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Monsoon model for early forecast

New Delhi, Dec. 2: A new method to forecast the monsoon might give the nation at least eight months’ advance notice about likely rainfall behaviour instead of just the two months available through current forecasts.

Scientists at the Indian Institute of Science, Bangalore, have devised a forecasting model that they claim relies on rainfall patterns in the past to predict future rainfall.

“It can give us a fairly reliable qualitative picture of the monsoon performance nearly a year in advance,” said R.. Iyengar at the Centre for Atmospheric and Oceanic Sciences at the institute who has developed the model jointly with a colleague, S.T.G. Raghukanth.

The new model has attracted special attention because it successfully “predicted” last year’s drought in India that had not been envisioned by any other monsoon forecasting model.

Iyengar used statistics to crunch all recorded rainfall data available for the past 130 years to come up with a model that “satisfactorily predicts” the total rainfall over India. The model had used only past rainfall data to predict the drought of 2002.

“This approach looks promising,” said J. Srinivasan, head of the Centre for Atmospheric and Oceanic Sciences. “After the failure of existing models to predict last year's drought, we’re looking for more effective models,” he said. “In fact, a successful prediction of the drought of 2002 might become a touchstone for future forecasting models.”

Iyengar cautions that the new model will need improvement to achieve better accuracy of forecasts. “In monsoon forecasting, 100 per cent accuracy is impossible, we’ll need to work with some uncertainty,” Iyengar said.

The standard long-range monsoon forecasting model that the Indian Meteorological Department (IMD) had used through the 1990s had failed to predict last year’s drought.

And earlier this year, the IMD discarded that long-range forecasting model and introduced a new model that relies on eight weather parameters to predict the monsoon over India.

Senior IMD officials had said they were ditching the old model because it had “outlived its utility” and the new “eight-parameter” model appeared to yield better forecasts.

The IMD models rely on remote weather parameters — such as surface temperatures over the Pacific Ocean or wind patterns over the oceans, or Eurasian snow cover — to predict the monsoon. IMD scientists say they have picked parameters that are physically-related to the Indian monsoon and are statistically stable over long periods of time.

Iyengar’s new model assumes that instead of looking at distant weather parameters, clues to future rainfall might lie hidden in previous years’ rainfall data.

The utility of the new technique would be the enormous advance notice that the nation might get to gear up for a possible bad monsoon.

While the current IMD strategy is to issue a preliminary long range forecast of the summer monsoon in April — barely two months before the monsoon onset in June — Iyengar’s model would allow scientists to predict next year’s rainfall at the end of the current monsoon.

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