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Science Week 2016: How does a weather radar “see” the rainfall?
16 November 2016

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.



Figure 1: Radar image showing a band of rainfall moving across the country from west to east, on 08 November 2016 at 1530 pm.

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.



Figure 2:  (a) Radio waves hit off targets and are returned back to the radar. (b) When the pulse hits a target it is scattered in all directions. (Source: University of Illinois WW2010 Project,  WW2010 – Radar).

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.

The radar is protected from wind and rain by a radome, which covers the antenna and dish. Radars are usually placed at high elevations in order to get good visibility over terrain and to prevent the radar beam from being blocked by higher terrain objects, for example by hilltops, buildings or mountains. If the radar beam is partially blocked in one direction, it can not “see” the precipitation, so the higher the radar can be placed the better. This is why radars are often placed on top of tall towers, so they are raised above all surrounding objects which could block their view.

For more information on weather radar, the COMET MetEd introductory course Weather Radar Fundamentals, or the radar section of the University of Illinois WW2010 Project, are great places to start. Check out the latest rainfall radar from Met Éireann, which shows an animation of rainfall over Ireland for the last 6 hours, and is updated every 15 minutes with new images as “seen” by the two radars in Dublin and Shannon airports.



Figure 3: (a) Basic scan pattern of a weather radar. (Source: N. Fitzpatrick, Met Éireann Technical Note No. 62). (b) The Met Éireann radome, covering the weather radar at Dublin Airport. (Source: Sarah Gallagher).

For further information contact Sarah Gallagher (Sarah dot Gallagher AT met dot ie).

 
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