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Forecasting the monsoon

Every year, the nation holds its collective breath as the venerable India Meteorological Department (IMD) issues its forecast of how the south-west monsoon is likely to turn out. Such concern is a reflection of how much the monsoon continues to dominate the Indian economy and rules the lives of people across this vast and diverse land. What everyone dreads is the prospect of a drought. A monsoon is considered to have ended in a drought when the rainfall received countrywide is less than 90 per cent of the long-term average. In the last 130-odd years, such droughts have occurred in just 21 years. However, an analysis published by Indian scientists recently showed that in the current state of the economy even a moderate drought (when the rainfall deficit ranges from 10 per cent to 15 per cent) can reduce the GDP by Rs.50,000 crore or more and slash foodgrain production by about 10 million tonnes. For the forthcoming monsoon, the IMD has predicted that the countrywide rainfall would be just five per cent below the long-term average, which would classify the monsoon as a `normal' one. This year's forecast is based on a spanking new statistical method that the IMD scientists hope will improve the accuracy of their predictions. Will the technique be able to give advance warning of a drought? It is believed that the new method could have successfully predicted the drought of 2004 and given at least an early indication of trouble during the 2002 monsoon, which failed badly and produced one of the worst droughts in a century.

An issue of practical concern is that rainfall during the monsoon is unevenly distributed in time and across regions. Some places can be subjected to bouts of heavy rain while other parts of the country receive hardly any rain at all; these are aberrations that averages do not capture. The IMD is receiving a long overdue upgradation of its instrumentation, with plans to install a network of automatic rain gauges, wind-measuring devices, and advanced weather radars. Timely availability of data from these instruments can help to greatly increase the accuracy with which dynamical models that simulate processes in the atmosphere and ocean are able to predict the course of the monsoon a few days in advance. To further increase the lead time for forecasts, however, the dynamical models will themselves have to be improved. But such improvements will be possible only if there is a better understanding of factors influencing the monsoon. The monsoon's links to the oceans surrounding India are, for instance, just getting recognised. Investing in science that will increase our understanding of the monsoon holds the key to better forecasts that will immensely benefit the people of India.

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