent.
In absolute terms, West Delhi has recorded 1 mm rainfall against 183 mm recorded in North West Delhi. The distance between IMD’s automatic weather station (AWS) in Pusa, West Delhi, and the one in Pitampura, North West Delhi is a mere 12 km.
IMD has a network of 12 AWS, one automatic rain gauge, four part time observatories and five synoptic manual observatories to observe the weather.
The rainfall distribution in the rest of the capital has also been skewed as North East Delhi district has deficit rainfall of 64 per cent while not too far away, North Delhi and New Delhi have 84 per cent and 75 per cent more rains than the normal, respectively.
South Delhi has a deficit rainfall of 23 per cent while South West Delhi (-11 per cent) and Central Delhi (-9 per cent) have received what the IMD defines as ‘normal rainfall’ between June 1 and July 10. The total rainfall of Delhi stands at 11 per cent excess as on July 10.
The monsoon arrived in Delhi with great strength on June 27-28 in the presence of multiple other weather systems, including a western disturbance and incursion of moisture from the Bay of Bengal and Arabian Sea, leading to a ‘mesoscale convective system’, according to IMD. A mesoscale convective system is a combination of thunderstorms which could carry cloud cover of up to 100 km.
This caused extreme rainfall in the early morning hours of June 28, which deluged the city. IMD’s observatory in Safdarjung recorded a single day rainfall of 228.1 mm, the second highest June rainfall on record. The flooding and related incidents killed 14 people, according to media reports. Since the deluge, Delhi has received intermittent rainfall.
While the reasons for this dramatic distribution of rainfall over such a small area still needs to be studied, scientists point to complex interactions between atmospheric warming, differential land surface heating, surface friction faced by winds leading to them slowing down along with moisture, green cover, presence of water bodies and pollution as factors that can influence the distribution of rainfall over a city.
“Some of these factors can increase rainfall in some areas such as the presence of green cover and aerosols (pollution) and so would excess surface heating while other factors such as direction and strength of winds could shift the rainfall from one place to another,” V Vinoj, associate professor at the School of Earth, Ocean and Climate Sciences at the Indian Institute of Technology (IIT), Bhubaneswar, told Down To Earth (DTE).
Heating of land surfaces in the presence of moisture leads to the creation of low pressure areas which has the ability to cause rainfall. The surfaces of buildings create friction and resistance for the flow of winds which can slow down, holding moisture at certain places.
“Pollution causes something called as ‘aerosol invigoration’ which leads to the formation of thunderstorm clouds. Given sufficient time, such clouds can grow and cause thunderstorms and heavy rainfall in certain areas”, Vinoj added.
All these factors are further influenced by atmospheric warming and local instabilities. In turn, the factors can also influence warming, which is visible in the urban heat island effect.
“To delineate the exact impact of these factors and their correlations with each other, detailed city- based studies need to be carried out and each city needs to be dealt with uniquely,” says Vinoj.
He and his colleagues at IIT Bhubaneswar are studying various cities such as Bhubaneswar, Bengaluru and Dehradun to understand how the combination of regional weather and climatic changes along with the impacts of urbanisation are affecting day to day weather and climate over longer time periods at the smallest of scales — cities in this case.
“The impacts of climate change and urbanisation can either amplify or suppress each other, leading to large variations in different aspects of weather such as temperatures and rainfall. But we do not currently know how this happens. It is a new and active area of study for India and the world,” said Vinoj.
But some insights have been found. For instance, in Bhubaneswar, the areas with a little forest and small hillocks have received more rainfall than the rest of the city. “Even small variations in topography, presence of green cover and water bodies can lead to large variations in the spatial distribution of rainfall and is often overlooked. We have found — and some other studies also indicate — that there is a possibility of redistribution of rainfall due to urbanisation, especially in the downwind areas,” he noted.
The direction from which the wind over a city or any region originates is known as the ‘upwind direction’ and the direction towards which wind flows is known as the ‘downwind direction’.
Another study on Kolkata published in the journal Atmospheric Chemistry and Physics in January 2024 found that urban land use enhanced and even induced storms through thermodynamic instability in the atmosphere, which enhanced mean monthly rainfall by 14.4 per cent.
The modelling study also found that the induction of the storm happened due to the urban surface’s disturbance of the winds flowing close to the surface, leading to dynamically produced kinetic energy.
The primary reason for the lack of such microclimatic studies over cities is the lack of observational data. “Most of our current studies are based on computer models. But even they require certain amount of observational data, without which they will not be able to recreate the real world occurrences,” said Vinoj. In fact, current models also do not have the necessary resolution to be able to predict the spatial distribution of rainfall at the most granular level.
Satellite observations are made only once or twice over a city in a day. Weather stations in most cities are also sparse. Delhi has 22 automatic and manual weather stations to measure rainfall and temperature, but many smaller cities such as Bhubaneswar have far less number of stations.
“What we perhaps need is a city-based weather radar that would be constantly measuring the rainfall and other parameters over a city,” Vinoj explained. Weather radars such as doppler radars can measure rainfall and winds at the minutest of scales and are currently deployed to monitor and track tropical cyclones along the coast or cloud bursts in the mountains.