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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.

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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.

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Luca Baldini and William Emery
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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.

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Luca Baldini and William Emery
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Eugenio Gorgucci, V. Chandrasekar, and Luca Baldini

Abstract

The recent advances in attenuation correction methodology are based on the use of a constraint represented by the total amount of the attenuation encountered along the path shared over each range bin in the path. This technique is improved by using the inner self-consistency of radar measurements. The full self-consistency methodology provides an optimization procedure for obtaining the best estimate of specific and cumulative attenuation and specific and cumulative differential attenuation.

The main goal of the study is to examine drop size distribution (DSD) retrieval from X-band radar measurements after attenuation correction. A new technique for estimating the slope of a linear axis ratio model from polarimetric radar measurements at attenuated frequencies is envisioned. A new set of improved algorithms immune to variability in the raindrop shape–size relation are presented for the estimation of the governing parameters characterizing a gamma raindrop size distribution.

Simulations based on the use of profiles of gamma drop size distribution parameters obtained from S-band observations are used for quantitative analysis. Radar data collected by the NOAA/Earth System Research Laboratory (ESRL) X-band polarimetric radar are used to provide examples of the DSD parameter retrievals using attenuation-corrected radar measurements. Retrievals agree fairly well with disdrometer data. The radar data are also used to observe the prevailing shape of raindrops directly from the radar measurements. A significant result is that oblateness of drops is bounded between the two shape models of Pruppacher and Beard, and Beard and Chuang, the former representing the upper boundary and the latter the lower boundary.

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Eugenio Gorgucci, Luca Baldini, and V. Chandrasekar

Abstract

In a previous study, Gorgucci et al. showed the potential advantage of using together polarimetric radar measurements of reflectivity factor, differential reflectivity, and specific differential propagation phase, in order to gather information about the calibration of radar systems. Scarchilli et al. generalized this concept in the self-consistency principle, which stated that, given a drop-shape model to describe the form of raindrops, the corresponding radar measurements are constrained on this three-dimensional surface. In this work the self-consistency principle is collapsed onto a two-dimensional domain defined by the variables: 1) the ratio between specific differential phase and reflectivity factor, and 2) differential reflectivity. In this space the scatter of drop size distribution (DSD) variability is minimized in such a way that drop-shape variability shows up. This methodology is used to observe for the first time the predominant shape of raindrops directly from the radar measurements. The radar polarimetric data were collected in two different climatological regions as central Florida and northern Italy. The significant result shows that the underlying mean axis ratio approaches the model established by Pruppacher and Beard, and the relationship described by Beard and Chuang forms a sort of border for the sphericity of the drop shape.

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Elisa Adirosi, Luca Baldini, and Ali Tokay

Abstract

A well-designed deployment of well-maintained surface instruments as well as abundant rainfall provided an excellent dataset with which to evaluate the Micro Rain Radar (MRR) performance for estimating raindrop size distribution (DSD) and its integral rainfall parameters with respect to the consolidated devices during the Iowa Flood Studies (IFloodS) field campaign. The MRR was collocated with two-dimensional video disdrometer (2DVD) and Autonomous Parsivel2 Unit (APU) at three different sites located at 5–70-km distances from the National Aeronautics and Space Administration’s S-band dual-polarization Doppler radar (NPOL). A comparative study between MRR, 2DVD, APU, and NPOL was conducted including all rainy minutes as well as minutes of stratiform rain and convective rain. Considering 2DVD as a primary reference, a good agreement was evident for reflectivity between MRR’s lowest reliable height and 2DVD with an absolute bias of less than 2 dB even in convective rain except for one site. For rainfall rate, the percent absolute bias between MRR and 2DVD ranged between 25% and 35% in stratiform rain and about 10% higher in convective rain. Agreement for mean mass-weighted raindrop diameter was good (bias less than 0.1 mm), whereas MRR overestimated the normalized intercept parameter of the gamma DSD [mean bias among the three sites was −0.13 log(mm−1 m−3)]. The agreement between MRR and APU was slightly worse than the one between MRR and 2DVD. When the horizontal and differential reflectivities of NPOL were compared with the ones derived from the MRR DSD resampled within the radar volume, we found an absolute bias of approximately 3 and 0.4 dB, respectively.

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Eugenio Gorgucci, V. Chandrasekar, and Luca Baldini

Abstract

A method is proposed to retrieve raindrop shape–size relations from the radar measurements of reflectivity factor Zh, differential reflectivity Z dr, and specific differential phase K dp at S band. This procedure is obtained using a domain defined by the two variables K dp/Zh and Z dr where the drop size distribution (DSD) variability is collapsed onto a line and any variation is essentially due to the drop shape variability. To obtain information on the raindrop shape–size relation underlying a set of radar observations, this domain is studied in conjunction with another domain describing the relation between the drop axial ratio (or shape) and its equivolumetric diameter. Using an initial drop shape and choosing a set of DSDs described by a normalized gamma model, polarimetric radar measurements are produced by simulation. An averaged curve of K dp/Zh versus Z dr is obtained and compared with the same curve obtained from the radar data. By changing the initial axial ratio relation, a procedure of minimization between the two curves is developed to derive the underlying drop shape–size relation governing the radar measurements under consideration. Three sets of radar data collected in different climatic regions are analyzed to evaluate whether there is a unique shape–size relation.

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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.

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