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Monsoon forecast: A shade below normal

New Delhi, April 19: Rainfall this year will be 5 per cent below average, weather scientists predicted today after adopting a new forecasting strategy and abandoning their decades-old, trouble-ridden method.

Releasing the first forecast for 2007, senior Indian Meteorological Department (IMD) officials said the rainfall for the entire country is likely to be 95 per cent of the average monsoon rainfall of 89 cm.

This means the rainfall will be near-normal. According to the IMD’s definition, rainfall within 10 per cent of the long-period average is normal.

The long-range forecast of the monsoon is based on statistical links between rainfall in India and weather “predictors” such as sea surface temperatures, air pressures and wind conditions from across the world. Such forecasts do not yield information about how rainfall will be distributed in different regions or through the four-month monsoon season.

The new method, called “ensemble-based forecasting”, uses multiple models and several combinations of eight weather predictors, two of them not used in previous forecasts.

“It is superior to the traditional forecasting method,” said Madhavan Rajeevan, director of the IMD climate studies centre in Pune and head of the long-range monsoon forecasting team.

Monsoon forecasts have been off the mark in recent years. Last year, the IMD predicted 92 per cent of the average rainfall, but it was 100 per cent. In 2004, actual rainfall was 13 per cent below average and off the mark.

The new method was used on a trial basis in 2005 and 2006 and delivered correct forecasts, Madhavan said.

However, it still cannot account for the drought of 2002, he conceded. Scientists will be able to evaluate the reliability of the new model only after two or three years of forecasts.

In another departure from tradition, the ensemble-based forecasting technique was validated through scientific peer review and publication in an international journal before it was applied for operational forecasts. The earlier forecasting methods had been evaluated in-house by the IMD and in certain instances by external experts only after they had been introduced for forecasts.

“We consulted experts at the Indian Statistical Institute and the Indian Institute of Science in Bangalore before taking a decision to introduce it for a real forecast this year,” Madhavan said.

The IMD said it would update its full-season forecast and issue separate forecasts for the rainfall in July — important for crops — and the rainfall across the four geographical regions this June.

One predictor is Pacific Ocean surface temperatures — temperatures higher than average can depress the monsoon — also known as the El Nino effect. But, the IMD said, the warm temperatures disappeared around February this year, and current weather trends suggest surface ocean temperatures are likely to drop in the coming months — also known as the La Nina effect.

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