1,721,000 research outputs found
A Simple Parametrization for the Concentration Variance Dissipation in a Lagrangian Single-Particle Model
No full textNew Boundary Conditions for Positive and Negative Skewed Turbulence in Fluctuating Plume Models
No full textInteraction of Submeso Motions in the Antarctic Stable Boundary Layer
No full textSubmesomotions add complexities to the structure of the stable boundary layer. Such motions
include horizontal meandering and gravity waves, in particular when the large-scale flow is
weak. The coexistence and interaction of such submeso motions is investigated through the
analysis of data collected in Antarctica, in persistent conditions of strong atmospheric stratification.
Detected horizontal meandering is frequently associated with temperature oscillations
characterized by similar time scales (30 min) at all levels (2, 4.5 and 10 m). In contrast, dirty
gravity waves superimposed on horizontal meandering are detected only at the highest level,
characterized by time scales of a few minutes. The meandering produces an energy peak in
the low-frequency spectral range, well fitted by a spectral model previously proposed for
low wind speeds. The coexistence of horizontal and vertical oscillations is observed in the
presence of large wind-direction shifts superimposed on the gradual flow meandering. Such
shifts are often related to the variation of the mean flow dynamics, but also to intermittent
events, localized in time, which do not produce a variation in the mean wind direction and
that are associated with sharp decreases in wind speed and temperature. The noisy gravity
waves coexisting with horizontal meandering persist only for a few cycles and produce bursts
of turbulentmixing close to the ground, affecting the exchange processes between the surface
and the stable boundary layer. The results confirm the importance of sharp wind-direction
changes at lowwind speed in the stable atmosphere and suggest a possible correlation between
observed gravity waves and dynamical instabilities modulated by horizontal meandering
Submeso Motions and Intermittent Turbulence Across a Nocturnal Low-Level Jet: A Self-Organized Criticality Analogy
No full textOne of the hallmarks of the stable boundary layer is the switching between turbulent (active) and non-turbulent (passive) states. In very stable conditions, the boundary layer becomes layered with fully-developed turbulence confined to a shallow region near the surface. In the quiescent region above this near-surface layer, the turbulence is weak, intermittent and detached from the ground. These conditions promote the development of a low-level jet that re-energizes the turbulence through an elevated shear layer. The Monin–Obukhov similarity theory fails in the layered stable boundary layer thereby making the quantification of mixing and transport properties challenging for numerical models. In the present study, multi-level time series from a tall (140 m) meteorological tower are analyzed using the telegraphic approximation to investigate analogies with a general class of intermittency models that include self-organized criticality. The analogy between turbulence and self-organized criticality is restricted to clustering properties of sign changes of flow variables for describing switching between turbulent and non-turbulent states. The telegraphic approximation provides a new perspective on clustering and on external and internal intermittency for periods dominated by turbulent motions, a low-level jet and submeso motions. Some of these periods are characterized by the absence of turbulence but occasionally punctuated by bursts of intermittent turbulent events. The switching probability of active–inactive states and the lifetimes of inactive states (related to intermittent turbulent bursts) show evidence of self-organized-criticality like behaviour in terms of scaling laws. The coexistence of self-organized criticality and intermittent turbulence may offer new perspectives on the genesis of scaling laws and similarity arguments, thereby improving the performance of numerical models in the stable boundary layer
Concentration Fluctuations and Odor Dispersion in Lagrangian Models
No full textIn this paper, a review of the Lagrangian stochastic models developed in the last decades for the simulation of the concentration–fluctuation dispersion is presented. The main approaches available in the literature are described and their ability in reproducing the higher order moments of the probability density function is discussed. Then, the Lagrangian approaches for evaluating of the odor annoyance are presented. It is worth to notice that, while Lagrangian stochastic models for mean concentrations are well-known and their ability in correctly reproducing the observation is well assessed, concerning concentration fluctuations the approaches are often new and unknown for most of the scientific community
Evaluation of high-order concentration statistics in a dispersing plume
No full textIn this paper we analyze the concentration higher-order moments of a dispersing plume in a neutral wind tunnel boundary layer. They are evaluated through a fluctuating plume model in which the plume barycenter is directly calculated from the measured mean concentration field. The reliability of the model is firstly verified by comparing the simulated and measured moments up to the fourth order. Then the relationships between higher and lower-order moments are investigated seeking for a proper scaling of the fourth, fifth and sixth-order normalized moments. We found that a general behavior appears when they are expressed as a function of the third-order moments. In this case each order calculated or measured at different distances collapses on the same curve
Model chain for buoyant plume dispersion
No full textA new original software interface between the WRF mesoscale meteorological model and the SPRAYWEB dispersion model has been developed. The model chain was designed such a highly responsive tool for risk assessment and emergency-response purposes. The model interface reads the wind and temperature fields provided by WRF and interpolates them on a fixed-in-Time grid, which is the input to the dispersion model. Furthermore, it calculates the turbulence-parameter vertical profiles, based on the surface-layer data provided by WRF. In this work we simulate the dispersion of a high-buoyancy plume. The model chain performances were tested against the Bull-Run dataset
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
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