Prediction of the upper tail of concentration distributions of a continuous point source release in urban environments

The peak values observed in a measured concentration time series of a dispersing gaseous pollutant released continuously from a point source in urban environments, and the hazard level associated with them, demonstrate the necessity of predicting the upper tail of concentration distributions. For the prediction of concentration distributions statistical models are preferably employed which provide information about the probability of occurrence. In this paper a concentration database pertaining to a field experiment is used for the selection of the statistical distribution. The inverses of the gamma cumulative distribution function (cdf) for 75th–99th percentiles of concentration are found to be more consistent with the experimental data than those of the log-normal distribution. The experimental values have been derived from measured high frequency time series by sorting first the concentrations and then finding the concentration which corresponds to each probability. Then the concentration mean and variance that are predicted with Computational Fluid Dynamics-Reynolds Averaged Navier–Stokes (RANS) methodology are used to construct the gamma distribution. The proposed model (“RANS-gamma”) is included in the framework of a computational code (ADREA-HF) suitable for simulating the dispersion of airborne pollutants over complex geometries. The methodology is validated by comparing the inverses of the model cdfs with the observed ones from two wind tunnel experiments. The evaluation is performed in the form of validation metrics such as the fractional bias, the normalized mean square error and the factor-of-two percentage. From the above comparisons it is concluded that the overall model performance for the present cases is satisfactory.     

Exaggerated orientation scatter of nocturnal passerine migrants close to breeding grounds: comparisons between seasons and latitudes

Using tracking radars, we investigated the variability of flight directions of long-distance nocturnal passerine migrants across seasons (spring versus autumn migration) and sites at the southern (56° N) and northern (68° N) ends of the Scandinavian Peninsula (Lund versus Abisko). Whilst most migrants at Lund are on passage to and from breeding sites in Fennoscandia, the majority of the migrants at Abisko are close to their breeding sites, and migration at Abisko thus to a large degree reflects initial departure from breeding sites (autumn) or final approach to breeding destinations (spring). The radar data were used to test predictions about differences in orientation and wind drift effects between adult and juvenile birds (a large proportion of autumn migrants consists of juvenile birds on their first journey), between situations far away from or near the goals and between different phases of migration (initial departure, en route passage, final approach to goal). The concentrations (both total and within-night concentrations) of flight directions differed significantly between seasons as well as sites, with the highest concentration at Lund in spring (mean vector length of track directions, r = 0.79) and lowest at Abisko during spring (r = 0.35). Partial wind drift and partial compensation were recorded at Lund, with a similar effect size in spring and autumn, whilst possible wind drift effects at Abisko were obscured by the large directional scatter at this site. The results from Lund support the prediction that the high proportion of juveniles in autumn contributes to increase the directional scatter during this season, whilst there was no support for predictions of differential wind drift effects between seasons and situations with different goal distances. The most striking and surprising result was the exceedingly large scatter of flight directions at Abisko, particularly in spring. We suggest that such an exaggerated scatter may be associated with final approach orientation, where migrants reach their specific goals from all various directions by final navigation within a more wide-ranging goal region. The larger scatter of autumn flight directions at Abisko compared to Lund may be due to exploratory flights in variable directions being more common at initial departure from breeding sites than later during migratory passage. These surprising results highlight the importance of studying and analysing orientation during final approach to (and initial departure from) migratory goals for understanding the orientation systems of migratory birds.     

Migratory orientation of red-eyed vireos,Vireo olivaceus,in relation to energetic condition and ecological context

How and when migrants integrate directional information from different sources may depend not only on the bird’s internal state, including fat stores, but also on the ecological context during passage. We designed experiments to (1) examine the influence of stored fat on the decision to migrate and on the choice of migratory direction and (2) investigate how the integration of orientation cue information is tied to energetic status in relation to migration across an ecological barrier. Migratory orientation of red-eyed vireos (Vireo olivaceus) at twilight was recorded using two different techniques, orientation cage experiments and free-flight release tests, during both fall and spring migration. During fall migration, the amount of stored fat proved decisive for directional selections of the vireos. Fat birds chose directions in accordance with migration across the Gulf of Mexico. Lean birds oriented either parallel to the coast line (cage tests) or moved inland (free-flight releases). Whereas only fat birds showed significant responses to experimental deflections of the geomagnetic field, lean birds displayed a tendency to shift their activity in the expected direction, making it difficult to evaluate the prediction that use of the magnetic compass is context dependent. Fat loads also had a significant effect on the decision to migrate, i.e., fat individuals were more likely to embark on migration than were lean birds (true for both cage and release experiments). During spring migration, a majority of experimental subjects were classified as lean, following their arrival after crossing the Gulf of Mexico, and oriented in seasonally appropriate directions. The vireos also showed significant responses to experimental deflections of the geomagnetic field regardless of their energetic status. Free-flight release experiments during spring migration revealed a significant difference in mean directions between clear sky and overcast tests. The difference may indicate a compensatory response to wind drift or possibly a need for celestial cues to calibrate the magnetic compass. Finally, this is the first demonstration of magnetic compass orientation in a North American vireo. Received: 15 December 1995/Accepted after revision: 24 March 1996     

Large-eddy simulation of turbulent flow and dispersion over a complex urban street canyon

Turbulent flow and dispersion characteristics over a complex urban street canyon are investigated by large-eddy simulation using a modified version of the Fire Dynamics Simulator. Two kinds of subgrid scale (SGS) models, the constant coefficient Smagorinsky model and the Vreman model, are assessed. Turbulent statistics, particularly turbulent stresses and wake patterns, are compared between the two SGS models for three different wind directions. We found that while the role of the SGS model is small on average, the local or instantaneous contribution to total stress near the surface or edge of the buildings is not negligible. By yielding a smaller eddy viscosity near solid surfaces, the Vreman model appears to be more appropriate for the simulation of a flow in a complex urban street canyon. Depending on wind direction, wind fields, turbulence statistics, and dispersion patterns show very different characteristics. Particularly, tall buildings near the street canyon predominantly generate turbulence, leading to homogenization of the mean flow inside the street canyon. Furthermore, the release position of pollutants sensitively determines subsequent dispersion characteristics.     

LES validation of urban flow,part II: eddy statistics and flow structures

Time-dependent three-dimensional numerical simulations such as large-eddy simulation (LES) play an important role in fundamental research and practical applications in meteorology and wind engineering. Whether these simulations provide a sufficiently accurate picture of the time-dependent structure of the flow, however, is often not determined in enough detail. We propose an application-specific validation procedure for LES that focuses on the time dependent nature of mechanically induced shear-layer turbulence to derive information about strengths and limitations of the model. The validation procedure is tested for LES of turbulent flow in a complex city, for which reference data from wind-tunnel experiments are available. An initial comparison of mean flow statistics and frequency distributions was presented in part I. Part II focuses on comparing eddy statistics and flow structures. Analyses of integral time scales and auto-spectral energy densities show that the tested LES reproduces the temporal characteristics of energy-dominant and flux-carrying eddies accurately. Quadrant analysis of the vertical turbulent momentum flux reveals strong similarities between instantaneous ejection-sweep patterns in the LES and the laboratory flow, also showing comparable occurrence statistics of rare but strong flux events. A further comparison of wavelet-coefficient frequency distributions and associated high-order statistics reveals a strong agreement of location-dependent intermittency patterns induced by resolved eddies in the energy-production range. The validation concept enables wide-ranging conclusions to be drawn about the skill of turbulence-resolving simulations than the traditional approach of comparing only mean flow and turbulence statistics. Based on the accuracy levels determined, it can be stated that the tested LES is sufficiently accurate for its purpose of generating realistic urban wind fields that can be used to drive simpler dispersion models.     

Description and preliminary validation of the PMSS fast response parallel atmospheric flow and dispersion solver in complex built-up areas

Noxious atmospheric releases may originate from both accidents and malicious activities. They are a major concern for public authorities or first responders who may wish to have the most accurate situational awareness. Nonetheless, it is difficult to reliably and accurately model the flow, transport, and dispersion processes in large complex built-up environments in a limited amount of time and resources compatible with operational needs. The parallel version of Micro-SWIFT-SPRAY (PMSS) is an attempt to propose a physically sound and fast response modelling system applicable to complicated industrial or urban sites in case of a hazardous release. This paper presents and justifies the choice of the diagnostic flow and Lagrangian dispersion models in PMSS. Then, it documents in detail the development of the parallel algorithms used to reduce the computational time of the models. Finally, the paper emphasizes the preliminary model validation and parallel performances of PMSS based on data from both wind tunnel (Evaluation of Model Uncertainty) and in-field reduced-scale (Mock Urban Setting Test) and real-scale (Oklahoma City) experimental campaigns.     

Weak-wind mesoscale meandering in the nocturnal boundary layer

This study examines the strength and statistical behavior of mesoscale motions on time scales up to 1 h using eight data sets over different surface types. The mesoscale motions include internal gravity waves, microfront-like structures, horizontal modes, and a complex variety of other signatures, perhaps resulting from superposition of different modes. With weak large-scale flow, the mesoscale motions lead to meandering of the wind direction, as found in previous studies. However, the meandering often takes the form of sudden wind shifts rather than oscillation of wind direction. The relative strength and impact of such mesoscale motions are examined in terms of the constancy of the wind vector, the within-record standard deviation of the wind direction and the ratio of a meso-velocity scale to the speed of the large-scale flow. The strength of the mesoscale flow varies by an order of magnitude between nights at a given site and varies systematically between sites. The statistics of the vertical structure of such motions are examined for two of the data sets, both with sonic anemometers at seven levels.     

Nitrogen saturation in stream ecosystems

The concept of nitrogen (N) saturation has organized the assessment of N loading in terrestrial ecosystems. Here we extend the concept to lotic ecosystems by coupling Michaelis-Menten kinetics and nutrient spiraling. We propose a series of saturation response types, which may be used to characterize the proximity of streams to N saturation. We conducted a series of short-term N releases using a tracer (15NO3-N) to measure uptake. Experiments were conducted in streams spanning a gradient of background N concentration. Uptake increased in four of six streams as NO3-N was incrementally elevated, indicating that these streams were not saturated. Uptake generally corresponded to Michaelis-Menten kinetics but deviated from the model in two streams where some other growth-critical factor may have been limiting. Proximity to saturation was correlated to background N concentration but was better predicted by the ratio of dissolved inorganic N (DIN) to soluble reactive phosphorus (SRP), suggesting phosphorus limitation in several high-N streams. Uptake velocity, a reflection of uptake efficiency, declined nonlinearly with increasing N amendment in all streams. At the same time, uptake velocity was highest in the low-N streams. Our conceptual model of N transport, uptake, and uptake efficiency suggests that, while streams may be active sites of N uptake on the landscape, N saturation contributes to nonlinear changes in stream N dynamics that correspond to decreased uptake efficiency.     

Effectiveness of multiple release sites in reintroduction of Persian fallow deer

Releasing animals in more than one location may increase or decrease the probability of success of a reintroduction project, yet the question of how many release sites to use has received little attention. We used empirical data from the reintroduction program of the Persian fallow deer (Dama mesopotamica) (Galilee region in northern Israel) in an individual-based spatially explicit simulation model to assess the effects of releasing deer from multiple sites. We examined whether multiple release sites increase reintroduction success, and if so, whether the optimal number of sites for a given scenario can be determined and whether the outcome differs if animals are released alternately (i.e., the location of the release alternates yearly between sites) or consecutively (i.e., one release site is used for several years, then another is used, and so forth). We selected 8 potential release sites in addition to the original site and simulated the release of 180 individuals at a rate of 10 individuals per year in different combinations of the original site and 1-4 additional sites. In our model, releasing animals into the wild at multiple sites produced higher population growth and greater spatial expansion than releasing animals at only one site and a consecutive-release approach was superior to an alternate-release approach. We suggest that through the use of simulation modeling that is based on empirical data from previous releases, managers can make better-informed decisions regarding the use of multiple release sites and greatly improve the probability of reintroduction success.     

Statistical properties of concentration fluctuations in two merging plumes

The statistics of the fluctuating concentration field within a plume is important in the analysis of atmospheric dispersion of toxic, inflammable and odorous gases. Previous work has tended to focus on concentration fluctuations in single plumes released in the surface layer or at ground level and there is a general lack of information about the mixing of two adjacent plumes and how the statistical properties of the concentration fluctuations are modified in these circumstances. In this work, data from wind tunnel experiments are used to analyse the variance, skewness, kurtosis, intermittency, probability density function and power spectrum of the concentration field during the mixing of two identical plumes and results are compared with those obtained for an equivalent single plume. The normalised variance, skewness and kurtosis on the centre-lines of the combined plume increase with distance downwind of the stack and, in the two-source configuration, takes lower values than those found in the single plumes. The results reflect the merging process at short range, which is least protracted for cases in which the sources are in-line or up to 30 \(^{\circ }\) off-line. At angles of 45 \(^{\circ }\) and more, the plumes are effectively side-by-side during the merging process and the interaction between the vortex pairs in each plume is strong. Vertical asymmetry is observed between the upper and the lower parts of the plumes, with the upper part having greater intermittency (i.e. the probability that no plume material is present) and a more pronounced tail to the concentration probability distribution. This asymmetry tends to diminish at greater distances from the source but occurs in both buoyant and neutral plumes and is believed to be associated with the ‘bending-over’ of the emission in the cross-flow and the vortex pair that this generates. The results allowed us to identify three phases in plume development. The first, very near the stack, is dominated by turbulence generated within the plume and characterised by concentration spectra with distinct peaks corresponding to scales comparable with those of the counter-rotating vortex pair. A second phase follows at somewhat greater distances downwind, in which there are significant contributions to the concentration fluctuations from both the turbulence internal to the plume and the external turbulence. The third phase is one in which the concentration fluctuations appear to be controlled by the external turbulence present in the ambient flow.     

Tradition without social learning: scent-mark-based communal roost formation in a Neotropical harvestman (Prionostemma sp.)

Most models of habitat selection assume that individual animals choose and either reuse or abandon sites based on a constant reassessment of site quality. When survival is a function of the presence of conspecifics, however, the benefits of returning to traditional sites may override resource assessment. Many animals form roosting aggregations at what appear to be traditional sites. At our study site in Nicaragua, the harvestman Prionostemma sp. forms diurnal roosting aggregations on a small subset of the available spiny palm trees. With respect to physical characteristics and microclimate, the spiny palms used by the harvestmen resembled a random sample of those available, yet the same subset of trees was used in two different years (2001, 2003). This suggests that the location of aggregation sites is traditional, not a product of habitat limitation. Individual harvestmen were not faithful to particular roost sites, however, which raises the question of how the tradition could be maintained over time. In this paper, we present evidence, derived from a series of small-scale field experiments, that the harvestmen mark roosting sites chemically and enter marked sites preferentially when searching for places to roost. We also show that the harvestmen are sensitive to changes in site quality (the presence of spines) but will continue to use degraded traditional sites when no intact spiny palms are nearby. This system provides an example of how animal traditions could be maintained over multiple generations without learning. Site-labeling can be viewed as an external form of social memory.     

A General Model for Analyzing Data from Mark-recapture Experiments with an Application to the Pacific Halibut

A general model is developed to examine the patterns of the regional movement of tagged and released fish from mark-recapture experiments. It is a stochastic model that incorporates fishing mortality, natural mortality, fish movement, tag-shedding, and different rates of reporting. A likelihood function is constructed for estimating its parameters. We used this model to analyze data on the Pacific halibut from mark-recapture experiments conducted by the International Pacific Halibut Commission (IPHC), with a total of 36,058 releases from 1982 to 1986 and 5,826 recoveries from 1982 to 2000. We estimated their rates of movement among IPHC management areas, along with their instantaneous rates of natural and fishing mortalities. Our analysis revealed that fish movement was not significant among areas, with a resident probability of > 0.92. This lends support to the IPHC catch-at-age stock assessment model (which has no built-in movement components). The estimated instantaneous rate of natural mortality (0.198 year⁻¹) lies between that assumed in all IPHC stock assessments before 1998 (0.20 year⁻¹) and that from 1999 onwards (0.15 year⁻¹). The estimates of the instantaneous rates of fishing mortality were consistent with those from the IPHC stock assessment model. Received: April 2003 / Revised: May 2005     

Filament-Based Atmospheric Dispersion Model to Achieve Short Time-Scale Structure of Odor Plumes

This article presents the theoretical motivation, implementation approach, and example validation results for a computationally efficient plume simulation model, designed to replicate both the short-term time signature and long-term exposure statistics of a chemical plume evolving in a turbulent flow. Within the resulting plume, the odor concentration is intermittent with rapidly changing spatial gradient. The model includes a wind field defined over the region of interest that is continuous, but which varies with location and time in both magnitude and direction. The plume shape takes a time varying sinuous form that is determined by the integrated effect of the wind field. Simulated and field data are compared. The motivation for the development of such a simulation model was the desire to evaluate various strategies for tracing odor plumes to their source, under identical conditions. The performance of such strategies depends in part on the instantaneous response of target receptors; therefore, the sequence of events is of considerable consequence and individual exemplar plume realizations are required. Due to the high number of required simulations, computational efficiency was critically important.     

Evaluation of wind environment around a residential complex using a PIV velocity field measurement technique

Wind-tunnel simulations were employed to evaluate the wind environment around a tested residential area located near industrial complexes. The scaled-down geomorphological model of the test area was placed in the test section of a boundary layer wind tunnel. Particle image velocimetry (PIV) measurements were made in five vertical planes and one horizontal plane around the test area for two prevailing wind directions. The results showed that the wind speed decreased in the near surface layer and the velocity fluctuations increased in the upper region due to the presence of hills and high-rise buildings around the test area. Regions of flow separation and low-speed flow were found inside the test area for both the wind directions. The result suggests that the high-rise buildings should be well arranged with respect to the main wind directions to increase the natural ventilation inside the residential complex at the initial design stage.     

Multiple stressors and amphibian declines: dual impacts of pesticides and fish on yellow-legged frogs.

More than 40% of Earth's 5700+ amphibian species have undergone recent declines. Despite the likely involvement of multiple factors in driving these declines, most studies continue to focus on single stressors. In California (USA), separate studies have implicated either introduced fish or pesticides as causal agents. To date, however, no study has simultaneously evaluated the respective roles of these two potential stressors nor attempted to assess their relative importance, information critical for the development of effective conservation efforts and environmental policies. We examined the role and relative effect of fish and pesticides on the mountain yellow-legged frog (Rana muscosa) using unusually detailed data sets for a large portion of R. muscosa's historic range in California's Sierra Nevada. Habitat characteristics and presence/absence of R. muscosa and fish were quantified at each of 6831 sites during field surveys. Pesticide use upwind of each site was calculated from pesticide application records and predominant wind directions. Using generalized additive models, we found that, after accounting for habitat effects, the probability of R. muscosa presence was significantly reduced by both fish and pesticides, with the landscape-scale effect of pesticides much stronger than that of fish. The degree to which a site was sheltered from the predominant wind (and associated pesticides) was also a significant predictor of R. muscosa presence. Taken together, these results represent the strongest evidence to date that windborne pesticides are contributing to amphibian declines in pristine locations. Our results suggest that amphibian declines may have complex multi-factorial causes, and caution that single-factor studies that demonstrate the importance of one factor should not be used as evidence against the importance of other factors.     

不同环境场所夏季空气负离子浓度分布特征及其与环境因子的关系

大气离子是由许多自然和人为的原因产生,并且它们的浓度在不同环境场所下差别很大。以我国南部沿海某省份作为研究区域,挑选了近10处有代表性的环境场所进行为期一周的实测和研究,收集了空气正、负离子浓度、风速、空气温度、相对湿度等数据。研究显示,空气负离子浓度与风速、水、植物、相对湿度等有较为密切的关系,其中最主要的影响因素是水,其次是风,最小的是气温。为了改善城市生态环境,建议在城市规划建设中采取有效措施以提高空气负离子浓度。     

Modeling leaf dispersal in mixed hardwood forests using a ballistic approach

In mixed-species stands, modeling leaf litter dispersal is important to predict the physical and chemical characteristics of the forest floor, which plays a major role in nutrient cycling and in plant population dynamics. In this study, a spatially explicit model of leaf litterfall was developed and compared with two other models. These three models were calibrated for a mixed forest of oak and beech using litterfall data from mapped forest plots. All models assumed that an allometric equation described individual leaf litter production, but they strongly differed in the modeling of the probability density of leaf shedding with distance from source trees. Two models used a negative exponential function to account for leaf dispersal with distance, and this function was allowed to vary according to wind direction in one of them. In contrast, our approach was based on a simple ballistic equation considering release height, wind speed, wind direction, and leaf fall velocity; the distributions of wind speeds and wind directions were modeled according to a Weibull and a Von Mises distribution, respectively. Using an independent validation data set, all three models provided predictions well correlated to measurements (r > 0.83); however, the two models with a direction-dependent component were slightly more accurate. In addition, parameter estimates of the ballistic model were in close agreement with a foliar litter production equation derived from the literature for beech and with wind characteristics measured during leaf litterfall for both species. Because of its mechanistic background, such a spatially explicit model might be incorporated as a litterfall module in larger models (nutrient cycling, plant population dynamics) or used to determine the manner in which patch size in mixed-species stands influences litter mixture.     

Design and Testing of a Turbulence Probe for Harsh Flows

The force of wind on the ground created by turbulent eddies is commonly used to describe the horizontal flux of material during wind erosion. Here we present the Murdoch Turbulence Probe, an instrument for use in both clean and eroding flows which uses pressure differences to measure the three components of wind velocity. Correlation techniques calculate the forces near the ground and turbulence statistics in nearly real time, including turbulent velocity fluctuations from less than 0.1 Hz to 200 Hz, mean flow velocities, Reynolds stresses as well as the integral length and time scales. In the portable wind-tunnel used by Agriculture Western Australia, turbulence statistics were recorded over stable surfaces and in blowing sand from the initiation of erosion up to the time the sand supply was exhausted. Estimates of the friction velocity derived from the turbulence probe were compared with estimates obtained from the wind speed profile measured with a rake of pitot and static tubes. The Murdoch Turbulence Probe appears to work well in sandblasting conditions. Relative turbulence intensities ranged from 0.11 to 0.2 and are in close agreement with values in the literature. The ratio of the turbulence to the friction velocity (3 to 3.2) is at the high end of the reported range. The Reynolds stress measurements agree closely with predictions of the threshold friction velocities of the sand and estimates from the wind speed profile with a von Kármán constant of about 0.3, lower than the commonly accepted value of 0.4. We suggest that the wind-tunnel profile represents the `outer layer' of the boundary-layer that may best be described by a `Wake Law' or `Defect Law'. At about 54 mm above the surface, the friction velocity decreases from 0.64 m/s to 0.39 m/s and the mean velocity increases from 9.6 m/s to 11.6 m/s as the supply of sand is depleted. In addition to the friction velocity, other scales may be necessary to characterise the overriding effect of the wind and in extending wind-tunnel results to the field.     

Emerging infectious disease as a proximate cause of amphibian mass mortality

A newly discovered infectious disease of amphibians, chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis, is implicated in population declines and possible extinctions throughout the world. The purpose of our study was to examine the effects of B. dendrobatidis on the mountain yellow-legged frog (Rana muscosa) in the Sierra Nevada of California (USA). We (1) quantified the prevalence and incidence of B. dendrobatidis through repeat surveys of several hundred R. muscosa populations in the southern Sierra Nevada; (2) described the population-level effects of B. dendrobatidis on R. muscosa population abundance; and (3) compared the mortality rates of infected and uninfected R. muscosa individuals from pre- through post-metamorphosis using both laboratory and field experiments. Mouthpart inspections conducted in 144 and 132 R. muscosa populations in 2003 and 2004, respectively, indicated that 19% of R. muscosa populations in both years showed indications of chytridiomycosis. Sixteen percent of populations that were uninfected in 2003 became infected by 2004. Rana muscosa population sizes were reduced by an average of 88% following B. dendrobatidis outbreaks at six sites, but at seven B. dendrobatidis-negative sites, R. muscosa population sizes increased by an average of 45% over the same time period. In the laboratory, all infected R. muscosa developed fatal chytridiomycosis after metamorphosis, while all uninfected individuals remained healthy. In the field experiment in which R. muscosa tadpoles were caged at infected and uninfected sites, 96% of the individuals that metamorphosed at infected sites died vs. 5% at the uninfected sites. These studies indicate that chytridiomycosis causes high mortality in post-metamorphic R. muscosa, that this emerging disease is the proximate cause of numerous observed R. muscosa population declines, and that the disease threatens this species with extirpation at numerous sites in California's Sierra Nevada.