Rain forecast accuracy for Delhi: IMD's work in progress
Rain forecast accuracy for Delhi: IMD's work in progress

Photo Credit: IANS


New Delhi, Oct 7 (IANS) Except for one month in one year, the India Meteorological Department's (IMD) month-wise forecast for rainfall in Delhi during the peak monsoon season, June to September, during last 10 years was never 100 per cent accurate.

The 100 per cent accurate forecast was in July 2012, the same year when the forecast accuracy for the south-west monsoon season as a whole for Delhi was as high as 92 per cent while for rest other years, it was less than 85 per cent, according to a compilation by the IMD.

In what can be termed as a solid measure of transparency for the scientific work, the IMD has put out this compilation on its website for stakeholders.

The number of days that the national capital gets rains is limited compared to several other states. But given the changing climatic conditions that have made the precipitation more erratic, it is all the more necessary to have an accurate, or as near to accurate forecast as possible.

Said former Secretary, Ministry of Earth Sciences, Madhavan Rajeevan, "IMD's accuracy of forecast has improved a lot, especially the short-range forecast. For example, 48-hour forecast accuracy is almost 80-85 per cent. That is improved substantially over last few years because IMD has been using improved models for forecasting."

This year, IMD modified the existing two-stage forecasting strategy. The new strategy is based on the existing statistical forecasting system and the newly developed Multi-Model Ensemble (MME)-based forecasting system. The MME approach uses the coupled global climate models (CGCMs) from different global climate prediction and research centres, including IMD's own Monsoon Mission Climate Forecasting System (MMCFS).

Rajeevan, however said, any running model will have chances of errors. "All these forecasts are dependent on how much initial data goes initially into the model. The quantum of this initial data will decide the quality of the forecast. We accumulate as much data as possible. Huge volume of data is accumulated from satellites, surface observatories, from ships, from aircraft. This data accumulation too has gone up substantially for last 7-8 years," he told IANS.

There are agreements for getting data from China and European Union and that too adds to the data accumulated.

IMD's compilation showed that in the last 10 years, i.e., between 2011 to till August 2021, the forecast accuracy was 90 per cent or above in August 2020, August 2021, September 2012, September 2015, September 2018, and September 2020 (September 2021 data was not available yet).

The forecast accuracy was between 80 to 90 per cent for June twice (2011 & 2020); for July thrice (2011, 2019 and 2021); for August it was five times (2012, 2017, 2019, 2020 and 2021) and for the month of September, the forecast accuracy between 80 to 90 per cent was in the years 2011, 2013, 2014, 2016 and 2017, the data showed.

Similarly, the forecast accuracy was between 70 to 80 per cent for the month of June in years 2017, 2018 and 2021; twice in July (in 2016 and 2017); twice in August too (in 2014 and 2016) while it was only once that the forecast accuracy was recorded between 70 to 80 per cent for September in 2019.

The IMD data showed that the forecast accuracy during the Southwest monsoon season could not surpass even 50 per cent mark in July 2014 (42%). For rest, it ranged between 60 to 70 per cent. Surprisingly, the accuracy for rainfall forecast for Delhi was better in non-monsoon months, i.e., in winters.

Asserting that accuracy in India is comparable to the global level, Rajeevan further said, "We cannot ideally compare Indian weather system with, say, the US weather system. India is a tropical country. The weather forecast accuracy is slightly less than what one gets in the mid-latitude or the polar region. Amongst the tropical countries, we are the best."

IMD's former Director General Ranjan Kelkar said, "Spatial distribution over Delhi is uneven. People are now willing to interpret IMD's forecast in terms of practical actions. This is the change in the modern world, because the accuracy is good, the civic body is able to take appropriate action. If your forecast is believable, other agencies can take appropriate action and also inform people accordingly."

He cites example of people travelling from Noida to Gurugram or Ghaziabad to Airport who really benefit from the improved IMD forecast.


Pd/ Niv


Update: 07-October-2021