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21.
Pavlos Protopapas Raul Jimenez Charles Alcock 《Monthly notices of the Royal Astronomical Society》2005,362(2):460-468
We present an algorithm that allows fast and efficient detection of transits, including planetary transits, from light-curves. The method is based on building an ensemble of fiducial models and compressing the data using the moped compression algorithm. We describe the method and demonstrate its efficiency by finding planet-like transits in simulated Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) light-curves. We show that our method is independent of the size of the search space of transit parameters. In large sets of light-curves, we achieve speed-up factors of the order of 103 times over an optimized adaptive search in the χ2 space. We discuss how the algorithm can be used in forthcoming large surveys like Pan-STARRS and the Large Synoptic Survey Telescope (LSST), and how it may be optimized for future space missions like Kepler and COROT where most of the processing must be done on board. 相似文献
22.
P. A. Kassomenos A. K. Paschalidou A. Vlachogianni 《Stochastic Environmental Research and Risk Assessment (SERRA)》2013,27(2):561-572
The adverse health effects associated with exposure to CO range from the more subtle cardiovascular effects at low concentrations to death after acute or chronic exposure to higher concentrations. The forecasting of the daily CO maximum levels is therefore essential in every attempt for protecting and improving public health in urban areas. The objective of this work is to create a suite of statistical models for predicting the one-day-ahead maximum CO levels based on both the meteorological and the pollutant data recorded in six monitoring sites in the greater area of Athens, Greece. The meteorological variables used as input consist of hourly values of the surface air Temperature, the Relative Humidity, the Wind Speed and the Wind Direction, while the pollutant parameters consist of hourly concentrations of nitrogen oxide, nitric dioxide, ozone and sulfur dioxide, all corresponding to the 7-year-period between 2001 and 2007. The models were developed on a seasonal (warm vs. cold period) and hebdomadal (workdays vs. weekends) basis and revealed that the influence of the air pollution levels recorded one day before (day m?1) on the maximum CO concentrations of day m is quite variable and depends on the site/type of the station, the local meteorology and the emission sources. Additionally, the analysis revealed that the CO concentrations are influenced by both local and/or wider area CO sources, suggesting a strong persistence of the CO levels, while only local meteorology (e.g. in the vicinity of the station and especially during working days) plays a role in the formation of present day’s CO levels. The derived models were validated against an independent yearlong data set (2008) through the use of a classical set of validation parameters known as the Model Validation Kit. Indices assessing the ability of the models to predict the CO exceedances of the EC limit value were also used. On the whole, it was found that the prognostic models introduced here manage to predict the CO maximum daily values in a satisfactory level, with Pearson’s correlation coefficients ranging between 0.62 and 0.76 during the warm period and between 0.51 and 0.80 during the cold period of the year. Similarly the index of agreement ranges between 0.50–0.95 during the warm period and 0.57–0.81 during the cold period of the year, revealing a rather adequate model performance. 相似文献
23.
Bartosz Hawelka Izabela Sitko Euro Beinat Stanislav Sobolevsky Pavlos Kazakopoulos Carlo Ratti 《制图学和地理信息科学》2014,41(3):260-271
Pervasive presence of location-sharing services made it possible for researchers to gain an unprecedented access to the direct records of human activity in space and time. This article analyses geo-located Twitter messages in order to uncover global patterns of human mobility. Based on a dataset of almost a billion tweets recorded in 2012, we estimate the volume of international travelers by country of residence. Mobility profiles of different nations were examined based on such characteristics as mobility rate, radius of gyration, diversity of destinations, and inflow–outflow balance. Temporal patterns disclose the universally valid seasons of increased international mobility and the particular character of international travels of different nations. Our analysis of the community structure of the Twitter mobility network reveals spatially cohesive regions that follow the regional division of the world. We validate our result using global tourism statistics and mobility models provided by other authors and argue that Twitter is exceptionally useful for understanding and quantifying global mobility patterns. 相似文献
24.
Environmental study for pollution in the area of Megalopolis power plant (Peloponnesos,Greece) 总被引：1，自引：0，他引：1
Apostolos Sarris Eleni Kokinou Eleni Aidona Nikolaos Kallithrakas-Kontos Pavlos Koulouridakis Georgia Kakoulaki Kassiani Droulia Ourania Damianovits 《Environmental Geology》2009,58(8):1769-1783
The main purpose of the present study is to investigate the suitability of magnetic properties in correlation to geochemical
measurements as a pollution-monitoring tool and study the metal transmission factors in an area around a major local source,
namely a lignite-burning power plant. Surface soil samples were collected in the wide area of the power plant of Megalopolis
(Peloponnesos, Greece). The magnetic susceptibility and other magnetic properties of the surface soils were originally measured
and mapped. Loci of high values of magnetic susceptibility within the study area gave rise to further analyze the soil samples
for metal concentrations. GIS techniques were used for mapping all the chemical constituent concentrations and the magnetic
measurements on the various topographic and geological features of the area. Maps were created through interpolation algorithms
indicating the spatial distribution of the above measurements. Spatial tools and statistical analysis through the calculation
of Pearson’s coefficients proved the correlation between magnetic properties, metal concentrations and the terrain attributes
(especially the geological structure and the wind currents) of the region. 相似文献
25.
The water piercing missile launcher (WPML) is a new concept for launching missiles from submerged platforms. The WPML employs a high speed gas jet, using rocket exhaust as the gas source, to create a dry path underwater through which a missile may pass without contacting water. The gas jet is deflected due to cross flow and the gas jet trajectory is computed through a semi-empirical relationship commonly used to describe single phase jet deflection. This relationship, which requires an experimentally determined constant to predict jet deflection, is computed using experimental data. Uncoupled simulations of rocket exhaust and missile dynamics are shown to demonstrate how such a launcher could be utilized in launching a generic artillery missile. Although the results indicate the optimal launch depth, in terms of maximizing the launch depth while minimizing missile restraint time, is 14 m given an 8 m/s submarine speed it may be possible to launch a missile from a moving submarine at a speed of 5 m/s at a maximum depth of 20 m. 相似文献
26.
C. P. Jacovides P. Kassomenos N. A. Kaltsunides 《Theoretical and Applied Climatology》1996,53(4):211-220
Summary This study reports a 37-year long record of direct beam spectral irradiance measurements made in Athens, Greece. An analysis of aerosol effects on the spectral distribution of solar radiation through effective optical depths, are presented. Thus, spectrally resolved aerosol optical depths were calculated and analyzed for the period 1954–1990. Summertime aerosol optical depths were found to be larger than winter values, while their seasonal variations were related to varying weather conditions throughout the year. The interrelationships between effective optical depths were found to be linear and were related strongly to microphysics of aerosol loading in the atmosphere. For the period 1962–1983 as wavelength exponent
0 values ranged between 0.76–1.14 the spectrally resolved optical depths were found to increase markedly with respect to remaining periods 1954–1961 and 1984–1990 in which
0 values ranged between 1.16–1.39. A minimum in aerosol optical depths, believed to be near background levels, was reached during period 1954–1957, while there was some indication that both optical depths continued to decrease reaching background levels at the end of the study period. From the long-term variation of aerosol effective optical depths some interesting information on the time evolution of air quality in Athens was gained. In addition, their frequency distribution, temporal daily variations and some remarks on photosynthetically active radiation for plant development, are presented and discussed.With 7 Figures 相似文献
27.
The aim of this study was to identify local and exogenous sources affecting particulate matter (PM) levels in five major cities of Northern Europe namely: London, Paris, Hamburg, Copenhagen and Stockholm. Besides local emissions, PM profile at urban and suburban areas of the European Union (EU) is also influenced by regional PM sources due to atmospheric transport, thus geographical city distribution is of a great importance. At each city, PM10, PM2.5, NO2, SO2, CO and O3 air pollution data from two air pollution monitoring stations of the EU network were used. Different background characteristics of the selected two sampling sites at each city facilitated comparisons, providing a more exact analysis of PM sources. Four source apportionment methods: Pearson correlations among the levels of particulates and gaseous pollutants, characterisation of primal component analysis components, long-range transport analysis and extrapolation of PM size distribution ratios were applied. In general, fine (PM2.5) and coarse (PM10) particles were highly correlated, thus common sources are suggested. Combustion-originated gaseous pollutants (CO, NO2, SO2) were strongly associated to PM10 and PM2.5, primarily at areas severely affected by traffic. On the contrary, at background stations neighbouring important natural sources of particles or situated in suburban areas with rural background, natural emissions of aerosols were indicated. Series of daily PM2.5/PM10 ratios showed that minimum fraction values were detected during warm periods, due to higher volumes of airborne biogenic PM coarse, mainly at stations with important natural sources of particles in their vicinity. Hybrid single-particle Lagrangian integrated trajectory model was used, in order to extract 4-day backward air mass trajectories that arrived in the five cities which are under study during days with recorded PM10 exceedances. At all five cities, a significantly large fraction of those trajectories were classified in short- and medium-range clusters, thus transportation of particulates along with slow moving air masses was identified. A finding that supports the assumption of long-range transport is that, at background stations, long-range transportation effects were stronger, in comparison to traffic stations, due to less local particle emissions. Short-range trajectories associated to PM transport in Stockholm, Copenhagen and Hamburg were mainly of a continental origin. All three cities were approached by slow moving air masses originated from Poland and the Czech Republic, whereas Copenhagen and Stockholm were also influenced by short-range trajectories from Germany and France and from Jutland Peninsula and Scandinavian Peninsula, respectively. London and Paris are located to the north-west part of Europe. Trajectories of short and medium length arrived to these two megacities mainly through France, Germany, UK and North Atlantic. 相似文献
28.
G. A. Efstathiou C. J. Lolis N. M. Zoumakis P. Kassomenos D. Melas 《Theoretical and Applied Climatology》2014,115(1-2):259-279
Raingauge data from four meteorological stations in Chalkidiki peninsula (Greece) were used to identify the characteristics of the synoptic circulation patterns associated with cold-season heavy rainfall events and corresponding flooding over the area. Precipitation climatology over the complex topography of Chalkidiki is characterized by limited annual rainfall, but in the occurrence of heavy rainfall episodes daily accumulations are exceptionally high with increased precipitation rates, leading often to severe flooding. Fifty-five cases of high daily accumulations for the period of 1997 to 2010, mostly observed during December and October, were classified into eight clusters by applying S-Mode Factor Analysis and Cluster Analysis to the ERA-Interim grid point reanalysis data. The results revealed that, in most cases, intense rainfall and flooding is produced by synoptic scale disturbances that generate and sustain cyclonic activity over south Italy, the Ionian, and less frequently over the Aegean Sea. In particular, the atmospheric circulation patterns associated with heavy rainfall are characterized by the presence of a southeasterly–easterly low-level humid flow over Chalkidiki in conjunction with the potentially unstable lower troposphere influenced by mid-level cyclonic vorticity advection and enhanced low-level convergence over the complex terrain. 相似文献
29.
Dae-Won Kim Pavlos Protopapas Charles Alcock Yong-Ik Byun Federica B. Bianco 《Monthly notices of the Royal Astronomical Society》2009,397(1):558-568
We present a detrending algorithm for the removal of trends in time series. Trends in time series could be caused by various systematic and random noise sources such as cloud passages, changes of airmass, telescope vibration, CCD noise or defects of photometry. Those trends undermine the intrinsic signals of stars and should be removed. We determine the trends from subsets of stars that are highly correlated among themselves. These subsets are selected based on a hierarchical tree clustering algorithm. A bottom-up merging algorithm based on the departure from normal distribution in the correlation is developed to identify subsets, which we call clusters. After identification of clusters, we determine a trend per cluster by weighted sum of normalized light curves. We then use quadratic programming to detrend all individual light curves based on these determined trends. Experimental results with synthetic light curves containing artificial trends and events are presented. Results from other detrending methods are also compared. The developed algorithm can be applied to time series for trend removal in both narrow and wide field astronomy. 相似文献
30.
P. Kassomenos H. A. Flocas S. Lykoudis M. Petrakis 《Meteorology and Atmospheric Physics》1997,62(3-4):249-256
Summary Serious events of radiative frost were reported in an important agricultural area of central Macedonia, the valley of Lagadas, at the end of March 1994. Since the phenomenon has a strong local character and no surface observations are available, a simulation is performed using a three dimensional numerical model in order to investigate the horizontal and vertical thermal structure of the lower troposphere during the frost event. The synoptic analysis reveals that the background flow was weak, associated with anticyclonic circulation and insignificant pressure gradient in the lower troposphere. Under these synoptic conditions, it was demonstrated that the meteorological model represents successfully the temperature and wind fields over the valley of Lagadas during the frost event. It appears that the model can serve as an alternative method to analyse the characteristics of the radiative frost that occurs very frequently over the examined area associated with substantial economic repercussion.With 10 Figures 相似文献