Soon after radar was developed in the 1930s for detecting enemy aircraft, it was realised that radar pulses could also detect rainfall droplets, snow particles and hail. The Met Office has been developing rainfall radar since the 1950s, with the first operational rainfall radar brought online in 1974. Today it operates 15 out of the 18 radars that give continuous coverage of the British Isles.
Rainfall radar coverage over the British Isles
For real-time operational use, radar provides a unique means of obtaining widespread, spatially continuous measurements of precipitation location and intensity at scales of hundreds of metres. Rainfall radar products are used directly by weather forecasters, and are fed into forecast models. In the areas of aviation and flood forecasting (in partnership with the Environment Agency in England and Wales, and the Scottish Environmental Protection Agency in Scotland) they are crucial for the protection of life and property.
Weather radars observe rain, hail and snow, but drizzle can be more difficult to detect because the droplets are so small. Doppler radar functions are also used for the detection of dangerous wind conditions (e.g. wind shear) which constitute a significant hazard to aviation safety.
How a rainfall radar works
Each radar sends out pulses of microwave radiation and detects the return signals reflected by particles of precipitation, whether liquid or frozen. In simple terms, a weather radar sends out a pulse at a wavelength of 5.6 cm, which is reflected by precipitation (this is then compared to a number of rain gauges and adjusted accordingly).
The strength of the return signal may be used to estimate precipitation intensity, and its delay is a measure of distance from the radar site. The radar generates polar (circular) measurement maps by rotating through a full 360 degrees in azimuth, while transmitting pulses concentrated in a narrow beam. Several of these 'scans' are taken at a number of low elevation angles above the horizon. A scanning cycle takes 5 minutes providing data out to 255 km from the site with a resolution up to 1 km.
Data from all 18 radars with coverage in the UK are sent to the Met Office at Exeter for immediate processing; the Channel islands and the Republic of Ireland also contribute to the UK composite, with data supplied by the Jersey Meteorological Department & Met Éireann in accordance with the regulations of ECOMET.
The resulting composite picture provides estimates of rainfall intensity over the whole of the British Isles and the surrounding sea areas at a resolution of up to 1 km, every 5 minutes. Processing at the Met Office normally removes:
- permanent echoes or reflections from hills and buildings known as clutter;
- anomalous echoes from a radar beam reflected from the ground after being bent downwards by the atmosphere (this is known as 'anaprop' and is typical in anticyclonic conditions, i.e. high pressure);
- the strong echoes produced when falling snowflakes start to melt (appearing to the radar like giant raindrops)
However, we currently do have a mean of removing all clutter, for example, wind farms in line-of-sight of a weather radar can cause significantly detrimental impacts to operations.
The radar beam is readily reflected off rain, hail and snow particles, but drizzle can be more difficult to detect because the droplets are so small. To improve the accuracy of radar estimates, values are compared with rainfall amounts measured by rain gauges and appropriate adjustments are made. The radars have Doppler capability, enabling them to track the movement of precipitation particles and provide an estimate of the wind. Additional dual-polarisation capability gives information about the particle shapes and hence precipitation type.
Having a maximum range of typically 200-300 km, radars provide coverage out to sea and may span national boundaries. For this reason, many European nations have collaborated in the routine exchange of radar data. By gathering the data together, a composite picture of rainfall emerges covering a large part of the European land mass and many of the surrounding sea areas.