TAB E - Methodology for Modeling a Possible Chemical Warfare Agent Release

A. Background

In 1996, the Central Intelligence Agency (CIA) reported on computer modeling that it had used to simulate possible releases of chemical warfare agents from several sites. However, because the CIA used only a single model approach, its results reflected the strengths and weaknesses of only that model. On November 2, 1996, to improve computer modeling over the earlier CIA results, the Department of Defense asked the Institute for Defense Analyses (IDA) to convene an independent panel of experts in meteorology, physics, chemistry, and related disciplines.[89] The panel reviewed previous modeling analyses, and recommended using several atmospheric models and data sources for future modeling[90] to generate a better result than a single model produces. Consequently, the Special Assistant agreed to new modeling to estimate the areas of possible exposure to chemical warfare agents that may have been released during Gulf War air operations and post-war demolitions.

The staff of the Special Assistant uses computer simulations because on-site measurements of chemical warfare agent exposure were unavailable and the local weather conditions were not always measured or recorded. To implement the recommendations of the IDA panel, the DoD and the CIA asked other agencies with extensive modeling experience to participate in the modeling process. The modeling team consists of scientists from the Defense Threat Reduction Agency (formerly the Defense Special Weapons Agency), the Naval Research Laboratory, the Naval Surface Warfare Center, the National Center for Atmospheric Research, and Science Applications International Corporation (supporting the CIA and the Defense Threat Reduction Agency). This team uses existing high-quality computer models, as recommended by the IDA panel, to develop potential exposure areas, specific to each incident under investigation. The team combines these models (called an ensemble) to compensate for the bias inherent in each model to produce a better result by maximizing the strengths of each and minimizing its weaknesses.

B. Methodology

To assess the possible dispersion of chemical warfare agents, we adopted the IDA panel’s recommendation to use an ensemble of weather and dispersion models, combined with global data sources. The methodology for modeling a chemical warfare agent release uses:

Figure 16 depicts this methodology.

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Figure 16.  Process for modeling possible chemical warfare agent releases

The methodology uses two types of models—weather models and dispersion models. Weather models allow us to simulate the weather conditions in specific areas of interest by approximating both global and regional weather patterns. Given the weather generated by a global model, a regional weather model predicts local weather conditions near a possible chemical warfare agent release. Actual, although quite limited, weather measurements from the Persian Gulf and surrounding regions supplement the global and regional weather modeling.

Dispersion models allow us to simulate how chemical warfare agents may move and diffuse in the atmosphere given the predicted local weather conditions. These models combine the source characteristics of the agent (including the amount and type of agent, the release location, and the release rate) with the local weather from the regional models to predict how the agent might disperse. Running one dispersion model using the weather conditions predicted by one regional model results in one projected downwind hazard area. Running each dispersion model using the weather from each of the different regional weather models results in a set of unique hazard areas. We overlay all these hazard areas to create a union or composite of the various projections. The resulting composite graphic provides the most credible array of potential agent vapor hazard areas for determining where military units might have been exposed.[91] This is the basic process for all of our modeling efforts.

C. Global Models and Database

Global weather models forecast atmospheric conditions over the globe. To perform calculations, the models mainly rely on observations collected from a global network of land-based weather stations, sea-based ships or buoys, and remote sensing satellites. Agencies such as the World Meteorological Organization distribute these data for civilian agencies. Because of their vast domain (i.e., the whole globe), global models provide only general weather information.

We use three sources for global weather data:

These sources collect and process weather observations differently, and maintain historical archives that provide historical and simulated global weather data for our analyses.

D. Regional Models

Regional weather models, sometimes called mesoscale meteorological models, estimate local weather conditions in the detail dispersion models require. To predict detailed local atmospheric conditions, regional models take the outputs from global models to yield weather estimates with a resolution within a few miles.

We use three regional models:

These models simulate atmospheric conditions for advanced, high-resolution weather forecasting. They can simulate local weather conditions to a few square miles. Although these models operate differently, they all produce the detailed meteorological data needed to run the dispersion models. Because of differences in input data, modeling processes, and the physical process assumptions, each regional model produces different results. However, careful analysis of each simulation has shown that the regional models’ outputs generally are consistent.

E. Dispersion Models

Dispersion models predict possible downwind hazard areas, which indicate how chemical warfare agents disperse over time due to prevailing local atmospheric conditions. Dispersion model results depend on both the local weather descriptions created by the regional weather models and other modeling assumptions, including:

We use two dispersion models:

Because of their different inherent assumptions, these two dispersion models generate slightly different results even with the same weather inputs and source characterization—in much the same way that different weather models produce different forecasts even when using the same observed data. As a result, combining all regional weather models with each dispersion model can create several distinct hazard areas.

The IDA technical review panel hypothesized that because of the uncertainty of modeling and the consistency of the simulations, each simulation’s results are equally likely to accurately reflect what really may have occurred.[101] Therefore, we combined all hazard projections generated by HPAC or VLSTRACK to create a single hazard area combining all exposure areas from all dispersion model runs. This approach assures a high probability that the exposure area includes all units possibly exposed. Figure 17 depicts the process for creating a composite hazard projection.

F. Unit and Personnel Locations

We send the hazard projection graphics derived from the dispersion models to the US Army Center for Health Promotion and Preventive Medicine,[102] which overlays the hazard projection graphics with data on US unit locations to create an exposure plot showing the areas and levels of possible exposure. Two Department of Defense organizations, the US Armed Services Center for Unit Records Research and the Defense Manpower Data Center provide the databases used to determine unit locations during the Gulf War and who was in each unit in the possible exposure hazard areas. We cross-reference, validate, refine, and update the unit location data to increase the accuracy of whether a unit was in an exposure area at a specific time.

G. Modeling Considerations

Weather models represent our best attempts to approximate actual atmospheric conditions. They do not replicate reality with absolute certainty, but modern modeling techniques enable us to generate reasonably close approximations. We simulate regional weather conditions in the Gulf using weather modeling. Dispersion models then work with the simulated weather to project the overall agent distribution in the area. The composite hazard projection area represents an average picture. Since the atmosphere is inherently turbulent, the actual concentration of agent within the hazard area might vary throughout the projected area. Therefore, modeling predicts that the concentration of chemical warfare agent is at the exposure threshold throughout the hazard area, even though the agent may not necessarily be everywhere in the area. We can only conclude that people within the hazard projection area may have been exposed to the calculated concentration of agent multiplied by the time of exposure across the entire hazard area.

The CIA estimated the quantity, type, and storage configuration of chemical warfare agents stored at the sites under investigation. The source characterizations tend to overstate the size of the release to minimize risk of failing to identify all the agent that might have been released. Presenting a composite of the different modeling results is another way to minimize the risk of missing veterans who may have been in a hazard area. If we are to err, we would prefer to identify a veteran who may not have been exposed, rather than fail to recognize a veteran who had been exposed.

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