An improved C-band rainfall attenuation correction for operational weather radars using a radiometric constraint on reflectivity and polarimetry

Abstract

Ground-based radar networks have the unique ability to monitor rainfall rates close to the ground every few minutes, but in heavier flood-producing rain, when the radar observations are most useful, the radar return is attenuated by the rain. We report on rainfall derived using a consensus approach for estimating attenuation, based on three techniques: (i) A new “emission” technique to estimate this attenuation by monitoring the increased noise in “empty” gates at long range where the radar beam is above any precipitation. Attenuating rain will emit as a blackbody and raise the noise floor along the entire ray. For Met Office C-band radars, an increase of 20K is equivalent to a path integrated attenuation (PIA) of ∼1dB, but we need the distribution of attenuation over range so we appeal to: (ii) The empirical gate-by-gate correction scheme which tends to be unstable and uses a climatological relation between attenuation by rain and radar reflectivity (Z); and (iii) A polarimetric constraint: the differential phase shift between the H & V returns. The new scheme achieved reductions in the root mean square error of hourly radar based rainfall of up to 39% (compared with co-located rain gauges). These results, combined with a neutral impact on light rainfall events during both summer and winter, suggest that the new scheme is beneficial across the Met Office radar network. Emissions also have the unique potential to correct for attenuation by hail, but this is hard to evaluate in the UK.