Nearly all global weather models failed to predict the extremely heavy rains that Chennai received on December 30.

Water logging due to rain near Marina in Chennai Image credit: PTI
news Chennai Rains Friday, January 07, 2022 - 12:06

Unexpected heavy rains in Chennai on December 30, 2021, once again sent alarm bells ringing in the city. It was the highest rainfall Chennai has received in a single day since 2015. However, nearly all the weather prediction models in India and around the world failed to forecast it. By the afternoon of that day, the India Meteorological Department (IMD) forecasted moderate precipitation in and around Chennai and neighbouring districts. However, the city was already seeing extremely heavy showers by then. The IMD then monitored and updated its data.

Following the rains, a strong demand to upgrade weather prediction technology at IMD Chennai (Regional Meteorological Centre) has emerged. DMK MP Dayanidhi Maran had also written to Prime Minister Modi regarding fixing the S Band Doppler Weather Radar in the Chennai Port Trust – the city’s oldest – in December 2020 and again in November 2021. He tweeted the same in December, after the heavy rains.

Dysfunctional radar

“Almost a year ago, on December 2, 2020, I had written to you to bring to your urgent attention the pathetic condition of the S Band Doppler Weather Radar at Chennai port trust (which has a range of 500 km), which is dysfunctional and old, because of which the Karaikal and Sriharikota radars were being used to supplement data for Chennai. The Union government had promised that a new X Bank weather radar at the National Institute of Ocean Technology (NIOT), Pallikaranai, will also be operationalised and ready to use. Almost a year later, the S Band radar has not been fixed, the X band radar, too, has not been fully calibrated,” Maran had written in 2021. 

TNM spoke to the IMD Director General of Meteorology, Dr Mrutyunjay Mohapatra, who confirmed that the S Band radar was not in full use. 

“The S Band radar at Port Trust is the oldest radar in Chennai. It can provide observations on weather conditions up to 500 km in range, based on which weather models can make predictions. The S Band Radar was repaired by ISRO. Now it is being operated as and when required. It is not being used round the clock like earlier, when it collected data every 10 minutes. However, we are working towards fixing the radar to make it fully operational,” Dr Mohapatra said. 

Improve operational systems and weather models 

The IMD further added that upgrading technology is an ongoing process. It cannot be specific to a region. “Five years ago, our weather prediction efficiency was only 40-50%. But today, we have brought it up to 80%. However, not all weather events can be forecast accurately. The Chennai rain [on December 30] was one such adverse event which was missed by even international models,” Dr Mohapatra said.  

That being said, the IMD says that it plans to improve weather forecasting technology by making upgrades to operational systems and weather modelling systems. Operational systems include radars, satellites and weather stations – the hardware used to collect observations and data on the weather. The modelling systems are then fed this data, in order to forecast. 

Upgrading resolution in weather modelling systems

One of the major upgrades planned is on the weather modelling systems. The IMD says it has six weather modelling systems under the Ministry of Earth Sciences. This includes the Global Weather Modelling System which has a resolution of 12 km. The lower the range (distance it is able to forecast for) of a weather model, the higher its resolution, and the greater the precision. For example, a 12-km resolution is lower than a 2-km resolution, which will be much more precise. Generally, the higher the resolution, the smaller or finer the grid size, making it possible to zoom in and predict weather hyper-locally.

 “We plan to improve the 12-km resolution weather modelling system to a 5-6 km resolution. This will help improve forecasting location accuracy of a weather event,” Dr Mohapatra says, including in Chennai.

High-Resolution Rapid Refresh model introduced

In 2021, the IMD also introduced the High-Resolution Rapid Refresh (HRRR) model. The HRRR is a high frequency, hourly updating atmospheric model.

“The Rapid Refresh Model is calibrated at a resolution of 2 km. This means that weather forecasts can be provided for every two kilometres in all the regions, including in Chennai and other parts of Tamil Nadu. This was introduced in 2021,” Dr Mohapatra added. The IMD says that it plans to improve the resolution of the model to every 1 km. Another weather model called the mesoscale model (which has a resolution of 2-3 km), will be strengthened to a higher resolution.

Specifically for Chennai, the Urban Forewarning systems and Urban Meteorological Services will be strengthened. “This means that information and forecasts on location specific weather will be relayed faster. We are also going to improve Impact Based Forecasting – a technique used to predict expected impact due to expected weather events,” he added.

All of this will be done by improving operation systems such as setting up automatic weather stations and automatic rain gauges. Upgrading of existing radars and satellites will also be done. The IMD did not comment on when the S Band radar would be restored to 24-hour operational capacity.

Become a TNM Member for just Rs 999!
You can also support us with a one-time payment.