Range Effects in Polarimetric Measurements at the S, C and X Bands in Brazil

Many important applications of radars such as in hydrology are crucially dependent on the accuracy of rainfall estimates. When coverage of large areas with a limited number of radars is demanded, as is presently the case in Brazil, operation to far ranges is required. The issue of range effects is then emphasized; degradation of quality of radar measurements and respective rainfall estimates with distance is a well-known fact. Growing interest in polarimetry stems, partly, from providing a reduction of the effect of drop size variability in rainfall estimates, thus allowing progress on radar rainfall estimation and on hydrometeorological applications. In an operational setting, however, the promises on improvements of radar rainfall accuracy are still to be completely proven due, mainly, to the combination of the geometry of radar measurements with the spatial structure of the rainfall systems. In fact, results from a recent study have shown an increase around 20% on average in the differences (in terms of NSE) between radar (at C-band) and ground measurements along 60 km, directly attributable to beam broadening (Gorgucci and Baldini, 2015). Significant biases of ZDR, PHIDP and RHOHV may be produced by cross-beam gradients of Z, ZDR and PHIDP within the resolution volume. These biases increase with range as a result of progressive beam broadening, and are larger at shorter wavelengths (Rhykhov, 2007). This paper deals with the evolution as a function of range of radar rainfall CDF(probability distribution function) retrieved from polarimetric algorithms. Beam broadening affects both conventional and polarimetric radars, and a previous work (Calheiros et al, 2014) was carried out with data then available, comparatively verifying range effects on measurements made with a conventional S-band radar and a polarimetric X-band, in the state of São Paulo. In the present work measurements from three standalone polarimetric radars, i.e., an S-band situated at Cascavel (-24.8755°,-53.5253°) in the state of Paraná, a C-band radar at Morro do Elefante (-19.9452°,-44.4344°) in the state of Minas Gerais, and the X-band utilized in the previous work before mentioned, at Sao Jose dos Campos (-23.208702°,-45.952817°) in the state of São Paulo, provided the radar data base. Beam width is 1° for all three radars. Both S and C-band data extend to 250 km range while X-band data is limited to 100 km range. All data are from the most intense period within the wet season, in summer, with the occurrence of strong tropical convection featuring elevated gradients of rainfall. Radar rainfall data are derived through polarimetric algorithms from the literature and CDF curves are generated. CDF are stratified in range to the farthest quantifying distance with range rings defined to balance capture of range effects and sample size. Then, a further stratification of the CDF is performed according to daily interval, reflecting the evolution of hourly rainfall under each radar umbrella. CDF from gage and distrometer for the S and X band coverage area and from gage only for the C band, were also derived and paired with the corresponding radar CDF for reference purposes. Continuing work involves use of CDF based on polarimetric algorithms specifically derived for each radar coverage area.