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Explained: As monsoon advances, how Doppler radars help track and forecast weather

Observing the time required for the beam to be transmitted and returned to the radar allows weather forecasting departments to "see" raindrops in the atmosphere, and measure their distance from the radar.

Doppler Radar at Colaba near RC Church, Mumbai (Express Archives)

The India Meteorological Department’s (IMD) only Doppler radar in Mumbai, which surveys weather patterns and forecast, stopped working again on Wednesday afternoon, when the city was witnessing rainfall. By 8.30 pm, it started functioning partially.

On May 17, a day before Cyclone Tauktae brushed past the city’s coastline, the Doppler Radar had become defunct. It had resumed operations only recently, after nearly a two-month repair work. Doppler radars are crucial for gauging the intensity of rainfall and impact area in real-time.

In the absence of the radar located at IMD’s Colaba observatory – which can carry out weather surveillance up to a radius of 450-500 km – satellite pictures and wind profiles are used for forecast, said the met department.

“It is an old machine and is under maintenance. The radar is not working owing to technical problems. Engineers are looking into it. It should be noted that there is no impact on the forecast because of this… we have high-resolution satellite images,” said Dr Jayanta Sarkar, head, Regional Meteorological Centre, Mumbai.

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How does a Doppler radar work?

In radars, a beam of energy– called radio waves– is emitted from an antenna. When this beam strikes an object in the atmosphere, the energy scatters in all directions, with some reflecting directly back to the radar.

The larger the object deflecting the beam, the greater is the amount of energy that the radar receives in return. Observing the time required for the beam to be transmitted and returned to the radar allows weather forecasting departments to “see” raindrops in the atmosphere, and measure their distance from the radar.

What makes a Doppler radar special is that it can provide information on both the position of targets as well as their movement. It does this by tracking the ‘phase’ of transmitted radio wave pulses; phase meaning the shape, position, and form of those pulses. As computers measure the shift in phase between the original pulse and the received echo, the movement of raindrops can be calculated, and it is possible to tell whether the precipitation is moving toward or away from the radar.

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In India, Doppler radars of varying frequencies — S-band, C-band and X-band — are commonly used by the IMD to track the movement of weather systems and cloud bands, and gauge rainfall over its coverage area of about 500 km. The radars guide meteorologists, particularly in times of extreme weather events like cyclones and associated heavy rainfall. An X-band radar is used to detect thunderstorms and lightning whereas C-band guides in cyclone tracking.

With the radar observations, updated every 10 minutes, forecasters can follow the development of weather systems as well as their varying intensities, and accordingly predict weather events and their impact.

Why are they called ‘Doppler’ radars?

The phase shift in these radars works on the same lines as the “Doppler effect” observed in sound waves– in which the sound pitch of an object approaching the observer is higher due to compression of sound waves (a change in their phase).

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As this object moves away from the observer, the sound waves stretch, resulting in lower frequency. This effect explains why an approaching train’s whistle sounds louder than the whistle when the train moves away. The discovery of the phenomenon is attributed to Christian Doppler, a 19th-century Austrian physicist.

As per the US National Weather Service, in an hour, a Doppler radar transmits a signal for only over seven seconds, and spends the remaining 59 minutes and 53 seconds listening to returned signals.

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Doppler radars on India’s coastline

India’s east coast, which is frequently affected by cyclones formed in the Bay of Bengal, has radars operational at eight locations — Kolkata, Paradip, Gopalpur, Visakhapatnam, Machilipatanam, Sriharikota, Karaikal and Chennai.

Along the west coast, there are radars at Thiruvananthapuram, Kochi, Goa and Mumbai. Other radars are operating from Srinagar, Patiala, Kufri, Delhi, Mukteshwar, Jaipur, Bhuj, Lucknow, Patna, Mohanbar, Agartala, Sohra, Bhopal, Hyderabad and Nagpur.

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In June, the IMD said it will install seven new Doppler radars in Maharashtra, including Mumbai, this year, and plans to have a network of 55 Doppler radars across the country. The department also announced an upgrade for the Chennai radar, which is presently defunct.

First published on: 15-07-2021 at 10:00:18 pm
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