Correcting for Attenuation: Can Emission Provide and Absolute Constraint?

C-band operational networks are common outside the USA and suffer from severe attenuation so that in heavy rainfall events the radar often underestimates rainfall totals by more than 50%. All attenuators are emitters, so the total attenuation of the radar return may be derived from the increased background noise level detected by the radar receiver. We report results from the emission technique which is now being rolled out across the UK operational radar network. This approach has the advantage that it is an absolute measurement and no assumptions need be made regarding the microphysical properties (rain, wet hail….) of the attenuating hydrometeors; it also provides, for the first time, an accurate real-time estimate of the attenuation due to a wet radome. This robust measurement of total attenuation can then be used as a constraint to either the notoriously unstable gate-by-gate correction techniques or the newer polarisation methods; for example, it is well known that the coefficient linking differential phase shift to attenuation can vary by a factor of two in heavy rainfall.

Some care is needed to implement the technique. A calibrated noise source is needed for accurate results and spurious returns due to point targets in distant gates must be rigorously excluded. The stability of emission from ‘glowing' ground clutter in low elevation beams must be monitored. Observations show that, for a radome which is several years old, even light rain of 2-3mm/hr can produce an attenuation of 2-3dB at C-band. This attenuation varies a great deal with azimuth, rain rate and wind speed, so empirical corrections are not reliable. The technique is able to measure total attenuation from the radome and from distant storms over the range of 0.5 to 20dB.