Joint Centre for Mesoscale Meteorology (JCMM)
JCMM Internal Report No.89:Correction for attenuation of radar reflectivity using polarisation data by T J Smyth & A J Illingworth
Attenuation by heavy rain is a major operational problem in radar estimation of rainfall rates which is increasingly severe at wavelengths shorter than 10cm. Gate-by-gate correction algorithms, including those using both reflectivity (ZH) and differential reflectivity (ZDR), are inherently unstable. In addition, values of (ZH) and (ZDR) are affected by hail, which causes little attenuation. Methods dependent upon differential phase shift (KDP) do not give unique solutions but depend upon rain drop size distributions. A new technique which uses both KDP and differential attenuation between the horizontally and vertically polarised radiation (AH-V) is proposed. The total AH-V is measured from the negative ZDR in light precipitation behind the attenuating region. Theory and observation show that corrections based on the KDP - AH-V scheme should be unaffected by hail, stable and accurate. A simpler algorithm may be feasible at C-band in which the total attenuation is derived from the observed total differential attenuation. It appears that the most severe cases of attenuation result from heavy rain consisting of large oblate raindrops. Accordingly, the attenuation would be appreciably reduced if operational radars used vertical polarisation rather than the commonly employed horizontal polarisation.