1,721,011 research outputs found
Raindrop Size Distribution under Drop Break-up: Implications for GPM DPR Algorithm
No full textThe NASA's Global Precipitation Measurement (GPM) mission dual-frequency precipitation radar retrieval has adopted a three-parameter gamma distribution to retrieve the raindrop size distribution (DSD) from dual-frequency reflectivity radar (DPR) measurements. The disdrometer-based DSD measurements collected during GPM ground validation (GV) field experiments shows that the three-parameter gamma distribution represents the observed size spectra quite well most of the time. A possible exception to this are the size spectra in the presence of drop break-up. In the presence of drop break-up, the DSD shows a secondary maxima that occurs at around 2.2 mm in diameter following a local minimum at around 1.4 mm. This feature is observed at rain rates above 5 mm h-1 and is result of collisions between large and small drops. Basically, the collisional kinetic energy is not absorbed as the drops collide. This results in a depletion of drops around 1.4 mm with an increase in small drops (< 1 mm). This study investigates the presence of drop break up using observations from two-dimensional video and Parsivel disdrometers. An automated algorithm based on the slope of the size distribution between 1.0 and 2.5 mm suggests that break-up occurs in 8% or fewer of the cases depending on the nature of the storm. The study also determines the appropriateness of the gamma fitting through analyzing the correlation between the measured and gamma DSDs in the presence and absence of drop break-up. The parameters of the gamma distribution under break-up conditions differ from the parameters under non break-up conditions. These differences may play a role in the accuracy of the DSD parameters retrieved from DPR measurements
Comparison of different fittings of experimental DSDs
No full textThe knowledge of drop size distribution (DSD) of rain, namely the frequency distribution of drop equivolume
diameters, has a wide range of applications in earth sciences such as precipitation physics, hydrology and agricultural
and soil sciences, it is also important in precipitation remote sensing, especially in radar meteorology for
relationships among rainfall rate and radar measurements such as the radar reflectivity factor. In general, retrieval
of parametric DSDs would aim to best model the largest portion of measured drop spectra, as a consequence, there
is no guarantee that the selected distribution will adequately model some DSD portions, such as the tail. However,
for characterising physical quantities such as the liquid water content and radar reflectivity, the right tail is critical
because large drops play a much more important role than small droplets. In order to study the influence of various
tail-types, four different one-sided continuous distributions (the Pareto, the Lognormal, the Gamma and the
Weibull distributions) have been fitted both to the large drops only and to the entire sample of the measured spectra.
Observational data consist of 1-min spectra collected by two-dimensional video disdrometer (2DVD). One
dataset was measured during the first special observation period of the hydrological cycle in the Mediterranean
experiment (HyMeX) field campaign in Rome from September to November 2012, while the second one during
the Mid-latitude Continental Convective Clouds Experiment (MC3E) field campaign in Oklahoma from April to
June 2011. The results obtained for the two different datasets are consistent. Results of this preliminary analysis
show that considering the whole fitting the Weibull distribution seems to fit the highest percentages of the measured
drop spectra (37% for HyMex and 42% for MC3E), on the other end this distribution is closely followed
by the Gamma and the Lognormal distribution, with approximately 30% of success. While for the tail fitting the
performances of the Weibull and Lognormal distributions increase to the detriment of the Gamma distribution;
the Weibull distribution has the highest percentage of success for the Hymex dataset, while for the MC3E dataset
the Lognormal distribution fits the highest number of measured spectra. For both the datasets, when the Weibull
distribution performs the best fitting, the shape parameter of the distribution is greater than one
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A Field Study of Footprint-Scale Variability of Raindrop Size Distribution
No full textA network of seven two-dimensional video disdrometers (2DVD), which were operated during the Midlatitude
Continental Convective Clouds Experiment (MC3E) in northern Oklahoma, are employed to investigate
the spatial variability of raindrop size distribution (DSD) within the footprint of the dual-frequency
precipitation radar (DPR) on board the National Aeronautics and Space Administration’s Global Precipitation
Measurement (GPM) mission core satellite. One-minute 2DVD DSD observations were interpolated
uniformly to 13 points distributed within a nearly circular DPR footprint through an inverse
distance weighting method. The presence of deep continental showers was a unique feature of the dataset
resulting in a higher mean rain rate R with respect to previous studies. As a measure of spatial variability for
the interpolated data, a three-parameter exponential function was applied to paired correlations of three
parameters of normalized gamma DSD, R, reflectivity, and attenuation at Ka- and Ku-band frequencies of
DPR (Z_Ka, Z_Ku, k_Ka, and k_Ku, respectively). The symmetry of the interpolated sites allowed quantifying
the directional differences in correlations at the same distance. The correlation distances d0 of R, k_Ka,
and k_Ku were approximately 10 km and were not sensitive to the choice of four rain thresholds used in this
study. The d0 of Z_Ku, on the other hand, ranged from 29 to 20 km between different rain thresholds. The
coefficient of variation (CV) remained less than 0.5 for most of the samples for a given physical parameter,
but a CV of greater than 1.0 was also observed in noticeable samples, especially for the shape parameter
and Z_Ku
Rainfall microphysical characterization over the Mediterranean area during the GPM era
No full textThe NASA/JAXA Global Precipitation Measurement (GPM) Core Observatory (CO) was launched on 27th February, 2014. It carries, for the first time, a Dual-frequency Precipitation Radar (DPR) designed to provide insights into the 3-D structure of precipitating clouds and rain intensity by using its Ka- and Ku-band frequencies. In addition to characterize the 3-D structure of precipitation, the DPR is used as calibrator for the GPM Microwave Imager (GMI). Single-frequency (SF) (both Ku- and Ka-only) and double-frequency (DF) based products provide, among the others, particle-size distribution (PSD) parameters (namely the mass-weighted mean diameter Dm and the normalized intercept parameter Nw), as well as precipitation rates. This book chapter focuses on reliability of the PSD parameters (limited to rainfall events, in this case we'll talk about of DSD - drop size distribution) over the Mediterranean area by taking as reference the DSD parameters estimated by ground-based radar measurement. Before of this, an inter-comparison between the SF and DF DPR outputs considering five years of data from GPM-CO mission is carried out. The goal is to investigate the reliability of SF-based products by assessing their quality compared to the DF-based ones, treated as a reference
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
