The Use of Emission and Differential Phase Shift to Correct for Attenuation at C-band

Attenuation is a major problem in estimating high rain rate with C-band radars such as those used in operational networks in Europe, Canada and Japan. At C-band differential phase shift should be approximately linearly related to the attenuation but in practice the coefficient linking the two becomes rather uncertain in heavy rain when Mie scattering becomes significant. Such uncertainties in the coefficient become serious in severe rainfall events when the attenuations may be above 20dB. In addition, it now appears that wet hail can produce both considerable attenuation and phase shift. Because the shape and size of the hail and the melting water on the surface of the hail are both uncertain, then the link between phase shift and attenuation for wet hail is poorly defined. Accordingly, we have revived an earlier suggestion by Fabry, that relies on the well known physical principle that good absorbers are good emitters. In other words if the hydrometeors in the radar path are attenuating the C-band radiation, then these hydrometeors will appear to ‘glow' at this wavelength, and this continuous emission will be visible as an increased noise level at distant empty gates for attenuating beams. The emission technique is very appealing because it is a direct measurement of the attenuation and, once the brightness temperature of the emission is known, then attenuation can be calculated directly and no knowledge of the characteristics of the hydrometeors is needed. In practice there are other effects which make it difficult to measure the increase in noise. Firstly the contribution of the receiver to the noise must be established, secondly emission from other sources must be quantified. Whereas the emission from the water vapour and other gases along the path can, in theory, be derived from an operational model or, by comparing the emission at various beam elevations, the emission from the variable ground clutter can be a more serious challenge. In this paper we will present data taken during the severe flooding event over London which occurred on 20 July 2007 and some more recent observations from the operational network which has now been modified to improve its capability to observe emission from attenuating storms.