Paper PREDICTIONS OF DISPERSION AND DEPOSITION OF FALLOUT FROM NUCLEAR TESTING USING THE NOAA-HYSPLIT METEOROLOGICAL MODEL Brian E. Moroz,* Harold L. Beck,’ André Bouville,* and Steven L. Simon* were used in a limited fashion to support the dose reconstruction described in companion papers within this volume. Health Phys. 99(2):252-269; 2010 Abstract—The NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory Model (HYSPLIT) was evaluated as a research tool to simulate the dispersion and deposition of radioactive fallout from nuclear tests. Model-based estimates of fallout can be valuable for use in the reconstruction of past exposures from nuclear testing, particularly where little historical fallout monitoring data are available. The ability to makereliable predictions about fallout deposition could also have significant importance for nuclear events in the future. Weevaluated the accuracy of the HYSPLIT-predicted geographic patterns of deposition by comparing those predictions against known deposition patterns following specific nuclear tests with an emphasis on nuclear weaponstests conducted in the Marshall Islands. We evaluated the ability of the computer code to quantitatively predict the proportion of fallout particles of specific sizes deposited at specific locations as well as their time of transport. In our simulations of fallout from past nucleartests, historical meteorological data were used from a reanalysis conducted jointly by the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR). We used a systematic approach in testing the HYSPLIT model by simulating the release of a rangeof particle sizes from a rangeof altitudes and evaluating the numberandlocation of particles deposited. Our findings suggest that the quantity and quality of meteorological data are the most important factors for accurate fallout predictions and that, when satisfactory meteorological input data are used, HYSPLIT can produce relatively accurate deposition patterns and fallout arrival times. Furthermore, when no other measurement data are available, HYSPLIT can be used to indicate whetheror not fallout might have occurred at a given location and provide, at minimum, crude quantitative estimates of the magnitude of the deposited activity. A variety of simulations of the deposition of fallout from atmospheric nuclear tests conducted in the Marshall Islands (mid-Pacific), at the Nevada Test Site (U.S.), and at the Semipalatinsk Nuclear Test Site (Kazakhstan) were performed. The results of the Marshall Islands simulations Key words: Marshall Islands; nuclear weapons; fallout; modeling, meteorological INTRODUCTION CoMPUTER MODELS have been both influential and beneficial in predicting fallout dispersion and deposition. These models have been usedhistorically for such diverse tasks as producing quick fallout estimates necessary for immediate health assessments, extending exposure estimates downwind beyond ground-based measurements in retrospective dose and risk assessments (Cederwall and Peterson 1990; Hoecker and Machta 1990), and projecting potential physical damage, including atmospheric effects such as smokeproduction from regional nuclear conflicts and individual acts of nuclear terrorism (Toon et al. 2007). Computer codes used for such purposes were developed and applied by the scientific and defense communities as early as the 1960’s (Rowland 1994). Modeling the transport and deposition of particles released from a nuclear weaponstest is both a complex and highly uncertain exercise. This is true even when the meteorological data used in the simulation are accurate. Furthermore, in order to simulate the deposition density of specific radionuclides or total radioactivity, a modelis required for the spatial distribution of radionuclides in the initial debris cloud as well as the distribution of activity as a function of particle size. The most computationally burdensome factors in performing the simulations are the large size of the debris cloud and,therefore, the large numberof particles and particle sizes that are needed to conducta realistic fallout simulation over long distances. An additional difficulty is presented when modeling wet removal processes. Both in-cloud and below-cloud wet removal processes may be of great importance to accurately simulating deposition when precipitation occurred downwind. The data available * Division of Cancer Epidemiology and Genetics, National Insti- tutes of Health, National Cancer Institute, Bethesda, MD, 20892; * New York, NY. For correspondence contact: Steven L. Simon, National Cancer Institute, National Institutes of Health, 6120 Executive Blvd., Bethesda, MD 20892, or email at ssimon@mail.nih.gov. (Manuscript accepted 22 June 2009) 0017-9078/10/0 Copyright © 2010 Health Physics Society DOI: 10.1097/HP.0b013e3 18 1b43697 252