1,720,961 research outputs found
Seasonal sensitivity of a VIS-NIR-IR rain-no rain classifier
No full textMid-latitude precipitation characteristics are influenced by
the seasonal cycle: general circulation patterns, moisture
distribution and cloud type occurrence vary throughout
the year over a wide range of different structures. Since
radiation in the visible-infrared part of the spectrum is
sensitive to the cloud upper layers, the seasonal variability
of the cloud structure is expected to affect the capabilities
of satellite measurements to infer the precipitation at the
ground. This work aims to assess and quantify the seasonal
sensitivity of a statistical rain-no-rain classifier applied to
data from the moderate resolution imaging spectroradiometer
(MODIS) collected for summer and winter seasons
over the UK region. In the first part, the satellite radiance
measurement distributions for the two seasons were compared
and discussed. Then, the comparison between satellite
and ‘‘true’’ rain-no rain classification was carried out in
term of statistical parameters (such as the Equitable Threat
Score: ETS), showing their dependence on the dry to wet
ratio of the statistical ensemble considered. Finally, by considering
summer and winter datasets, the seasonal variability
of MODIS rain-no rain classifier performance has been
established and discussed. The sensitivity of the algorithm
to the number and wavelengths of the channels used has
been addressed, showing the high impact of the 1.6 mm
channel if combined with one visible channel. The best
performance was reached with six channels (0.85, 1.6,
3.9, 7.3, 8.5, and 12 mm), plus the solar zenith angle as
additional input, for which the computed ETS is about
45% for summer and 37% for winter, keeping a fixed dry
to wet ratio of 6. The use of an ‘‘annual’’ algorithm, trained
with ensemble of summer and winter pixels, and applied on
independent summer and winter ensembles, led to similar
values for both summer and winter
RAIN-RATE ESTIMATION FROM SEVIRI/MSG AND AMSR-E/AQUA. VALIDATION AND COMPARISON BY USING U.K. WEATHER RADARS
No full textThe satellite rainfall estimation algorithm here proposed is based on a statistical approach (Artificial Neural Network: ANN): it needs only radiation satellite measurements as input data and provides, at 5 km of spatial resolution, surface rain-rate classification onto five classes of precipitation: [less than 1/32] mm/h (no rain), [1/32, 0.125] mm/h (slight rain), [0.125, 0.5] mm/h (slight/moderate rain), [0.5, 2.0] mm/h (moderate), [more than 2.0] (heavy rain).
The algorithm works for U.K. area, daytime and summer season and adopts a cascade method where at first a rain no-rain classification is computed and then a similar yes-no classification is computed for the other pair of classes. It has been developed and validated with the use of U.K. weather radar rainfall estimates for both SEVIRI (on the geosynchronous Meteosat-8 satellite) and AMSR-E (on the low earth orbit AQUA satellite) sensors.
To assess the performance against radar rainfall estimation some skill indicators are computed: the Equitable Threat Score (ETS) and BIAS are used for pair of classes of precipitation whereas the Heidke Skill Score (HSS) is used for the four raining classes.
The validation procedure (over U.K. area and for June, July and August 2004, at noon time), shown that the nine channels SEVIRI classifier provides performances very close to the ones provided by the twelve channels AMSR-E classifier. The advantage of using AMSR-E measurements is more evident when only sea area is considered.
The analysis also show which are the best sets of channels (among the nine SEVIRI channels and the twelve AMSR-E channels) that give the most important contribute to the above performances
Multisensor analysis of convection in Mediterranean cyclones
No full text.A recent climatological study (Trigo et al., 1999) shown that the cyclonic activity is a key feature of the meteorology of the Mediterranean basin during the whole year. In particular, during warm months, short-living cyclones (averaged lifetime about 30 hours) rise from well known birthplaces (Gulf of Genoa, Gibraltar area, Atlas Mountains are the most effective) and affect the weather in the Mediterranean area. Following a different approach Porcú et al. (1997) shown that very often such cyclones force convective initialisation and development. Moreover those episodes are responsible for the most severe rainfall events sometime related with flood/flash flood occurrence in coastal and continental areas. The combination of long- lasting, moderate precipitation with heavy showers is the more common mechanism originating such floods. The cloud systems related to the cyclonic structure often develop over the sea: the scarcity of conventional observation available in the Mediterranean basin makes satellites peculiar points of view for a detailed study of these events. A complete analysis will include the use of ECMWF fields to assess the larger scale setting: in particular we'll consider the presence and intensity of Potential Vorticity Anomalies (PVAs) related to the cyclonic depression. PVAs are suspected to be precursor of heavy rainfall episodes (Massacand et al., 1999) and are powerful tools in classify cyclonic systems (Georgev, 1999). A further approach will be developed will include combined SSM/I-Meteosat water vapour retrieval over cloud-free areas, in order to evaluate the potential of convective initiation. In this work we present preliminary results of a multi-sensor, multi-frequency analysis of convective patterns in cyclonic structure. We modified a cloud classification algorithm originally developed for visible-infrared (VIS-IR) data (Porcù and Levizzani, 1992) to include also microwave radiances, to increase the informative content of the classification. In particular the SSM/I brightness temperature at 85 GHz is used together with the equivalent black body temperature as measured by the Meteosat infrared channel. The results show potential in understanding of the convective patterns, especially if embedded in cyclonic cloud bands, as it is common at mid-latitude
On the use of a SEVIRI-based statistical rainfall classification technique calibrated with TRMM-PR over southern Mediterranean
No full textA SEVIRI rainfall estimation technique based on Artificial Neural Networks (ANN) has recently been developed at the University of Ferrara. The algorithm makes use of the multispectral capabilities and the increased resolution of the new sensor on board the Meteosat Second Generation satellites. The present version of the algorithm provides rain maps in five rainrate levels, at 5 km of spatial resolution and 15 minutes of time resolution. It is tested and validated over U.K. area and for summer and winter season by using the Met Office Nimrod radar precipitation maps as ground truth. Performance are evaluated by calculating the Equitable Threat Score (ETS) and BIAS for rain – no rain classification.
The algorithm performances strongly depend upon the characteristics of the training dataset, such as season, climatic regime and latitude: to export the technique on a given target area is thus highly recommended to train the ANN with data from that area. In the frame of an EU Community initiative programme INTERREG III B ARCHIMED the focus of the project RiskMed are the Central and Western Mediterranean areas, where a reliable, high-resolution, ground radar based precipitation dataset, large enough to train the ANN, is not available. Nevertheless, almost the half of the domain is covered by the TRMM-PR overpasses. The TRMM product 2A25, in this first approach, is therefore used to assess the possibility to re-calibrate the U.K. trained algorithm by using a limited PR-based data set. TRMM-PR data collected over six months are used to train the ANN over the areas for two different seasons (winter and summer), and the performances are discussed
La precipitazione da satellite: un prodotto alternativo per le applicazioni agrometeorlogiche di monitoraggio territoriale
No full textLa piena operatività del sensore SEVIRI (Spinning Enhanced Visibile and Infra-Red Imager) in orbita sul METEOSAT-8
(Meteosat Second Generation, MSG) ha aperto nuove possibilità per il monitoraggio della precipitazione dallo spazio. Molte
tecniche sono allo studio per sfruttare pienamente il nuovo dataset disponibile rivolgendosi a differenti classi di possibili
utilizzatori, ciascuna con particolari specifiche richieste per il prodotto di stima di precipitazione da satellite. Le applicazioni
per l’agrometeorologia spaziano dalla previsione di intense precipitazioni distruttive (grandine) al continuo monitoraggio
anche di leggere precipitazioni. In particolare, la stima dei tempi di bagnatura fogliare richiede di poter monitorare anche
livelli di precipitazione bassi, comparabili con la rugiadazione, che di solito non sono rilevanti per le altre applicazioni delle
mappe di precipitazione da satellite, ma che hanno notevole impatto sullo stato della superficie fogliare.
Per un evento di precipitazione che ha interessato il Friuli nel maggio del 2004 si sono elaborati i dati del radar di Fossalon
di Grado e del sensore SEVIRI ottenendo mappe di precipitazione attraverso uno schema a rete neurale. Il confronto tra le
mappe radar e satellite viene qui effettuato al fine di fornire un’alternativa alla disponibilità di stima ad alta risoluzione
spazio-temporale prodotta dai radar polarimetrici laddove essi non siano disponibili o sufficientemente accurati. I risultati
ottenuti nella classificazione pioggia-non pioggia possono essere considerati accettabili (ETS 30% circa) rendendo tale tecnica
utilizzabile da sola o in sinergia con le misure radar
Deep convection in Mediterranean cyclones
No full text.The aim of this paper is to review the problem of deep convection in the Mediterranean basin with special emphasis on phenomena where convection is linked with larger scale structures. It is commonly recognized that cyclonic structures trigger often convective systems in the Mediterranean, and the interaction between such cloud systems and cyclones has not yet been properly investigated. As a major finding of the EU Project MEFFE (Prodi, 2000), a short term climatology of flood events in Europe has shown that the most severe flood episodes occurring in Europe over recent years were caused by cyclonic systems. In particular, two kind of cyclones have been defined, different in their basic structures: Atlantic and Mediterranean (Porcú et.al., 1997a). The first class includes events where the depression is at high latitude (above 50N), presents well defined frontal structures, lasts for several days and produces mainly stratified precipitation. Conversely, Mediterranean cyclones are mainly generated in Southern Europe as a perturbation of a polar front, are usually short lived (about one day) and always develop deep convection. While the first class has been studied extensively and conceptual models describing the main features have been developed and are widely used in the operational forecasting environment (see Browning (1982) for a summary), the study of the Mediterranean class is still at an early stage. Isolated and organized convection has been intensively studied in Europe over the last decade, and it has been seen that Europen convective systems show new features with respect to the well known systems in the U.S. and tropics. Nevertheless, only recently has the close connection between cyclogenesis and deep convection been pointed out. Since such cloud systems develop mainly over the sea, it seems that the extensive use of ground radar must be excluded for such studies: a space-view approach is needed. A review of satellite techniques for studying convection is presented, and some new results of their application to Mediterranean systems are summarized. The structure of this paper is as follows: Section 2 gives an overview of the research activity on convection in Europe; Section 3 outlines some techniques used for the study of convection from space; Section 4 presents a kind of classification of convective systems occurring in the Mediterranean with a possible interpretation of some mechanisms involved
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
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
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