Doppler Weather radar (DWR)

•  In a bid to improve weather forecasting services, Indian Space Research Organisation has commissioned a C- band Polarimetric Doppler weather radar (C-DWR) at the Vikram Sarabhai Space Centre (VSSC), Thiruvanthapuram.
• It is the first of its kind in South India.
• The data generated by the DWR would be shared with the India Meteorological Department to supplement its prediction.
• It would be made available to researchers and other user agencies also.
• The only other C- DWR in the country is at Chirapunji.
• The DWR is expected to aid rainfall estimation and detect thunderstorm, cyclones and other severe weather events within a radius of 400 km.

About Doppler Effect

• When wave energy like sound or radio waves travels from two objects, the wavelength can seem to be changed if one or both of them are moving. This is called the Doppler effect.
• The Doppler effect causes the received frequency of a source to differ from the sent frequency if there is motion that is increasing or decreasing the distance between the source and the receiver. This effect is readily observable as variation in the pitch of sound between a moving source and a stationary observer.

About Doppler Weather Radar (DWR)

• It provides advance information, enhancing the lead-time so essential for saving lives and property, in the event of natural disaster associated with severe weather.
• Though the conventional radars are able to track and predict cyclones, the DWR provides detailed information on storm’s internal wind flow and structure. The severity of the weather systems can thus be quantitatively estimated more accurately than ever before and more precise advance warnings can be generated for saving human lives and property.
• The polarimetric capability of the Radar will significantly improve the accuracy of rainfall estimation leading to accurate and timely flash flood warnings.

Radar Bands

• L band radars operate on a wavelength of 15-30 cm and a frequency of 1-2 GHz. L band radars are mostly used for clear air turbulence studies.
• S band radars operate on a wavelength of 8-15 cm and a frequency of 2-4 GHz. Because of the wavelength and frequency, S band radars are not easily attenuated. This makes them useful for near and far range weather observation. The National Weather Service (NWS) uses S band radars on a wavelength of just over 10 cm. The drawback to this band of radar is that it requires a large antenna dish and a large motor to power it. It is not uncommon for a S band dish to exceed 25 feet in size.
• C band radars operate on a wavelength of 4-8 cm and a frequency of 4-8 GHz. Because of the wavelength and frequency, the dish size does not need to be very large. This makes C band radars affordable for TV stations. The signal is more easily attenuated, so this type of radar is best used for short range weather observation. The frequency allows C band radars to create a smaller beam width using a smaller dish. C band radars also do not require as much power as an S band radar. The NWS transmits at 750,000 watts of power for their S band, where as a private TV station such as KCCI-TV in Des Moines only broadcasts at 270,000 watts of power with their C band radar.
• X band radars operate on a wavelength of 2.5-4 cm and a frequency of 8-12 GHz. Because of the smaller wavelength, the X band radar is more sensitive and can detect smaller particles. These radars are used for studies on cloud development because they can detect the tiny water particles and also used to detect light precipitation such as snow. X band radars also attenuate very easily, so they are used for only very short range weather observation. Also, due to the small size of the radar, it can therefore be portable like the Doppler on Wheels. (DOW) Most major airplanes are equipped with an X band radar to pick up turbulence and other weather phenomenon. This band is also shared with some police speed radars and some space radars.
• K band radars operate on a wavelength of .75-1.2 cm or 1.7-2.5 cm and a corresponding frequency of 27-40 GHz and 12-18 GHz. This band is split down the middle due to a strong absorption line in water vapor. This band is similar to the X band but is just more sensitive. This band also shares space with police radars

By: Vishal