By Rob Thompson
Weather radar is a powerful tool for rainfall measurement. Raingauges measure rainfall at a site, but a weather radar can provide coverage over a wide area, up to 250 km from the radar site. The UK radar network of 16 radars, operated by the Met Office, has coverage over the whole UK mainland and nearby surrounding islands. These radars are in the process of undergoing a major technological upgrade to ‘dual-polarisation capability’, which allows the radar to make many more measurements at each pixel, providing information on shape of drops, consistency of positioning of drops and the delay caused by passing through non-spherical drops.
So, what does this mean? What will be the benefits?
The initial gains from dual polarisation will be in data quality. Using dual polarisation it is much easier to identify the echoes that are from precipitation and not caused by other effects such as the radar beam hitting the ground, aircraft, insects and interference from other sources. Precipitation type identification will allow identification of hail, snow, sleet etc., something currently difficult to establish, but of importance to forecasters and atmospheric models.
But more improvements are being developed. My own work, part of the FRANC FFIR project and in conjunction with the Met Office, is developing techniques using the new radars to allow for the effects of rainfall at the radar site (something previously not possible at all), and also the problem of attenuation. Attenuation is caused when the radar beam is reduced in power by the presence of precipitation itself. In light rain the effect is small, but in very intense rainfall the effect can be serious, so that attenuation becomes a serious problem just when accurate measurements are their most important (see Figure 1).
Figure 1: an example of radar reflectivity as observed during a flood event in London in 2007. A large area of attenuation can be seen where rainfall is under-recorded as a result. The Heathrow raingauge received 17 mm of rain in 15 minutes during this event, yet the radar return suggested less than 1 mm, due to attenuation of the radar beam by intervening heavy rainfall.
At Reading we are using high resolution raingauges and novel radar observations to obtain estimates of attenuation to apply corrections that are more stable than those in current systems, leading to better rainfall estimates. The project is looking to feed the improved radar data into the Met Office computer forecast models leading to better predictions of future weather. When combined with hydrological models, this has the potential to give better warnings of flash flooding (flooding from intense rainfall) to allow better reactions to the danger from authorities and the public (this is covered by the SINATRA project of FFIR).
In the future, the dual polarisation radar network will provide more accurate and more detailed information on precipitation rates. When used in prediction models, this is likely to lead to improved weather forecasts and timely warnings of severe weather.