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 and today it operates 15 out of the 18 radars that give continuous coverage of the British Isles.
In operational use, radar is the only means available for measuring the location and intensity of precipitation in real time on the scale of a few kilometres. Rainfall radar products are used directly by weather forecasters, and are fed into forecast models. In the areas of aviation and flood forecasting they are crucial for the protection of life and property.
Each radar sends out pulses of microwave radiation and detects the return signals reflected by particles of precipitation, whether liquid or frozen. 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 pulses are concentrated in a narrow beam which scans through a full 360° 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 in the network are sent to the Met Office at Exeter for immediate processing. 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. Processing at the Met Office normally removes:
Most clutter can be removed in the processing stage, but wind farms in the line-of-sight of weather radar can cause significant problems.
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. Some radars have Doppler capability enabling them to track the movement of precipitation particles, and hence provide an estimate of the wind. This is particularly important for the detection of dangerous wind shear near airports which constitutes a significant hazard to aviation safety.
This example of a rainfall radar picture, composited from all radars in the UK network, shows two active bands of rain crossing the country from the west. At the time of this picture a warm front extends from northwest England through Wales to southwest England, while a second cold/occluded front over western Scotland and Northern Ireland marks the boundary of colder air that lies to the northwest. Areas of yellow and red indicate moderate to heavy rain. The strong variability of rainfall intensity between locations only a small distance apart is a typical characteristic of many active frontal systems. The enhanced rainfall over high ground in western Scotland, the Lake District and Wales is very apparent. These areas have very high average annual rainfall.
Rainfall radar over the UK at 12.45 on 28 Sep 2005
Having a range of 255 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.
NOTE Radar in Jersey and the Republic of Ireland also contribute to the UK composite, with data supplied by the Jersey Meteorological Department & Irish Met Service in accordance with the regulations of ECOMET.
Last updated: 14 October 2015