A test bed for verification of a methodology to correct the effects of range dependent errors on radar estimates

Quantitative radar precipitation estimates are affected by error determined by many causes that include, among others, radar miscalibration, range degradation (including beam broadening and sampling of precipitation at increasing altitude), attenuation, ground clutter, variability of Z-R relation, variability of drop size distribution, hydrometeor water phase distribution within the sampled volume, vertical variability of the precipitation system, vertical air motion, precipitation drift, anomalous propagation and beam-blocking (Zawadzki, 1984; Koistinen and Puhakka, 1986; Saltikoff et al., 2000; Villarini et al., 2008; Berenguer and Zawadzki, 2009; Villarini and Krajewski, 2010). Several sources, such as attenuation, range degradation and radar sampling above the clouds, determine a range dependent behaviour of error. The aim of this work is to quantify the range-dependent influence of the above-mentioned sources of uncertainties on rainfall radar estimates, through comparison between radar and rain gauge network precipitation fields. To reach this objective, the G/R ratio was calculated against range, where G and R are the corresponding rain gauge and radar rainfall amount, respectively, computed at each rain gauge location. Radar data are processed to compensate calibration and attenuation effects. Finally, the range dependent error was modeled through an adjustment factor, derived for different elevation angles. Radar data were collected by the Polar 55C weather radar located in Rome (Italy) managed by the Institute of Atmospheric Sciences and Climate of the National Research Council (ISAC-CNR) of Italy in 2008. Rain gauges data were collected by the network of the Lazio regional administration located inside the radar scanning area. A subset of rain gauges appears as aligned along a given direction from the radar along a range of almost 120 km free from beam blocking effects is used to verify the effectiveness of the methodology. A set of five events is used to this purpose. This direction, which is almost parallel to the Tyrrhenian coast line, is also that along which intense convective cells tend often to organize themselves as a squall line. Such condition is verified in the considered dataset. In the next section the data selection methodology is detailed. In Sect. 3 characteristics of Polar 55C weather radar are described, as well as the methodologies followed to calibrate weather radar with rain gauges, to perform radar rainfall estimates and to correct radar sampling errors and attenuation. In Sect. 4 logarithm of G/R trends with range, obtained before and after each processing of radar data, were compared, referring to different elevation angles. In Sect. 5 the influence of the melting layer on radar estimates is treated. Finally Sect. 6 completes the paper with conclusions.