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The area of Rome represents a unique site for studies on Mediterranean precipitation both for the availability of instumentation and for its position in the Mediterranean basin. The radar meteorology group of CNR-ISAC of Rome manages the coherent C-band Doppler dual polarization radar Polar 55C. The radar has being working since 2001 at the Rome ISAC site, located 30 km northeast from the coast of the Tyrrhenian Sea and 20 km southeast of Rome downtown (41°50’24” North, 12°38’50” East, 102 m asl). Polar 55C can observe precipitation over the farthest part of the Tiber basin, the urban area of Rome, the Appennin Range (where snow precipitation can occur during winter season), and a significant portion of the Tyrrhenian sea. In the current configuration, Polar 55C can measure radar reflectivity factor, differential reflectivity, Doppler velocity, Doppler spectrum and differential phase shift. Polar 55C can play a key role in the study of the interaction between the local and the mesoscale circulation and for the observation of meteorological events involving the city of Rome. Being a research radar, it is operated according to scientific objectives, that can ranges from radar applications to hydrometeorological modelling of urban and rural areas to the ground validation of precipitation satellite measurements. Studies on quantitative precipitation estimation are focused on dual polarization radar techniques. Within the coverage area of Polar 55C radar is the network of telemetered raingauges and hydrographs managed by Servizio Idrografico Regionale (the regional hydrologic service) of the Lazio regional government. Average spatial density of the network raingauges is 1 per 100 km2 and become quite dense within the historic center of Rome. Most of the raingauges have a 5-minutes time resolution and some of them have a resolution of 1 min. In the year 2008 several interesting case sudies have been observed by Polar 55C. Some of them will be discussed to highlight the use of dual polarization techniques to overcome critical aspects of radar rainfall estimation. Example concerns also the analysis of the scaling properties of rainfall in space by using data from the raingauge network and Polar 55C over the area of Rome. The aim is the identification of spatial scaling regimes, their ranges of validity, and the evaluation of the corresponding scaling properties and the dependence structure of rainfall (time and space correlation between time series for different time scales).

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Estimation of raindrop size distribution over large spatial and temporal scales has been a long-standing goal of polarimetric radar. Algorithms to estimate the parameters of a gamma raindrop size distribution model from polarimetric radar observations of reflectivity, differential reflectivity, and specific differential phase are developed. Differential reflectivity is the most closely related measurement to a parameter of the drop size distribution, namely, the drop median diameter (D0). The estimator for D0 as well as other parameters are evaluated in the presence of radar measurement errors. It is shown that the drop median diameter can be estimated to an accuracy of 10%, whereas the equivalent intercept parameter can be estimated to an accuracy of 6% in the logarithmic scale. The estimators for the raindrop size distribution parameters are also evaluated using disdrometer data based simulations. The disdrometer based evaluations confirm the accuracy of the algorithms developed herein.

A methodology is proposed for estimating the parameters of a gamma raindrop size distribution model from radar measurements of Zh, Zdr, and Kdp at S band. Previously developed algorithms by Gorgucci et al. are extended to cover low rain-rate events where both Zdr and Kdp are noisy. Polarimetric data from the S-band Dual-Polarization Doppler Radar (S-Pol) during the Tropical Rainfall Measuring Mission (TRMM)/Brazil campaign are analyzed; specifically, the gamma parameters are retrieved for samples of convective and trailing stratiform rain during the 15 February 1999 squall- line event. Histograms of Nw and Do are retrieved from radar for each rain type and compared with related statistics reported in the literature. The functional behavior of Nw and Do versus rain rate retrieved from radar is compared against samples of 2D-video and RD-69 disdrometer data obtained during the campaign. The time variation of Nw, Do, and averaged over a 5 km × 5 km area (within which a network of gauges and a profiler were situated) is shown to illustrate temporal changes associated with the gamma parameters as the squall line passed over the network. The gauge- derived areal rainfall over the network is compared against radar using the areal dp method, and the concept of an effective slope of a linear axis ratio versus diameter model is shown to significantly reduce the bias in radar-derived rainfall accumulation.

In this paper radar rainfall estimates obtained from C-band Doppler polarimetric weather radar GPM 500C are compared with rain gauge measurements collected by three rain gauge networks during a two months period from September 1 to October 30, 1996 when many convective thunderstorms developed over the Po valley area. In order to verify the capability and the accuracy of radar rainfall estimates two different techniques of comparison with the rain gauges have been analyzed: the first one is based on pointwise comparison of conventional and/or multiparameter radar estimates with the rain gauges measurements, the second utilizes the matching of the cumulative distribution function observed by the two sensors. The results are discussed considering two different areas, where the rain gauges are at a distance less than 40 km and at a distance ranging between 40 and 80 km, respectively.