Inelastic scattering of light by a cold trapped atom: Effects of the quantum center-of-mass motion

The light scattered by a cold trapped ion, which is in the stationary state of laser cooling, presents features due to the mechanical effects of atom-photon interaction. These features appear as additional peaks (sidebands) in the spectrum of resonance fluorescence. Among these sidebands the literature has discussed the Stokes and anti-Stokes components, namely the sidebands of the elastic peak. In this manuscript we show that the motion also gives rise to sidebands of the inelastic peaks. These are not always visible, but, as we show, can be measured in parameter regimes which are experimentally accessible.Comment: 10 pages, 4 figures, submitted to Phys. Rev.

Extracting Atoms on Demand with Lasers

We propose a scheme that allows to coherently extract cold atoms from a reservoir in a deterministic way. The transfer is achieved by means of radiation pulses coupling two atomic states which are object to different trapping conditions. A particular realization is proposed, where one state has zero magnetic moment and is confined by a dipole trap, whereas the other state with non-vanishing magnetic moment is confined by a steep microtrap potential. We show that in this setup a predetermined number of atoms can be transferred from a reservoir, a Bose-Einstein condensate, into the collective quantum state of the steep trap with high efficiency in the parameter regime of present experiments.Comment: 11 pages, 8 figure

Large-Eddy Simulation of inhomogeneous canopy flows using high resolution terrestrial laser scanning data

The effect of sub-tree forest heterogeneity in the flow past a clearing is investigated by means of large-eddy simulation (LES). For this purpose, a detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately 190m length in the field site “Tharandter Wald”, near the city of Dresden, Germany. The scanning data are used to produce a high resolution plant area distribution (PAD) that is averaged over approximately one tree height (30m) along the transverse direction, in order to simplify the LES study. Despite the smoothing involved with this procedure, the resulting two-dimensional PAD maintains a rich vertical and horizontal structure. For the LES study, the PAD is embedded in a larger domain covered with an idealized, horizontally homogeneous canopy. Simulations are performed for neutral conditions and compared to a LES with homogeneous PAD and recent field measurements. The results reveal a considerable influence of small-scale plant distribution on the mean velocity field as well as on turbulence data. Particularly near the edges of the clearing, where canopy structure is highly variable, usage of a realistic PAD appears to be crucial for capturing the local flow structure. Inside the forest, local variations in plant density induce a complex pattern of upward and downward motions, which remain visible in the mean flow and make it difficult to identify the “adjustment zone” behind the windward edge of the clearing

Large-Eddy Simulation Study of the Effects on Flow of a Heterogeneous Forest at Sub-tree Resolution

Abstract The effect of three-dimensional plant heterogeneity on flow past a clearing is investigated by means of large-eddy simulation. A detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately 328m length and 172m width at the field site “TharandterWald”, near the city of Dresden, Germany. The scanning data are used to produce a highly resolved, three-dimensional plant area distribution representing the actual canopy. Hence, the vegetation maintains a rich horizontal and vertical structure including the three-dimensional clearing. The scanned plant area density is embedded in a larger domain, which is filled with a heterogeneous forest generated by the virtual canopy generator of Bohrer et al. (Tellus B 59:566–576, 2007). Based on forest inventory maps and airborne laser scanning, the characteristics of the actual canopy are preserved. Furthermore, the topography is extracted from a digital terrain model with some modifications to accommodate for periodic boundary conditions. A large-eddy simulation is performed for neutral atmospheric conditions and compared to simulations of a two-dimensional plant area density and an one-year-long field experiment conducted at the corresponding field site. The results reveal a considerable influence of the plant heterogeneity on the mean velocity field as well as on the turbulent quantities. The three-dimensional environment, e.g., the oblique edges combined with horizontal and vertical variations in plant area density and the topography create a sustained vertical and cross-flow velocity. Downstream of the windward forest edge an enhanced gust zone develops, whose intensity and relative position are influenced by the local canopy density and, therefore, is not constant along the edge. These results lead us to the conclusion that the usage of a three-dimensional plant area distribution is essential for capturing the flow features inside the canopy and within the mixing layer above

3D-Vegetationsmodell aus Laserscannerdaten für forstmeteorologische Anwendungen am Beispiel des TurbEFA-Projektes

3D-Vegetationsmodelle sind in der Forstmeteorologie ein essentieller Bestandteil um Modellierungen durchzuführen und Messmethoden zu validieren. Inhomogenitäten, wie Änderungen der Bestandshöhe und Lichtungen in Waldbeständen, beeinflussen die Entstehung und die Struktur von turbulenten Windfeldern. Auch die numerische Simulation hochturbulenter Strömungen erfordert eine enorme Rechenleistung unter Verwendung von Vegetationsdaten. Daher ist die Anwendung von Vegetationsmodellen unterschiedlicher Art dringend notwendig. Während bisher die Verteilung der Biomasse von Untersuchungsgebieten über herkömmliche Methoden (Forstinventur, Abgriff aus Forstkarten) erfasst wurde, ist das Flugzeuglaserscanning (ALS) sowie das terrestrische Laserscanning (TLS) ein interessantes Werkzeug zur detaillierten Vegetationserfassung (Maas, 2010). Der Beitrag gibt einen Überblick über die Möglichkeiten der Generierung von Vegetationsmodellen aus verschiedenen Laserscannerdaten. Während Flugzeuglaserscanner große Gebiete aus der Luft erfassen können, sind die terrestrischen Laserscanner aufgrund der bodennahen Standpunkte auf kleinere Gebiete begrenzt. ALS- und TLS-Datensätze sind insofern komplementär, als Flugzeuglaserscannerdaten primär Kronenbereiche erfassen, während TLS-Aufnahmen hochaufgelöste Informationen über Baumstämme und den unteren Kronenbereich liefern. Eine vergleichsweise neue Messmethode im ALS ist das Full- Waveform Laserscanning, bei der das gesamte Intensitätssignal der reflektierten Energie aufgezeichnet und digitalisiert wird. Dadurch ist eine Volumenrekonstruktion der vertikalen Bestandsschicht möglich. Das Verfahren zeigt höhere Genauigkeiten hinsichtlich der Biomasseabschätzung im Vergleich zu dem herkömmlichen Flugzeuglaserscanning. Die unorganisierten 3D-Punktwolken, wie sie beim terrestrischen und Flugzeuglaserscanning entstehen, sind in der Regel für numerische Simulationen nicht handhabbar. Durch geeignete 3D-Datenstrukturen werden Gitterstrukturen auf Basis der Punktwolken aufgebaut und für numerische Simulationsprozesse nutzbar gemacht. Durch die Projektion der Full-Waveform Flugzeuglaserscannerdaten in diese Datenstruktur ergibt sich eine Voxelraumrepräsentation eines Waldbestandes. Hochaufgelöste TLS-Vegetationsscans ermöglichen eine detailliertere Parametrisierung der Pflanzenarchitektur. Über statistische und punktverteilungsbeschreibende Parameter aus den Punkten einer Gitterzelle können u.a. Informationen über Pflanzenflächendichte-Verteilungen (PAD) abgeleitet werden. Die standpunktweise Reflexionswahrscheinlichkeit pro Voxel wird mit Ray-Tracing Methoden bestimmt und repräsentiert den PAD (Queck et., 2012). Die Verwendung dieser Daten zielt auf eine genauere Modellierung der Strömungseffekte an Waldrandkanten, als natürliche Inhomogenität, ab, welche Forschungsschwerpunkt des interdisziplinären Projektes TurbEFA (Turbulent Exchange processes between Forested areas and the Atmosphere) ist

Arrival Metering Precision Study

This paper describes the background, method and results of the Arrival Metering Precision Study (AMPS) conducted in the Airspace Operations Laboratory at NASA Ames Research Center in May 2014. The simulation study measured delivery accuracy, flight efficiency, controller workload, and acceptability of time-based metering operations to a meter fix at the terminal area boundary for different resolution levels of metering delay times displayed to the air traffic controllers and different levels of airspeed information made available to the Time-Based Flow Management (TBFM) system computing the delay. The results show that the resolution of the delay countdown timer (DCT) on the controllers display has a significant impact on the delivery accuracy at the meter fix. Using the 10 seconds rounded and 1 minute rounded DCT resolutions resulted in more accurate delivery than 1 minute truncated and were preferred by the controllers. Using the speeds the controllers entered into the fourth line of the data tag to update the delay computation in TBFM in high and low altitude sectors increased air traffic control efficiency and reduced fuel burn for arriving aircraft during time based metering

TurbEFA: an interdisciplinary effort to investigate the turbulent flow across a forest clearing

the atmosphere within turbulence closure models is mainly limited by a realistic three-dimensional (3D) representation of the vegetation architecture. Within this contribution we present a method to record the 3D vegetation structure and to use this information to derive model parameters that are suitable for numerical flow models. A mixed conifer forest stand around a clearing was scanned and represented by a dense 3D point cloud applying a terrestrial laser scanner. Thus, the plant area density (PAD) with a resolution of one cubic meter was provided for analysis and for numerical simulations. Multi-level high-frequency wind velocity measurements were recorded simultaneously by 27 ultrasonic anemometers on 4 towers for a period of one year. The relationship between wind speed, Reynolds stress and PAD was investigated and a parametrization of the drag coefficient CD by the PAD is suggested. The derived 3D vegetation model and a simpler model (based on classical forest assessments of the site) were applied in a boundary layer model (BLM) and in large-eddy simulations (LES). The spatial development of the turbulent flow over the clearing is further demonstrated by the results of a wind tunnel experiment. The project showed, that the simulation results were improved significantly by the usage of realistic vegetation models. 3D simulations are necessary to depict the influence of heterogeneous canopies on the turbulent flow. Whereas we found limits for the mapping of the vegetation structure within the wind tunnel, there is a considerable potential for numerical simulations. The field measurements and the LES gave new insight into the turbulent flow in the vicinity and across the clearing. The results show that the zones of intensive turbulence development can not be restricted to the locations found in previous studies with more idealized canopies

The Impact of Trajectory Prediction Uncertainty on Air Traffic Controller Performance and Acceptability

A Human-In-The-Loop air traffic control simulation investigated the impact of uncertainties in trajectory predictions on NextGen Trajectory-Based Operations concepts, seeking to understand when the automation would become unacceptable to controllers or when performance targets could no longer be met. Retired air traffic controllers staffed two en route transition sectors, delivering arrival traffic to the northwest corner-post of Atlanta approach control under time-based metering operations. Using trajectory-based decision-support tools, the participants worked the traffic under varying levels of wind forecast error and aircraft performance model error, impacting the ground automations ability to make accurate predictions. Results suggest that the controllers were able to maintain high levels of performance, despite even the highest levels of trajectory prediction errors