Explained: Working and functions of IMD's Doppler Radar

India Meteorological Department's (IMD) only Doppler radar is situated in Mumbai. It is useful in surveying weather patterns and forecasts. It has recently stopped working again, now when the city was witnessing rainfall. 

On May 17, a day before Cyclone Tauktae, brushed the city, the Doppler Radar had become defunct. It had resumed operations only recently, after nearly a two month repair work. 

Doppler radars can be used for gauging the intensity of rainfall and impact area, that too in real-time. In the absence of the radar that is located at the IMD's Colaba Observatory- weather surveillance can be carried out to a radius of 450-500 km. Satellite pictures and wind profiles are used in the forecast.

How does a Doppler radar work?

Radars emit a beam of energy called radio waves emitted from an antenna. When the beam strikes the object, the energy gets dispersed in every possible direction. Some energy reflects back to the radar. The larger the object the greater the amount of energy that gets deflected.  

The observation is made for the beam to be transmitted and returned to the radar allowing the weather forecasting departments to watch the raindrops in the atmosphere measuring their distance from the radar. 

What makes the radar special is that it can provide information on both the position or targets and their movements. 

Later the tracking of the phase of the radio transmitted waves is done. The computers are designed to measure the shift in this phase, that is the shift in the original pulse and the received echo through which the movement of the raindrops can be calculated. From this it becomes possible to find out the movement of precipitation towards or away from the radar. 

In our country the Doppler radars of varying frequencies like S band, C band and X band are commonly used by the IMD to track the movement of the weather systems and cloud bands. Also the gauge rainfall's track is measured over an area of 500 km. 

The radars are helpful in conditions like cyclones and associated heavy rainfalls. The X band radar is useful to detect thunderstorms and lightning whereas C band guides in cyclone tracking.  The radar readings are updated every 10 minutes. 

Doppler Radars Basics:

The discovery of the Doppler effect was done by Christian Doppler who was a 19th century physicist. According to the National Weather Service, in one hour the Doppler radar is capable of transmitting signals for only seven seconds and can spend the remaining 59 minutes and 53 seconds listening to returned signals. 

Where are Doppler radars on Indian coastline?

On the eastern coast, the Bay of Bengal has radars operational at 8 locations including Kolkata, Paradip, Gopalpur, Visakhapatnam, Machilipatanam, Sriharikota, Karaikal and Chennai. Along the west coast the radars were at Thiruvananthapuram, Kochi, Goa and Mumbai. 

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