Opinion - News Analysis

Monsoon and the weatherman's woes

N. Gopal Raj

Rainfall prediction is a risky business as local phenomena can upset all calculations.

THE FURY of the monsoon has been all too evident along the west coast in recent days, with Karwar in Karnataka receiving over 40 cm rain in a single day and Ratnagiri in neighbouring Maharashtra receiving one and half times as much a day later. The torrential downpour has inevitably revived memories of the ferocious rain that devastated Mumbai on July 26 last year.

Heavy rain in Mumbai, India's financial nerve-centre and one of the world's most populous cities, has the potential to cause enormous human suffering and also extract a huge economic toll. While July 26 may have been exceptional, storms capable of severely disrupting life in a city that never sleeps are not unusual. Between 1950 and 1990, almost every year there was at least one day during the monsoon when the city received 15 cm or more of rain, points out P.V. Joseph, a meteorologist retired from the India Meteorological Department (IMD).

Based on over 100 years of rainfall data, Rajendra Kumar Jenamani and fellow scientists at the IMD have computed that Mumbai had a 35 per cent to 48 per cent chance of being deluged with 20 cm or more of rain on at least one day during the monsoon. There was a 11 per cent to 13 per cent chance of the city being lashed with over 30 cm of rain some time during the monsoon, they said in a paper published in the latest issue of the journal Current Science.

The rain of July 26, 2005, were unprecedented. The Santacruz area received 94 cm of rain in a 24-hour period and Vihar Lake, 15 km to the north-east, nearly 105 cm of rain. At Santacruz, 70 per cent of the rain fell in just six hours, between 2-30 p.m. and 8-30 p.m., according to the IMD scientists.

"Mumbai is vulnerable to intense rainstorms during the monsoon season," they remarked in their Current Science paper.

That sort of vulnerability makes the ability to predict such storms all the more important. A paper that has been published recently by scientists of the National Centre for Medium Range Weather Forecasting (NCMRWF) at Noida near New Delhi holds out hope that powerful supercomputers running the right sort of model simulating processes in the atmosphere could help forewarn even of a storm as exceptional as the one that befell the city on July 26, 2005.

The problem

What makes such prediction difficult is that a combination of a localised storm system embedded in a large band of rain-bearing clouds can be what produces such excessively copious rain. Computer models may indicate rain from the cloud bands but miss the localised storm, thus failing to pick up the true magnitude of the impending rain.

In a paper published in Current Science, A.K. Bohra and fellow NCMRWF scientists compared how well the Numerical Weather Prediction models at their own centre, the European Centre for Medium-Range Weather Forecasts, the U.K. Meteorological Office, the U.S. National Centers for Environmental Prediction, as well as the Japan Meteorological Agency reproduced the July 26, 2005, rain over Mumbai.

The U.K. Met. Office model was the most successful, showing the highest rainfall and being closest in geographical location. Based on weather parameters of July 25, the model predicted 28 cm to 32 cm rain near Mumbai during a 24-hour period. With the initial conditions of July 26 morning, the model showed 20 cm to 24 cm of rain near Mumbai.

More impressively, when the U.K. Met. Office carried out a re-run with their higher resolution Southern Asia Model, they were able to get rainfall in excess of 80 cm.

The NCMRWF models fared badly as did the one at the European Centre. The NCMRWF model indicated just two cm of rain over Mumbai and 4 cm further south when using the initial conditions of July 26 and its skill deteriorated as the lead time increased. A higher-resolution model at NCMRWF fared marginally better. Similarly, the model at the European Centre showed only 4 cm to 8 cm rain near Mumbai.

The Japanese model showed 16 cm to 20 cm rain south of Mumbai and the U.S. model 12 cm to 16 cm of rain north of the city.

The U.K. Met Office models appear to respond better to conditions in and around India, thereby simulating monsoon circulation over the country more correctly, says Ravi Nanjundiah of the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science. But unless models could reliably reproduce exceptionally heavy rainfall events and not just the July 26, 2005, event over Mumbai, their output would have little predictive value, he cautions.

The U.K. Met. Office and NCMRWF were jointly diagnosing the performance of their models over the Indian region during monsoon 2005, say Dr. Bohra and fellow scientists in their paper. The two organisations were seriously considering a strategic partnership for improving the predictive capability over the Indian monsoon region, for tropical cyclone track and intensity prediction as well as in a number of other areas, they added.

Lack of adequate computing resources, dearth of skilled manpower, and need for high-quality observations in near-real-time had retarded the progress of numerical weather prediction in India, they lamented.

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