17th July, 2021 Science and Technology
- 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.
Decoding Doppler Effect
- Doppler Effect is the change in frequency of a wave in relation to an observer who is moving relative to the wave source.
- It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.
- A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer.
- Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.
- The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave crest is emitted from a position closer to the observer than the crest of the previous wave.
- Therefore, each wave takes slightly less time to reach the observer than the previous wave.
- Hence, the time between the arrivals of successive wave crests at the observer is reduced, causing an increase in the frequency.
- While they are traveling, the distance between successive wave fronts is reduced, so the waves "bunch together".
- Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency.
- The distance between successive wave fronts is then increased, so the waves "spread out".
- For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source is relative to the medium in which the waves are transmitted.
- The total Doppler Effect may therefore result from motion of the source, motion of the observer, or motion of the medium.
- Each of these effects is analyzed separately.
- For waves which do not require a medium, such as electromagnetic waves or gravitational waves, only the relative difference in velocity between the observer and the source needs to be considered, giving rise to the relativistic Doppler effect.
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.
Doppler Radars in India
- 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.
- 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.