Dispersion and deposition of fallout from nuclear testmg @B E Moroz er at from most meteorological archives are generally msufficient for accurately modelmg these processes (Draxler and Hess 1997, 1998) Therefore, simphfymg assumptions are usually incorporated mto wet removal algorithms, leading to predictions with low rehability Given thepresent nationalsecurity concerns,there 1s a need for the scientific and defense communites to be aware of the capabilities of available atmospheric transport models and their possible application to predict fallout m the case of future events This paper discusses one particle transport and dispersion model, the Hybrid Single-Particle Integrated Trajectory (HYSPLIT) model, and describes how the model was tested and evaluated for the purpose of reconstructing fallout resultmg from nuclear testmg Our mai evaluation was of U S nuclear tests conducted m the Marshall Islands (MI) In addition to the Marshall Islands nuclear tests, two other fallout events were simulated to test the model the 1953 Upshot-Knothole Harry test at the Nevada Test Site (NTS) and the first Soviet nuclear test conducted at the Semupalatinsk Test Site m 1949 In latter section ofthis paper, we discuss one application of the HYSPLIT model our use of the model to support deposition and dose estrmates in the Marshall Islands reported m compamion papers (Beck et al 2010, Bouville et al 2010, Simon et al 2010a, 2010b) Based on the test simulations and the application mentioned above, the potential use of HYSPLIT predictions for both past and future fallout events 1s discussed MATERIALS AND METHODS The HYSPLIT model HYSPLIT(Draxler and Hess 1997, 1998, Draxler 1999) was developed and 1s mamtaimed by the National Oceanic and Atmospheric Admimstration Arr Resources Laboratory (NOAA ARL) The HYSPLIT model com- putes the dispersion and deposition of particles origmatimg from smgle or multiple source locations upon a simultaneous release In this paper, HYSPLIT was used to stmulate the advection and dispersion of particles ma radioactive debris cloud over the time period of several hoursto several days following the nuclear test Here we recogmze that HYSPLIT was not developedas a predictive fallout model, 1t makes no attempt to simulate the dynamics of the debris cloud prior to stabilization, nor does it stmulate the radioactivity associated with a particular particle size However, by assuming the debris cloud 1s stabilized, and assummg reasonable distribution ofparticles and particle sizes within the stabilized cloud, multiple stmulationsof the transport ofparticles released at various altttudes, for a range ofparticle sizes, can be combined to approximate the total fallout deposited from 2383 a debris cloud as the particles from each altitude are transported downwmd Meteorological mput data used by HYSPLIT are gridded meteorological data fields generated and archived from other meteorological models, although 1t 1s possible to perform simulations with user-defmed wind data to a limited extent (Draxler and Hess 1997, 1998, Draxler 1999) Because it 1s common for different meteorological models to use different vertical coordmate systems, HYSPLIT Imearly interpolates the meteorological data at each horizontal grid pomt m the meteorological mput data to an imternal sub-gnd contamng a terram-followmg coordimate system whereall herghts are expressed relative to mean sea level (Draxler and Hess 1997, 1998) The default vertical resolution for HYSPLIT defines the model’s lowest level (level 1) at approxi- mately 10 m and level 2, which 1s considered the surface layer, at approximately 75 m above ground level (AGL) Vertical resolution contmually decreases away from the ground surface followmg a quadratic form It 1s possible to modify the model’s mtermal vertical resolution by modifymg the mternal parameters corresponding to the model’s mternal height mdex (Draxler and Hess 1997) In contrast, the horizontal resolution applied by the model 1s equivalent to the horizontal resolution of the meteorological mput data The spacing between the gnd pomts of the mput data mfluences the accuracy of model computations As a general rule, the grid resolution should, at mimmum, correspondto the scale and the purpose of the simulation Data gridded at a coarse resolution mayyield less precise results than desired In contrast, finely gridded data can improve model results, assummg that the meteorological mput data are accurate The small amount of gridded meteorological data at fme resolutions in the tropic zones ofthe Pacific durmg the period of U § nuclear testmg in the Marshall Islands was one lnmiting factor to our work HYSPLIT offers several particle or puff modehng approaches which are discussed m Draxler and Hess (1997, 1998) We used the three-dimensional particle model to compute the advection and dispersion of the debris cloud Advection 1s computed mdependently, or priorto, the dispersion calculation The dispersion rate 1s dependent upon the vertical diffusivity profile, wind shear, and the horizontal deformation of the wind field (Draxler and Hess 1997) The particle advection algorithm has two primary steps After hnearly mterpolating the velocity vectors (x, vy, w) to the current particle position and time,a displacementcalculation yields a first-guess position by integratimg the velocity component at the current position and timeover the duration of the trme-step Thefinal position is then calculated by averagmg the velocity components