James F. Heagy

User-Oriented Two-Dimensional Measure of Effectiveness for the Evaluation of Transport and Dispersion Models

Abstract A two-dimensional measure of effectiveness for comparing hazardous material transport and dispersion model predictions and field observations has been developed. This measure is used for comparing predictions and observations paired in space and time, and the components of this measure—overprediction, or false-positive fraction, and underprediction, or false-negative fraction—can illuminate strengths and weaknesses of a model in ways that many one-dimensional measures cannot. Comparisons of predictions of short-range field observations are used to illustrate features of this measure of effectiveness, including its computation based on dosages or based on a dosage threshold of interest. With this user-oriented measure of effectiveness, statistically significant resolution of transport and dispersion model performance differences as a function of downwind range and meteorological stability category grouping, as well as between different models, is described. Evaluation of probabilistic prediction o...

Comparisons of Transport and Dispersion Model Predictions of the Joint Urban 2003 Field Experiment

Abstract For a hazardous material release in a city or densely populated area, effective mitigation requires an understanding of the transport and dispersion of these hazards in the complex urban environment. Improved characterization and understanding of urban transport and dispersion will allow for more robust modeling. The Defense Threat Reduction Agency has developed a Hazard Prediction Assessment Capability (HPAC) that includes features to address hazardous releases within an urban environment. During the summer of 2003, a series of tracer gas releases were carried out in Oklahoma City, Oklahoma, and extensive meteorological and tracer concentration measurements were collected in a field experiment known as Joint Urban 2003 (JU03). This analysis uses the observations of JU03 to evaluate “Urban HPAC.” Twenty sets of simulations, or “predictions,” using four Urban HPAC modes and five meteorological input options, were created and compared using a variety of metrics. Strong consistency was found between...

Comparisons of Transport and Dispersion Model Predictions of the Mock Urban Setting Test Field Experiment

The potential effects of a terrorist attack involving the atmospheric release of chemical, biological, radiological, nuclear, or other hazardous materials continue to be of concern to the United States. The Defense Threat Reduction Agency has developed a Hazard Prediction Assessment Capability (HPAC) that includes initial features to address hazardous releases within an urban environment. Improved characterization and understanding of urban transport and dispersion are required to allow for more robust modeling. In 2001, a scaled urban setting was created in the desert of Utah using shipping containers, and tracer gases were released. This atmospheric tracer and meteorological study is known as the Mock Urban Setting Test (MUST). This paper describes the creation of sets of HPAC predictions and comparisons with the MUST field experiment. Strong consistency between the conclusions of this study and a previously reported HPAC evaluation that relied on urban tracer observations within the downtown area of Salt Lake City was found. For example, in both cases, improved predictions were associated with the inclusion of a simple empirically based urban dispersion model within HPAC, whereas improvements associated with the inclusion of a more computationally intensive wind field module were not found. The use of meteorological observations closest to the array and well above the obstacle array—the sonic anemometer measurements 16 m above ground level—resulted in predictions with the best fit to the observed tracer concentrations. The authors speculate that including meteorological observations or vertical wind profiles above or upwind of an urban region might be a sufficient input to create reasonable HPAC hazard-area predictions.

Synchronization and desynchronization in pulse coupled relaxation oscillators

Abstract We have studied the synchronization of an array of three globally pulse coupled relaxation oscillators. We vary the firing rate on one of the oscillators and see how this affects the synchronization of the oscillators for different coupling symmetrics. We find that certain types of coupling make the array more responsive to parameter changes but still robust against noise.

Dual driving of magnetostatic modes in yttrium–iron–garnet film experiments

We study experimentally the behavior of magnetostatic wave modes in a rectangular yttrium–iron–garnet film placed in an in‐plane magnetic field and subject to multiple drive excitations. Patterned slotlines and coplanar waveguides are used to drive the film with microwave (2–4 GHz) excitations corresponding to magnetostatic wave modes. For a single drive, we observe linear transmission at low drive powers and saturation above the Suhl instability. For sufficiently large powers above the instability, the transmitted power through the film displays periodic low‐frequency (kHz) auto‐oscillations. A second microwave drive applied to the sample is used to excite additional spin‐wave modes that interact with those arising from the original excitation. Such dual‐drive experiments can probe the nature of spin‐wave mode interactions at high amplitudes.

Riddled basins and other practical problems in coupled synchronized chaotic circuits

We investigate a number of phenomena which occur in synchronization in coupled or driven chaotic systems and which can cause difficulties in attaining synchronized states. We present direct experimental and numerical evidence for riddled basins of attraction, bursting phenomena, short wavelength bifurcations and size desynchronization effects. We show that typical Lyapunov stability exponents are not the optimal guide in designing such systems.

Assessment of HPAC urban modelling capabilities using data from the Joint Urban 2003 field experiment

An independent evaluation was conducted of four urban prediction models included in the US Defense Threat Reduction Agencys (DTRA) Hazard Prediction and Assessment Capability (HPAC) (Version 4.0). This evaluation focused on comparing HPAC predictions with data obtained during the Joint Urban 2003 (JU03) field experiment in Oklahoma City. These models were run using a variety of meteorological inputs, including the results of forecast models and meteorological observations from the JU03 experiment. A particularly significant finding was a difference in model performance for daytime vs. nighttime tracer gas releases. Consistent significant differences in performance between models were only observed for nighttime releases.

Effects of meteorological data thresholding on the quality of urban HPAC predictions of the Joint Urban 2003 field trials

Comparisons of field trial observations with predictions created by the US Defense Threat Reduction Agencys Hazard Prediction and Assessment Capability (HPAC) represent an ongoing effort. Intuitively, one might prefer to include meteorological inputs as close to the release location as possible. During Joint Urban 2003, SODARs measured vertical wind profiles?upwind, within the city centre near the releases, and downwind of Oklahoma City. Low altitude SODAR measurements fell within the urban canopy, while the upper altitude measurements were outside the canopy. In this paper, we demonstrate how the mix of lower altitude and upper altitude wind measurements affects HPAC predictions.

Evaluations of urban atmospheric transport and dispersion models using data from the joint urban 2003 field experiment

We have evaluated the performance of several urban Atmospheric Transport and Dispersion (AT&D) models by comparing model predictions to tracer gas concentrations measured during the Joint Urban 2003 field experiment in Oklahoma City, USA. These models include the Urban Canopy, Urban Dispersion Model (UDM), and Micro-SWIFT/SPRAY (MSS) modes within the HPAC modelling suite, QUIC-URB/QUIC-PLUME models, and the MESO/RUSTIC models. We discuss some of the results of these comparisons, including relative model performance according to bias and scatter metrics, differences in model behaviour for predictions of daytime vs. nighttime releases, and operational considerations such as runtime differences.

Comparison of two point-to-point protocols for transport and dispersion model evaluations

Historically, assessments of transport and dispersion models have involved comparisons to field trial data using quantities derived from observations – e.g., maximum concentrations or plume widths. Recently, several methodologies that compare observations and predictions paired in space and time have been developed. Two protocols designed to deal with low observed or predicted values have emerged. One requires that the observation and prediction exceed a threshold before being considered for comparison. The other protocol requires that either the observation or the prediction exceed the threshold before being considered for comparison. This paper compares the potential effects of these protocols to model assessments.