Science Week 2016: How does a weather radar “see” the rainfall?
Weather radar plays an important role in the detection and forecasting of rainfall in Ireland. Met Éireann operates two weather radars, located at Dublin and Shannon airports. Data from these radars are a vital tool for monitoring the development and movement of weather systems that affect Ireland, providing real-time rainfall estimates.
The term RADAR is short for “RAdio Detection And Ranging”. A weather radar works by transmitting a pulse of microwave energy, a type of radio wave, from a transmitter and listening for the return of the reflections or “echoes” from this pulse with a receiver. Objects in the path of this pulse are called targets. The wavelength of the radar beam is tuned to be scattered by precipitation (e.g. rain droplets) in the atmosphere. The pulses are scattered by the precipitation in all directions so only a small fraction of the power returns to the receiver (this is called back-scattered radiation). Rain, snow and hail (targets) all reflect radar waves. The more targets that are present, the more radio waves return to the radar and the larger the targets, the stronger the scatter signal. If we measure the time travelled and the power of the echoes we can get information about the location and intensity of the precipitation, which helps us to “see” the rainfall.
The radar antenna rotates on the horizontal plane through 360°, while transmitting rapid, narrow pulses to each segment, and listening for the return. This rotation is repeated over several elevation angles of the antenna, building up a 3-dimensional volume of precipitation data. Measurements can be made by Met Éireann’s radars at distances of up to 120 km in each direction from the radar unit, as the radar rotates (the radar range is 240 km). The radars scan every 5 minutes, with a 1 km resolution along the beam and a 1 degree angular (azimuth) resolution.
For further information contact Sarah Gallagher (Sarah dot Gallagher AT met dot ie).