In time for the monsoon,Mumbai to get a Doppler radar

The DWR is a remote sensing device useful in making forecasts at short range within six hours,experts say. Its ability to obtain weather-related information in three dimensions makes it highly valuable to weather researchers and atmospheric scientists. Explains Sachin Deshpande,a scientist at the physical meteorology and aerology division of the Indian Institute of Tropical Meteorology (IITM),Pune,“The Doppler radar can see inside storms,and can be used to assess storm severity using the reflectivity information as well as Doppler velocity information (what are called the winds within the storm). It provides unique information such as precipitation intensity,radial winds,etc.”

Unlike an active remote sensor,the DWR does not suffer from many of the observation problems associated with passive sensors,such as image displacement and changing illumination. Besides,the information is also available immediately for use.

Typically,there are three different types of weather radars used by weather scientists. The conventional radar,which gives information only about the rainfall estimation; the Doppler radar,which has the capability of measuring rainfall and winds in the atmosphere; and the polarisation radar (or multi-parameter radar),which can measure rainfall,winds,along with the shape and number of raindrops,which is important for understanding cloud and precipitation formation processes.

Doppler radar itself comes in different bands. L band radars operate on a wavelength of 15-30 cm and a frequency of 1-2 GHz they are mostly used for clear air turbulence studies; C band radars operate on a wavelength of 4-8 cm and a frequency of 4-8 GHz and are the most affordable radars and thus in use in TV stations; X band radars operate on a wavelength of 2.5-4 cm and a frequency of 8-12 GHz—because of the smaller wavelength,they are more sensitive and can detect smaller particles,hence they are used for studies on cloud development because they can detect the tiny water particles and to detect light precipitation such as snow; K band radars operate on a wavelength of 0.75-1.2 cm or 1.7-2.5 cm and a corresponding frequency of 27-40 GHz and 12-18 GHz.

The star of the moment,however,is the S-band radar,which Mumbai’s IMD will soon possess. This radar operates on a wavelength of 8-15 cm and a frequency of 2-4 GHz. However,experts at IIT Mumbai pointed out that because of their wavelength and frequency,S-band radars are not easily attenuated. “This makes them useful for near and far range weather observation. The drawback to this band of radar is that it requires a large antenna dish (25 feet) and a large motor to power it,” they said. IIT Mumbai is in the process of procuring two dual-polarisation DWRs—an X-band (for precipitation studies) and a K-band (for cloud studies) radar.

Sharma of IMD says the Met offices everywhere are going through intensive renovations. “As part of our first phase of modernisation,we have already got DWR in Kolkata,Visakhapatanam,Maslipatnam,Chennai,Delhi,Hyderabad,Bhuj and Mumbai. The device in Mumbai will cover Pune too. Our second phase will involve installing sophisticated devices in our centres in Ratnagiri,Nagpur and other offices.”