Simple yet reliable?

Scientists at the Bangalore-based Indian Institute of Science (iisc) have put to test a simple yet innovative forecasting model, which uses past rainfall data to predict monsoon. The model, developed by R N Iyengar and S T G Raghukanth, has been used for the first time this year to forecast the southwest monsoon. It has pegged the quantum of the monsoon rainfall at 80.34 centimetres, with a possible error of 3.3 centimetres. This forecast varies appreciably from that of a normal rainfall (more than 88 centimetres) made by the New Delhi-based India Meteorological Department.

The uniqueness of the new model is that it does not take into consideration weather phenomena around the world (what scientists call teleconnections) to arrive at its plausible results. Instead, it assumes that the imprint of such events is already ingrained in the annual monsoon rainfall data of the past, provided the information is available for a sufficiently long period of time.

To predict this year's rainfall, the iisc scientists used annual rainfall data of 118 years (from 1872 to1990). Explaining the procedure, Iyengar told Down To Earth that they have broken down the monsoon rainfall data into six data sets, depending on the predominant factors influencing rainfall. Thereafter, they have worked out the cyclic nature of each set, which varies from 2.7 to 60 years.

As per Iyengar, only the collation of the values of the six data sets can provide a realistic picture of the monsoon of a particular year.

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