Search

Femtosecond light pulses carrying optical angular momentums (OAMs), possessing intriguing properties of helical phase fronts and ultrafast temporal profiles, enable many applications in nonlinear optics, strong-field physics, and laser micromachining. While generation of OAM-carrying ultrafast pulses and their interactions with matters are intensively studied in experiments, 3D characterization of ultrafast OAM-carrying light beams in spatiotemporal domain has, however, proved difficult to achieve. Conventional measurement schemes rely on the use of a reference pulsed light beam, which needs to be well characterized in its phase front and to have sufficient overlap and coherence with the beam under test, largely limiting practical applications of these schemes. Here a self-referencing set-up is demonstrated based on a tilted interferometer that can be used to measure complete spatiotemporal information of OAM-carrying femtosecond pulses with different topological charges. Through scanning one interferometer arm, the spectral phase over the pulse spatial profile can be obtained using the tilted interference signal, and the temporal envelope of the light field at one particular position around its phase singularity can be retrieved simultaneously, enabling 3D beam reconstruction. This self-referencing technique, capable of measuring spatiotemporal ultrafast optical-vortex beams, may find many applications in fields of nonlinear optics and light-matter interactions. [ABSTRACT FROM AUTHOR]

• Study traffic dynamics in ride-hailing mobility networks. • Develop a mesoscopic model with high spatio-temporal resolutions. • Calibrate and verify the model by empirical data. • Conduct a set of sensitivity analyses to show the performance of the model. Ride-hailing vehicles, private vehicles, and passengers are integral components of ride-hailing markets. Accurately characterizing the traffic dynamics driven by the spatio-temporal variations of these traffic flows is crucial for formulating operational strategies to realize sustainable ride-hailing services. From the mesoscopic perspective, we develop an integrated simulation model with high spatio-temporal resolutions. In a multi-class cell transmission model, we embed aggregate-ratio based decision-making mechanisms and bilateral matching between waiting passengers and idle vehicles in a large-scale non-equilibrium ride-hailing mobility network. At the individual level, the simulation model can capture the entire trip chain of passengers. Simultaneously, it can describe the cruising strategy of idle vehicles and the routing strategy of reserved/occupied/private vehicles. At the network level, it can depict the real-time space distribution of these multi-class traffic flows in the ride-hailing mobility network. We use empirical data, including road network density data, ride-hailing order, and trajectory data, to calibrate and verify the proposed simulation model. Sensitivity analyses based on simulation experiments indicate that the matching strategy, fleet size, and background traffic have diverse and significant effects on the operation performance of ride-hailing services. [ABSTRACT FROM AUTHOR]

Drought can be considered one of the most severe and complex weather-related natural hazards. It is a relevant stressor for ecosystems, affecting vegetation, ecosystem productivity, and water and carbon cycles, with a complex web of related impacts. Despite the interlink between the spatial and temporal scales of droughts, these two aspects are often studied separately. In addition, studies generally focus on detecting the events, without trying to investigate similarities among them. In this work, we introduce a set of tools used to summarize the main properties of major droughts in Europe, with the goal of subdividing the events in groups characterized by similar properties. We used a European dataset of meteorological droughts (from 1981 to 2020) that detects events based on the Standardized Precipitation Index using an event-oriented spatio-temporal clustering algorithm. From the analysis, we identified three groups of major meteorological droughts: a first group that is comprised by warm-season events, characterized by a longer duration, a shorter early growing phase, and a longer exhaustion phase; a second group, less numerous, comprised by droughts occurring during the cold season, that tend to have a shorter duration, a longer early growing phase and a shorter exhaustion phase; and a third group comprised of droughts occurring across the two periods. This last class is characterized by a longer duration and a high variability in most of the other characteristics, suggesting that these events may be associated with a large range of driving mechanisms. The proposed procedure allows for a drought classification that can be used for better understanding the mechanisms behind spatio-temporal evolution of these events.

Agriculture residue (in the form of stubble) burning is commonly practised in the northern states of India along the Indian part of the Indo-Gangetic Plains (IGP), predominantly in the states of Punjab and Haryana. In the present study, we characterize spatiotemporal patterns of stubble burning in Punjab and Haryana states of India, using active fire data from Moderate Resolution Imaging Spectroradiometer (MODIS) and Suomi National Polar-orbiting Partnership-Visible Infrared Imaging Radiometer Suite (Suomi NPP-VIIRS) for kharif cropping season (September-November) 2014. Analysis of active fire locations derived from MODIS and VIIRS during September-November 2014 suggest intense practice of stubble burning in the study area with total fire detections going up to 15,222 (MODIS) and 15,568 (VIIRS). Comparative analysis of MODIS and VIIRS active fire detections suggested that VIIRS is more sensitive with higher detection capability. Further, grid-based (5 × 5 km) analysis of fire patterns, viz. total fire detections, fire frequency and total fire intensity using temporal (kharif season for 2004-2014) MODIS active fire locations suggest intense burning activity in the central and southern districts of Punjab during the study period. Temporal analysis of MODIS active fire locations (2004-2014) suggests a transition of decrease to increase in the total number of fire detections during October to November respectively, during 2004 through 2014. [ABSTRACT FROM AUTHOR]

Canopies are complex multilayered structures comprising individual plant crowns exposing a multifaceted surface area to sunlight. Foliage arrangement and properties are the main mediators of canopy functions. The leaves act as light traps whose exposure to sunlight varies with time of the day, date and latitude in a trade-off between photosynthetic light harvesting and excessive or photoinhibitory light avoidance. To date, ecological research based upon leaf sampling has been limited by the available technology, with which data acquisition becomes labour intensive and time-consuming, given the overwhelming number of leaves involved., In the present study, our goal involved developing a tool capable of measuring a sufficient number of leaves to enable analysis of leaf populations, tree crowns and canopies. We specifically tested whether a cell phone working as a 3D pointer could yield reliable, repeatable and valid leaf angle measurements with a simple gesture. We evaluated the accuracy of this method under controlled conditions, using a 3D digitizer, and we compared performance in the field with the methods commonly used. We presented an equation to estimate the potential proportion of the leaf exposed to direct sunlight ( SAL) at any given time and compared the results with those obtained by means of a graphical method., We found a strong and highly significant correlation between the graphical methods and the equation presented. The calibration process showed a strong correlation between the results derived from the two methods with a mean relative difference below 10%. The mean relative difference in calculation of instantaneous exposure was below 5%. Our device performed equally well in diverse locations, in which we characterized over 700 leaves in a single day., The new method, involving the use of a cell phone, is much more effective than the traditional methods or digitizers when the goal is to scale up from leaf position to performance of leaf populations, tree crowns or canopies. Our methodology constitutes an affordable and valuable tool within which to frame a wide range of ecological hypotheses and to support canopy modelling approaches. [ABSTRACT FROM AUTHOR]

The utilization of multiple antennas and space-time codes in multiple-input and multiple-output (MIMO) communication systems significantly improves the transmission channel capacity without using additional bandwidth and power. The improvement is achieved by decomposing the spatial structure of transmission channels and performing appropriate temporal and spatial multiplexing. In this paper, we propose a novel theoretical framework for MIMO channel modeling and characterization in order to facilitate the MIMO system design and performance evaluation. The channels are represented in space, time, wave vector, and frequency domains while the space-time and wave vector-frequency interdependences are considered. A realization of the theoretical framework, in a form of a practical framework, is also proposed to address the channel modeling and characterization at both transmitter and receiver sides. The utilization of the practical framework in MIMO communication system design is discussed to illustrate its applications in realistic scenarios. The angle of arrival estimation based on the proposed practical framework using field test measurement data is also presented as illustrative examples. [ABSTRACT FROM AUTHOR]

• The interaction of the electron beam with the all-optical image sensor has been simulated based on the Monte Carlo computational method for the first time. • An InP-based all-optical image sensor was firstly applied in spatio-temporal characterization of pulse laser beams and paved the way for applications in electron and other particle beams. • Fringes in the recorded images have been effectively removed with a filter method based on Fourier transformation. • A differential method has been developed to increase the temporal resolution over the duration time of probe beams. A method for analyzing the spatio-temporal intensity distribution of pulse beams, such as photons, electrons and other particles, with an all-optical image sensor has been developed. The image sensor measures the photon or particle intensity by probing the induced refractive index change due to their excitation in the semiconductor. In order to prove this scheme, the interaction of the electron beam with the image sensor has been simulated, indicating that the transient phase grating can be created in the semiconductor. The transient phase grating can be probed with a laser to evaluate the spatio-temporal intensity distribution of the incident electrons. In addition, an InP-based image sensor has been established for analyzing pulse laser beams. A pump-probe technique has been utilized to record images with varying delays between the analyzed and probe lasers and a differential method has been developed to improve the temporal resolution to be less than the duration time of probe lasers. Sequential images, representing the spatio-temporal distribution of the analyzed laser beams with several hundreds of picoseconds temporal resolution, have been obtained, indicating the ability of this scheme for analyzing the spatio-temporal distribution of pulse laser beams and paving the way for promising applications in characterization of pulse particle beams. [ABSTRACT FROM AUTHOR]

In this paper, we present a new approach of analysis and recognition of retinal vessel segments for the quantification of their shapes change due to alterations. This approach is based on a spatial followed by a temporal characterization of retinal fluorescein angiographic images. The spatial characteristics (coordinates and classification number i.e. numbering of segments) are associated to bifurcation points (bp), which are matched in temporal image pairs for vessel segments correspondence forming. The matching process uses spatial and temporal characteristics between the bp and their surrounded vessel segments by computing a coefficient of similarity measurement.The recognition of vessel segments will help ophthalmologists in quantifying changes in vessel shape parameters and detect the temporal evolution of some retinal pathologies as the Sickle Cell retinopathy in our case. [Copyright &y& Elsevier]

Statistical aspects of intense snowstorms over Austria are discussedbased on the daily fresh snow depth-recordings from a sample of 81 stations over a 19-year period from the 1970/71 to the 1988/89-winter seasons. The study focuses on heavy snowfall events (HSEs) defined asat least a 20 cm-intensity threshold for a 24-hour accumulation of new snow. Calculation of the geometric center of the area experiencingheavy snowfall allows for the estimation of storm spatial characteristics (e.g., its expansion, eccentricity, and the number of stations involved) and the objective assignment of storms to one of four distinct regions, located to the north and south of the Alpine main crest.Temporal characteristics investigated include the annual and seasonal frequencies of HSEs, whereas considerations of intensity put an emphasis on the average and maximum new snow-depth from intense storms. The biggest 24-hour snow accumulation within the period of study was 170 cm, measured at Sillian (Eastern Tyrol) in 1986. [ABSTRACT FROM AUTHOR]