Search Results

You are looking at 1 - 10 of 20 items for

  • Author or Editor: Eugenio Gorgucci x
  • All content x
Clear All Modify Search
Luca Baldini and Eugenio Gorgucci

Abstract

Features of the melting layer of precipitation such as height and thickness are important in many meteorological applications. Doppler radar observations with a vertically pointing antenna have often been used in order to derive these features and to investigate the physical processes governing formation of the precipitation. This paper presents a technique to detect the characteristics of the melting layer based on the standard deviation of polarimetric radar measurements taken at vertical incidence. Using two case studies with stratiform and convective precipitation, the proposed technique is compared with conventional techniques based on reflectivity and mean Doppler velocity profiles. Data presented in this paper were obtained by the coherent dual-polarization C-band radar, Polar 55C, in Rome, Italy, during the summer and the fall of 2004.

Full access
Eugenio Gorgucci and V. Chandrasekar

Abstract

Monitoring of precipitation using high-frequency radar systems, such as the X band, is becoming increasingly popular because of their lower cost compared to their S-band counterpart. However, at higher frequencies, such as the X band, the precipitation-induced attenuation is significant, and introduces ambiguities in the interpretation of the radar observations. Differential phase measurements have been shown to be very useful for correcting the measured reflectivity for precipitation-induced attenuation. This paper presents a quantitative evaluation of two attenuation correction methodologies with specific emphasis on the X band. A simple differential phase–based algorithm as well as the range-profiling algorithm are studied. The impact of backscatter differential phase on the performance of attenuation correction is evaluated. It is shown that both of the algorithms for attenuation correction work fairly well, yielding attenuation-accurate corrected reflectivities with a negligible bias.

Full access
Eugenio Gorgucci and Luca Baldini

Abstract

An assessment of the performance of a self-consistent numerical method for dual-frequency radar based on the retrieval of microphysical precipitation parameter profiles is presented. From the surface reference technique (SRT), the estimation of path-integrated attenuation (PIA) is performed at both wavelengths and reflectivity factors are corrected for attenuation. Then, solving numerically a system of two nonlinear differential equations, the drop size distribution (DSD) parameters are obtained. The method is applied only in the stratiform rain region, from the surface along the path upward to the brightband bottom.

Assuming a gamma DSD model to describe the distribution of precipitation found in nature, a methodology has been developed to transform the estimated DSD provided by a vertically pointing Micro Rain Radar to a profile given by a ground-based Ku- and Ka-band radar, and then in a spaceborne dual-frequency radar measurement profile.

Under ideal conditions in which the different errors that simultaneously affect the retrieval of precipitation microphysical parameters may be individually studied, particular emphasis has been placed on the incidence of variability due to the DSD shape parameter μ, the presence of uncertainties in PIA estimates, and radar signal fluctuations.

To achieve an appropriate level of confidence in the simulation outputs, a qualitative indirect method of validation was realized by comparing the results obtained by the simulation with the experimental ones and weighing how consistent they are with what the theory implies. GPM near-real-time data from an entire year (October 2014–September 2015) were used for this purpose.

Full access
Eugenio Gorgucci and Luca Baldini

Abstract

The quantitative estimation of rain rates using meteorological radar has been a major theme in radar meteorology and radar hydrology. The increase of interest in polarimetric radar is in part because polarization diversity can reduce the effect on radar precipitation estimates caused by raindrop size variability, which has allowed progress on radar rainfall estimation and on hydrometeorological applications. From an operational point of view, the promises regarding the improvement of radar rainfall accuracy have not yet been completely proven. The main reason behind these limits is the geometry of radar measurements combined with the variability of the spatial structure of the precipitation systems. To overcome these difficulties, a methodology has been developed to transform the estimated drop size distribution (DSD) provided by a vertically pointing micro rain radar to a profile given by a ground-based polarimetric radar. As a result, the rainfall rate at the ground is fixed at all ranges, whereas the broadening beam encompasses a large variability of DSDs. The resulting DSD profile is used to simulate the corresponding profile of radar measurements at C band. Rainfall algorithms based on polarimetric radar measurements were taken into account to estimate the rainfall into the radar beam. Finally, merit factors were used to achieve a quantitative analysis of the performance of the rainfall algorithm in comparison with the corresponding measurements at the ground obtained from a 2D video disdrometer (2DVD) that was positioned beside the micro rain radar. In this method, the behavior change of the merit factors in the range is directly attributable to the DSD variability inside the radar measurement volume, thus providing an assessment of the effects due to beam broadening.

Full access
Gianfranco Scarchilli, Eugenio Gorgucci, and Roberto Leonardi

Abstract

In this paper we explain the general theory of the excess bias measurement as a radar parameter suitable for deriving information of meteorological relevance from the fluctuation of weather echoes.

Since the excess bias is the difference between the estimates of the radar reflectivity factor within the radar measurement cell obtained with the logarithmic and simulated receivers, the integral and differential characteristics of this measurement are accurately analyzed.

It is shown that, for specific reflectivity models, the excess bias is a monotonic function of the reflectivity variation within the measurement cell, and the experimental data confirm a close correlation between these two parameters.

Finally, we assess the influence of the simulated receiver on detecting reflectivity variations through excess bias measurements. The study refers to the class of receiver responses for which the output y is related to the input power P by: y = aPb, where a and b are characteristic of the receiver. The optimum receiver responses varies with the reflectivity models considered. Whenever the reflectivity field exhibits linear variation in a logarithmic scale, the optimum response approaches the response of a quadratic receiver (b = 1), as much as the reflectivity variations increase. For the models which describe reflectivity steps at the edge of precipitation cells, the optimum b is smaller than the corresponding one for the linear model; the greater the fraction of the measurement cell filled with precipitation, the smaller becomes the optimum b.

Full access
Eugenio Gorgucci, Gianfranco Scarchilli, and V. Chandrasekar

Abstract

Specific differential propagation phase (K DP) is estimated as the slope of the differential propagation phase (ΦDP). The estimate of specific differential propagation phase is typically obtained over a path to minimize measurement error. It is shown that the estimate of specific differential phase over a nonuniform rainfall path is biased, and the bias increases with increased reflectivity variation along the path. It is also shown that the bias can be both positive and negative depending on the nature of nonuniformity in the propagation path. Three models for nonuniform reflectivity variation along the precipitation path are studied. A simple algorithm is presented to correct the bias in the estimation of K DP due to nonuniform rainfall paths. Multiparameter radar data collected over central Florida are analyzed and compared to the theoretical results developed in this paper.

Full access
Eugenio Gorgucci, V. Chandrasekar, and Gianfranco Scarchilli

Abstract

Conventional usage of multiparameter radar measurements for rainfall estimation has been associated with tracking the variability of the raindrop size distribution. The use of multiparameter radar measurements in a statistical framework to estimate rainfall is presented in this paper. The techniques developed in this paper are applied to the radar and rain gauge measurement of rainfall observed on 26 July 1991, during the Convection and Precipitation Electrification program. Conventional pointwise estimates of rainfall are also compared. The probability matching procedure, when applied to the radar and surface measurements shows that multiparameter radar algorithms can match the probability distribution functions better than the reflectivity based algorithms, thereby indicating the potential of multiparameter radar measurements for statistical approach to rainfall estimation.

Full access
Eugenio Gorgucci, V. Chandrasekar, and Luca Baldini

Abstract

New algorithms for rain attenuation correction of reflectivity factor and differential reflectivity are presented. Following the methodology suggested for the first time by Gorgucci et al., the new algorithms are developed based on the self-consistency principle, describing the interrelation between polarimetric measurements along the rain medium. There is an increasing interest in X-band radar systems, owing to the early success of the attenuation-correction procedures as well as the initiative of the Center for Collaborative Adaptive Sensing of the Atmosphere to deploy X-band radars in a networked fashion. In this paper, self-consistent algorithms for correcting attenuation and differential attenuation are developed. The performance of the algorithms for application to X-band dual-polarization radars is evaluated extensively. The evaluation is conducted based on X-band dual-polarization observations generated from S-band radar measurements. Evaluation of the new self-consistency algorithms shows significant improvement in performance compared to the current class of algorithms. In the case that reflectivity and differential reflectivity are calibrated between ±1 and ±0.2 dB, respectively, the new algorithms can estimate both attenuation and differential attenuation with less than 10% bias and 15% random error. In addition, the attenuation-corrected reflectivity and differential reflectivity are within 1–0.2 dB 96% and 99% of the time, respectively, demonstrating the good performance.

Full access
Eugenio Gorgucci, Gianfranco Scarchilli, and Roberto Leonardi

Abstract

In this paper we analyze a method of real-time radar receiver calibration by means of multiple processing of the same meteorological echoes collected during the routine measurements.

It is shown that the method is independent of all parameters which aided the mean input power estimates in the same way (e.g., attenuation, radar constants and range) and its accuracy can be increased by increasing the number of the averaged samples.

Through simulating typical logarithmic receivers by computer, it is shown that in the nonlinear region of the transfer function, the error in the estimate of the mean input power is less than 0.4 dBm; in the linear region the error is negligible.

Finally, the comparison between the mean input power estimates obtained from the method proposed here by utilizing the radar data and from the measurements given by the microwave signal generator is discussed.

Full access
Eugenio Gorgucci, Gianfranco Scarchilli, and V. Chandrasekar

Abstract

Rainfall estimation using specific differential phase (K DP) involves estimation of K DP over a propagation path. The choice of pathlength plays an important role in the performance of rainfall algorithms using K DP. The path-integrated nature of K DP-based rainfall estimates may involve inhomogeneous paths, thereby having potential implications on the pathlengths used in algorithms for rainfall estimation and measurement errors. The effect of inhomogeneous rainfall paths (over which K DP is estimated) on the choice of rainfall rate algorithms is studied as a function of rain pathlength. Rainfall estimation is biased when algorithms relating rainfall rate and K DP are used over nonuniform paths. This bias is evaluated and compared for different rainfall algorithms. Radar and rain gauge data collected during the Convection and Precipitation/Electrification Experiment (CaPE) are used to evaluate the performance of K DP-based rainfall algorithms, for different pathlengths.

Full access