1,721,019 research outputs found
Atmospheric Pollutant Dispersion over Complex Terrain: Challenges and Needs for Improving Air Quality Measurements and Modeling
No full textPollutant dispersion processes over complex terrain are much more complicated than over flat areas, as they are affected by atmospheric interactions with the orography at different spatial scales. This paper reviews recent findings and progress in this field, focusing on both experimental and modeling perspectives. It highlights open questions and challenges to our capability for better understanding and representing atmospheric processes controlling the fate of pollutants over mountainous areas. In particular, attention is focused on new measurement techniques for the retrieval of spatially distributed turbulence information and air quality parameters, and on challenges for meteorological and dispersion models to reproduce fine-scale processes influenced by the orography. Finally, specific needs in this field are discussed, along with possible directions for future research efforts
Atmospheric turbulence and surface energy exchange in urban environments : results from the Basel Urban Boundary Layer Experiment (BUBBLE)
Full text linkThe present experimental study addresses turbulence
and exchange processes in the urban roughness
sublayer, namely the region from street
canyon floor up to 2.5 times the mean building
height. Measurements with ultrasonic anemometerthermometers
from three urban full-scale towers
provided new insights into vertical profiles of
mean flow, Reynolds stress, turbulent kinetic energy
(TKE), dissipation rate, as well as exchange
processes of heat, and partially water vapor and
CO2. With the help of ensemble profiles, which are
a surrogate for a real horizontal average, results are
discussed in the frame of an ‘urban family portrait’.
For the majority of realizations, the plane mixing
layer analogy matches processes in the urban roughness
sublayer much better than the classical boundary
layer theory. The observed patterns suggest a
conceptual division of the urban roughness sublayer
into three parts, namely the canyon layer, the roof
layer, and an above-roof layer.
In the canyon layer, local mechanical and thermal
turbulence production are of minor importance. Turbulence
is dominated by large coherent structures,
it is very intermittent and highly uncorrelated. The
majority of TKE is imported by turbulent and pressure
transport from the roof layer. The well known
street canyon vortex is only found on average and
only for selected configurations. Upwind roof shape
was determined as an important factor affecting its
dynamics.
In the roof layer, profiles are characterized by
strongest gradients and exchange is more efficient.
Here, local shear production is a strong source of
TKE. The skimming flow over the street canyons
creates an inflected mean wind profile, from which
instabilities evolve. Notable amounts of TKE and
temperature variance are exported from the roof
layer by sweeps into the upper street canyon and
by ejections into the above-roof layer. As a consequence,
dissipation rate is lower than locally produced
turbulence and neutral limits of velocity variances
are slightly lower than predicted with classical
(local) approaches.
In the above-roof layer, the mean wind profile approximates
the well known logarithmic form valid
in the inertial sublayer. And, integral statistics approach
surface layer values. Turbulent transport
processes of momentum and heat are dominated by
ejections. While shear production is the main source
of TKE in the roof layer below, here both, buoyancy
and shear production are important.
Finally, a network of spatially distributed energy
balance measurements allowed a quantitative estimation
of the urban energy balance modification.
For this purpose, the surface energy balance was simultaneously
measured over different land uses (urban,
suburban, rural).
The impact of a lower urban albedo is roughly counterbalanced
by a stronger long-wave emission, resulting
in a nearly equivalent net radiation over urban
and rural surfaces. Urban surfaces are characterized
by a strong storage term and a high Bowen ratio.
At night, turbulent flux densities remain upward
directed in dense urban environments. This is explained
by a strong nocturnal release of stored heat.
As a consequence, the urban inertial sublayer and
the roughness sublayer are thermally unstable most
of the time
Observations and modelling of birch pollen emission and dispersion from an isolated source
Full text linkThe occurrence of allergic diseases in western countries increased during the last decades due to
greater awareness towards a hygienic lifestyle. The hygiene hypothesis relates the reduced expo-
sure to microbial pollution to an underdevelopment of the immune system, which in turn favours
the development of allergies. In order to provide information to affected individuals on adequate
pre-emptive measures, numerous studies on the health impact of allergenic pollen focus on their
atmospheric abundance and dispersion, including observations and simulation of emission and
transport. Prognostic models for the spatial distribution and concentration of different pollen
species on a regional scale are operational in many countries in order to identify highly affected
regions and allow health offices to announce warnings to the affected population. These models
are capable of predicting long-range transport in a full spatial resolution with respect to meteoro-
logical conditions. However, the initial abundance of airborne pollen in the models is determined
with empirically derived emission parameters, which are mostly based on long-term observation
averages with respect to large areas.
Field measurements and modelling work conducted in the framework of this thesis aimed at de-
scribing the emission and dispersion characteristics of an isolated natural birch pollen source in the
micro-scale, in order to improve the accuracy of the emission part in prognostic pollen transport
models. The basic approach was to infer the emission of the pollen source from downwind obser-
vations, with respect to meteorological conditions, by reproducing the observed pollen dispersion
with numerical simulations. Birch pollen are used, because they are among the most important
aeroallergens in Europe. In terms of quantifying the absolute pollen emission in speciffic cases,
however, the field observations of pollen concentrations were subject to various difficulties related
to sensor uncertainties and non-stationary conditions in the natural environment.
Firstly, the detailed investigation of pollen transport up- and downwind of the isolated source
relied on a large array of different instruments. In order to make the observations of birch pollen
concentrations comparable among different used instruments, a substantial part of this thesis is
dedicated to the description of performance and uncertainty of different pollen sampling methods.
Secondly, since naturally emitted pollen are used for tracers, instead of a controlled release of
artifficial particles, the observed pollen concentration can be biased by natural background con-
centration, which relates to emission from unknown sources upwind of the experiment site. The
wind
ow directed towards the birch canopy is substantially disturbed by its roughness and, addi-
tionally, a certain amount of airborne pollen is filtered by its vegetation elements. Observations
of undisturbed concentrations upwind of the windbreak thus fall short of describing the complex
pattern of downwind distribution. A computational
uid dynamics model, therefore, is used to
simulate Lagrangian-based trajectories of the pollen with respect to the disturbance of the wind
field. The results indicate that the portion of background concentration in the observed downwind
concentration is largely dependent on effects of accumulation due to deceleration of the wind
ow.
Deposition within the birch canopy is accounted for in a separate model, which is based on the
optical porosity of the windbreak. A combination of the two model approaches allows to eliminate
the portion of background concentration from the measured downwind concentrations, providing
information on the emissivity of the isolated birch pollen source.
Based on the corrected concentrations downwind of the windbreak, i.e. un-biased by background
concentration, a method of estimating the source strength of the isolated pollen source with a
Lagrangian particle model is assessed
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
Numerical simulation of fog and radiation in complex terrain : results from COST-722
Full text linkTwo high resolution numerical 1D models, namely COBEL and PAFOG, have been adapted to compute a probabilistic fog forecast. Major modifications were made to the COBEL model. It was coupled to the NOAH land surface model to take into account the effects of soil and vegetation and furthermore a parameterization of precipitation was added. To deal with the large uncertainty inherent to fog forecasts, a whole ensemble of 1D runs is computed using the two different numerical models and a set of different initial conditions in combination with distinct boundary conditions. Initial conditions are obtained from variational data assimilation, which optimally combines observations with a first guess taken from operational 3D models. The design of the ensemble scheme computes members that should fairly well represent the uncertainty of the current meteorological regime. Verification reveals that the probabilistic forecast can significantly improve the current methods used at Z¨rich u Unique airport. The complex topography in Switzerland further complicates fog forecasting. In order to simulate processes like advection, cold air drainage flows and cold air pooling, the NMM 3D model of NOAA/NCEP is modified and extended with detailed fog microphysics. The resulting 3D fog model runs at a horizontal resolution ofkm and a vertical resolution comparable to the 1D models. First results look very promising and are able to reproduce the spatial distribution of fog as it is seen by satellite. With increasing horizontal resolution of numerical weather prediction models, topographical effects on radiation gain importance. With a newly developed parameterization it is possible to consider slope angle, aspect angle, shadows and restricted sky view on the subgrid scale and with negligible computational costs. Verification reveals that RMS and mean error ofm temperature forecasts are generally improved by 0.5 toK
Variabilidad espacio-temporal de la Capa Límite atmosférica en el Valle de Aburrá: caracterización, procesos, interacciones multi-escala e impactos
Full text linkilustraciones, diagramas, mapas, tablasBoundary-Layer Meteorology refers to the theoretical, numerical, and experimental framework that focuses on studying the interactions between the earth's surface and the lower troposphere. These interactions are crucial for several applications in various temporal and spatial scales. A better understanding of these interactions implies studying the structure and variability of the Atmospheric Boundary-Layer as a result of processes that are better described in the realm of a combination of fluid dynamics and thermodynamics. By definition, the ABL is turbulent, which means that the processes of mixing and exchange of energy, momentum, and scalars from the Surface Layer to the rest of the atmosphere depend on turbulence, both thermal and mechanic. In this sense, the small-scale physical processes depicted by the different terms in Turbulent Kinetic Energy budget equation modulate the characteristics of large-scale phenomena. One of the main challenges in studying the ABL dynamics is the high heterogeneity of the underlying forcing imposed by the earth's surface. Ranging from abrupt changes in orography to a wide variety of land uses and roughness elements, complex terrain is the rule rather than the exception, especially when dealing with processes involving anthropogenic modified environments. Nevertheless, most of our understanding of turbulence in the ABL is based on an idealized picture of real-life scenarios, e.g., horizontally homogeneous and flat terrains, implying that most of the exchange is vertical, and horizontal contributions are small and negligible. The main goal of this work is to identify and gain insight into the main processes between the synoptic- and micro-scale ends of the spectrum of atmospheric motions (including their cross-scale interactions) that modulate the ABL structure and dynamics over a complex and highly urbanized low-latitude valley. The Aburrá Valley is located in the Colombian Andes. The work follows an experimental approach, using multiple data sources, in order to evaluate the potential contribution of these phenomena on the boundary-layer variability in the study area. The intention is to shed some light on open questions regarding boundary-layer characteristics and some of their environmental implications over such complex terrain. The structure of this document was decided considering that, once in final form, Chapters 3, 4 and 5 will be submitted to be considered for publication in peer reviewed journals (Chapter 2 is already published). For this reason, this thesis includes four main self-contained chapters (Chapters 2 to 5) with the results of the evaluation of the boundary-layer dynamics in a highly complex terrain at different spatio-temporal scales and some of the interactions between them.La Meteorología de la Capa Límite hace referencia al marco teórico, numérico y experimental que se enfoca en estudiar las interacciones entre la superficie terrestre y la tropósfera baja. Estas interacciones son fundamentales para muchas aplicaciones en varias escalas espacio-temporales. Un mejor entendimiento de dichas interacciones implica el estudio de la estructura y variabilidad de la Capa Límite Atmosférica como resultado de procesos que son mejor descritos en el ámbito de una combinación de dinámica de fluidos y termodinámica. Por definición, la Capa Límite es turbulenta, lo cual significa que los procesos de mezcla e intercambio de energía, momentum y escalares, desde la Capa Superficial al resto de la atmósfera, dependen de la turbulencia, tanto térmica como mecánica.En este sentido, los procesos físicos de pequeña escala descritos por los términos en la ecuación de balance de la energía cinética turbulenta modulen las características de los fenómenos a mayor escala. Uno de los más grandes desafíos a la hora de estudiar las dinámicas de la Capa Límite es la alta heterogeneidad de la superficie terrestre, que van desde cambios abruptos en la orografía, hasta una amplia variedad de usos del suelo y elementos rugosos. Así, los terrenos complejos más que la excepción son la regla, especialmente cuando se trabajan con procesos que involucran modificaciones antrópicas al ambiente. Sin embargo, la gran mayoría de nuestro conocimiento en turbulencia en la Capa límite es basado en una imagen idealizada de los escenarios de la vida real, es decir, terrenos planos y horizontalmente homogéneos, lo cual implica que la mayoría de los intercambios aon verticales, y las contribuciones horizontales son pequeñas y pueden ser despreciadas. El principal objetivo de esta investigación es identificar y comprender los principales procesos de las diferentes escalas atmosféricas de movimiento (incluidas las interacciones entre escalas) los cuales modulan la estructura y dinámica de la Capa Límite en el Valle de Aburrá, ell cual es un valle altamente urbanizado ubicado en los Andes Colombianos. El trabajo sigue un enfoque experimental, usando múltiples fuentes de datos, con el objetivo de evaluar la potencial contribución de estos fenómenos en la variabilidad de la Capa límite en la zona de estudio. La intención es dar algo de luz sobre preguntas que siguen abiertas relacionadas con las características de la capa límite y algunas de sus implicaciones ambientales en un terreno tan complejo. (Texto tomado de la fuente)DoctoradoDoctor en IngenieríaMeteorología de capa límiteÁrea Curricular de Medio Ambient
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