Steven G. Perry

AERMOD: A Dispersion Model for Industrial Source Applications. Part I: General Model Formulation and Boundary Layer Characterization

Abstract The formulation of the American Meteorological Society (AMS) and U.S. Environmental Protection Agency (EPA) Regulatory Model (AERMOD) Improvement Committee’s applied air dispersion model is described. This is the first of two articles describing the model and its performance. Part I includes AERMOD’s characterization of the boundary layer with computation of the Monin–Obukhov length, surface friction velocity, surface roughness length, sensible heat flux, convective scaling velocity, and both the shear- and convection-driven mixing heights. These parameters are used in conjunction with meteorological measurements to characterize the vertical structure of the wind, temperature, and turbulence. AERMOD’s method for considering both the vertical inhomogeneity of the meteorological characteristics and the influence of terrain are explained. The model’s concentration estimates are based on a steady-state plume approach with significant improvements over commonly applied regulatory dispersion models. Co...

AERMOD: A Dispersion Model for Industrial Source Applications. Part II: Model Performance against 17 Field Study Databases

Abstract The performance of the American Meteorological Society (AMS) and U.S. Environmental Protection Agency (EPA) Regulatory Model (AERMOD) Improvement Committee’s applied air dispersion model against 17 field study databases is described. AERMOD is a steady-state plume model with significant improvements over commonly applied regulatory models. The databases are characterized, and the performance measures are described. Emphasis is placed on statistics that demonstrate the model’s abilities to reproduce the upper end of the concentration distribution. This is most important for applied regulatory modeling. The field measurements are characterized by flat and complex terrain, urban and rural conditions, and elevated and surface releases with and without building wake effects. As is indicated by comparisons of modeled and observed concentration distributions, with few exceptions AERMOD’s performance is superior to that of the other applied models tested. This is the second of two articles, with the firs...

Screening Level Assessment of Risks Due to Dioxin Emissions from Burning Oil from the BP Deepwater Horizon Gulf of Mexico Spill

Between April 28 and July 19 of 2010, the U.S. Coast Guard conducted in situ oil burns as one approach used for the management of oil spilled after the explosion and subsequent sinking of the BP Deepwater Horizon platform in the Gulf of Mexico. The purpose of this paper is to describe a screening level assessment of the exposures and risks posed by the dioxin emissions from these fires. Using upper estimates for the oil burn emission factor, modeled air and fish concentrations, and conservative exposure assumptions, the potential cancer risk was estimated for three scenarios: inhalation exposure to workers, inhalation exposure to residents on the mainland, and fish ingestion exposures to residents. U.S. EPAs AERMOD model was used to estimate air concentrations in the immediate vicinity of the oil burns and NOAAs HYSPLIT model was used to estimate more distant air concentrations and deposition rates. The lifetime incremental cancer risks were estimated as 6 × 10(-8) for inhalation by workers, 6 × 10(-12) for inhalation by onshore residents, and 6 × 10(-8) for fish consumption by residents. For all scenarios, the risk estimates represent upper bounds and actual risks would be expected to be less.

Building resolving large-eddy simulations and comparison with wind tunnel experiments

We perform large-eddy simulations (LES) of the flow past a scale model of a complex building. Calculations are accomplished using two different methods to represent the edifice. The first method employs the standard Gal-Chen and Somerville terrain-following coordinate transformation, common in mesoscale atmospheric simulations. The second method uses an immersed boundary approach, in which fictitious body forces in the equations of motion are used to represent the building by attenuating the flow to stagnation within a time comparable to the time step of the model. Both methods are implemented in the same hydrodynamical code (EULAG) using the same nonoscillatory forward-in-time (NFT) incompressible flow solver based on the multidimensional positive definite advection transport algorithms (MPDATA). The two solution methods are compared to wind tunnel data collected for neutral stratification. Profiles of the first- and second-order moments at various locations around the model building show good agreement with the wind tunnel data. Although both methods appear to be viable tools for LES of urban flows, the immersed boundary approach is computationally more efficient. The results of these simulations demonstrate that, contrary to popular opinion, continuous mappings such as the Gal-Chen and Somerville transformation are not inherently limited to gentle slopes. Calculations for a strongly stratified case are also presented to point out the substantial differences from the neutral boundary layer flows.

Indirect sensing of plant canopy structure with simple radiation measurements

Abstract Easy and reliable estimates of plant canopy structure are needed for many applications but direct measurements are laborious and often destructive. A technique for indirectly sensing canopy structure from simple, manageable measurements of sunlight transmitted through the canopy is examined. The integral relationship between sunlight transmission and leaf area index and leaf angle distribution is discussed and a numerical inversion technique is described. An analysis of the kernels of the integral equation (relationship) reveals the relationship between the transmission measurement errors and the amount of canopy structural information contained in those measurements. Results of tests of the inversion technique with simulated transmission data characterizing that from a wide variety of plant canopies and with actual measurements in corn support the conclusions drawn from the information analysis. In all tests, with relative errors ranging from 0.5 to 4%, the true canopy type (angle distribution) is identified and the leaf area index is estimated to within 2% of the simulated inputs and to within 9% of the direct estimates in corn.

A complex terrain dispersion model for regulatory applications

Abstract This paper demonstrates the development of a model designed to estimate concentrations associated with a source situated in complex terrain. The model is designed to provide estimates of concentration distributions and is thus primarily suitable for regulatory applications. The model assumes that the concentration at a receptor is a combination of concentrations caused by two asymptotic states: the plume remains horizontal, and the plume climbs over the hill. The factor that weights the two states is a function of the fractional mass of the plume above the dividing streamline height. The model has been evaluated against data from four complex terrain sites. The evaluation shows that the model performs at least as well as CTDMPLUS (Perry, S.G., 1992. CTDMPLUS, a dispersion model for sources near complex topography. Part I: technical formations. Journal of Applied Meteorology 31, 633–645), a more comprehensive model designed for complex terrain applications.

CTDMPLUS: A Dispersion Model for Sources near Complex Topography. Part I: Technical Formulations

Abstract The Complex Terrain Dispersion Model (CTDMPLUS), a point-source, steady-state model for complex-terrain applications, is described. The model simulates the flow and plume distortion near user-selected, three-dimensional terrain features, yet retains simplicity by applying flow-distortion corrections to flat-terrain, Gaussian, and bi-Gaussian pollutant distributions. The algorithms for stable and near-neutral conditions are based on the demonstrated concept of a dividing streamline. These algorithms were developed using data from three major plume-impaction field studies and a number of fluid-modeling studies. The algorithms for plumes released into convective layers are based on recent understanding of the convective boundary layer through fluid modeling, numerical modeling, and field studies. The non-Gaussian nature of vertical dispersion is accounted for; lateral dispersion is modeled with the aid of convective scaling concepts. A terrain preprocessor and a meteorological preprocessor, which pr...

The effect of a tall tower on flow and dispersion through a model urban neighborhood: part 1. Flow characteristics.

Wind tunnel experiments were performed to examine the effect of a tall tower on the flow around an otherwise uniform array of buildings. Additionally, preliminary CFD simulations were run to visualize the flow with more resolution. The model used in both the wind tunnel and CFD studies was designed to simulate an area of Brooklyn, NY, USA, where blocks of residential row houses form a neighborhood bordering a major urban highway. This area was the site of a field study that, along with the work reported here, had the goal of improving the understanding of airflow and dispersion patterns within urban microenvironments. Results reveal that a tall tower has a dramatic effect on the flow in the street canyons in the neighboring blocks, enhancing the exchange between the street canyon flow and the freestream flow aloft. In particular, vertical motion down the windward side and up the leeward side of the tower resulted in strong flows in the lateral street canyons and increased winds in the street canyons in the immediate vicinity of the tower. These phenomena were visible in both the wind tunnel and CFD results, although some minor differences in the flow fields were noted.

Horizontal Coherence Decay Near Large Mesoscale Variations in Topography

Abstract A surface layer experiment is described which includes measurements of turbulent velocities at 2 m above the surface with an army of newly developed drag anemometers. The experiment site is located in central Pennsylvania where mesoscale topographic irregularities exist. The presence of a low mountain ridge near the site affects the estimated lateral scale of turbulence and the fluctuations of the lateral velocity component. A good correlation has been found between the variance spectrum of the lateral (or crosswind) velocity component and an estimate of the lateral Eulerian integral scale of the longitudinal velocity component. This can provide future estimates of the lateral scale from turbulent velocity measurements at a single location. A model for the decay of horizontal coherence which accounts for the stability, roughness and instrument separation has been suggested in a previous paper by Panofsky and Mizuno. The present data compare favorably with this model. The effect of stability on co...

AERMOD: The Developmental Evaluation

In 1991, the American Meteorological Society (AMS) and Environmental Protection Agency (EPA) initiated a formal collaboration to accelerate the inclusion of up-to-date science into regulatory dispersion models. A working group, the AMS/EPA Regulatory Model Improvement Committee (AERMIC), was established to facilitate this modeling activity (Perry et al., 1994: Weil, 1992).