Climate crisis forces shift in forecasting approaches

On the night of June 27, a cloud cluster formed over Delhi with the monsoon still approaching the city and multiple weather systems at play in different parts of the country.

Yet, weather models could not appropriately predict the unprecedented amount of rain that would lash the city in the next few hours.

Around 1.30am on June 28, the India Meteorological Department issued a nowcast warning that said intense rain was approaching the city in the next few hours. Hours later, the city woke up to the highest rain it had recorded in a day since 1936.

Between only 5am to 6am, some stations in Delhi recorded nearly 100mm of rain. By 8.30am, Delhi had recorded 230mm — the highest in a day since 1936.

Such events are unfolding across the country with increased frequency, when large, rain-filled convective clouds form and dump copious amounts of rain over relatively limited areas taking the authorities by surprise.

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For instance, a similar cloud cluster formed over Rajasthan on Saturday, and earlier in parts of Uttar Pradesh last week. These clouds are also emerging almost every monsoon over the Western Himalayas, affecting the hill states of Uttarakhand and Himachal Pradesh.

However, the only way to capture these events, and issue warnings is through nowcasting — “forecasting with local detail, by any method, over a period from the present to six hours ahead, including a detailed description of the present weather”, according to the World Meteorological Organisation. Weather models, instead, are an assessment based on past and some present data.

With impacts of climate crisis becoming more severe and frequent, meteorologists are focusing their energies on tracking these large, convective clouds that have a possibility of causing flooding rains over a region.

The problem is partly driven by intense heat leading to very high moisture availability which contributes to instability and sudden convective activity.

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“When there are continuous, extremely warm atmospheric conditions for several days, the moisture holding capacity of the atmosphere increases and so immediately after that rain can be unprecedented. That is how climate change impacts,” explained M Mohapatra, director general, IMD on July 1 .

With every degree of warming, the moisture holding capacity of the atmosphere increases by 7%, he said.

For at least two weeks before the unprecedented rainfall in Delhi, the city recorded extreme heat with maximum temperatures topping 45°C, accompanied with warm nights over 30°C.

But chasing these clouds poses a challenge, and warning of their arrival a few days in advance is nearly impossible.

“In a climate change scenario, nowcasting is more effective. If there is a low-pressure system that can be definitely predicted in advance, but in situations where extreme events are occurring without influence of low-pressure area or other major weather systems, nowcast is the solution,” said Mohapatra.

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“Radars will be used for nowcasting. They can capture mesoscale events. And nowcasting will become one of the most important tools for forewarning. They are already being used more than earlier. For example, when Delhi event occurred, we issued a nowcast around 1.30am that intense rain will occur. We should get prepared to take action in 3-4 hours,” he said.

Explaining the process, Mohapatra said: “Nowcasting is basically extrapolation based on clouds and other observations. Radars provide us high resolution, infrared images. Models are also used. That is enough to tell us what to expect in the next few hours. We are increasingly seeing mesoscale events during monsoon periods which have a very localised impact.”

Currently, the weather department has three radars, and is now planning to add three more for Delhi to address such unprecedented rain events.

Explaining the reason for the frequency of such events, Roxy Mathew Koll, climate scientist at Indian Institute of Tropical Meteorology, said that warmer air holds more moisture for a longer time. So it may not rain for some days, when then suddenly dump a month’s worth of rain in a few hours.

“The Cloud Cluster That Unleashed the Delhi Rains 228 mm. That’s three times the month’s rainfall in just a few hours. Warmer air holds more moisture for longer time. So, it may not rain for several days but when it rains, it dumps a month’s or a season’s rain in a few hours,” he said in a post on X, sharing a satellite image of the cloud cluster over Delhi, on June 29.

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Tropical weather systems are extremely dynamic, and predicting them days in advance is difficult.

“The weather systems in the tropics are often small, fast-moving, and short-lived, involving significant energy. Predicting these systems has always been a challenge. Now, climate change is adding more energy, producing localized extreme weather events that state-of-the-art models find even more difficult to predict. Our monitoring is also imperfect. We need monitoring stations at the panchayat level, with data readily available. With these, high-resolution models that accurately represent the climate system, and AI-based support systems, we should be able to monitor and predict hyperlocal weather extremes like cloudbursts and flash floods,” Koll explained.

The unpredictability of these systems is particularly true over India due to its geography and location, Koll said.

In India, monsoon has become more erratic over the years.

“Monsoon has become more erratic. The variability has become larger now. There are studies establishing this important fact. Therefore prediction becomes a big challenge,” said M Rajeevan, former secretary, ministry of earth sciences.

HT reported on June 25 that recent data from IMD shows a trend of weaker monsoon rainfall in June, followed by prolonged, heavy rainfall in September. This shift is accompanied by rising temperatures during monsoon months and an increase in the number of dry days over eastern India.

This is apparent in this year’s monsoon too. There is only a 2% excess in rain over the country with 0% deficiency over east and northeast India; 10% excess over northwest India; 12% excess over South Peninsula and 6% deficiency over central India. Gangetic West Bengal, a rice growing region has recorded a 51% deficit during this period and east Rajasthan, an otherwise arid region has recorded 29% excess rain since June 1.

“Inaccuracies in forecasting will only increase. Models are not able to capture more localised weather phenomena properly which is becoming a huge challenge. Any big system like a big cyclonic circulation, a low-pressure area development is being captured by models. But smaller cyclonic circulations, or even sudden popping up of convective clouds over a region cannot be captured by models. That is why nowcast is helping more. Radars can help, they have a radius of about 100 km. Unfortunately we do not have many tools to address this problem,” said Mahesh Palawat, vice president, climate and meteorology at Skymet Weather.