Dispersion and deposition of fallout from nuclear testmg @B E Moroz er at are provided m Draxler and Hess (1998) Draxler and Hess (1998) discussed a HYSPLIT simulation of the release from the Chermoby] reactor accident which took place m the former Soviet Umon m 1986 Resulong deposition contours and peaks were compared agamst those reported m Klug et al (1992) It was found that contour patterns were reasonably consistent and the HYSPLIT-computed deposition peaks were numerically ~10% higher than the reported values m the best case Two additional studies which used HYSPLIT as a research tool for modeling fallout processes mclude Kmser (2001) and Swanberg and Hoffert (2001), who simulated releases of '’Cs from the Chernobyl] reactor accident to mvestigate model-predicted wet deposition and resuspension as a source of '°’Cs m Europe Meteorological input data Highspatial density meteorological data are e1ther not available or are very sparse during the 1950’s for many areas mcluding the mid-Pacific Ocean where the Marshall Islands are located, the NTS, and the Semipal- atmsk Test Site For that reason, meteorological imput data from a reanalysis data set were used Thereanalysis was conducted as a collaborative effort between the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) with the purpose of recovering historical weather observations from many sources, providmg quahty controlofthe data, and compiling the results so as to fulfill the research needs for chmate momtormg and prediction The NCEP/NCARdatabase covers the period 1948-2008 and provides forecasts at regular temporal tervals of four times daily (though, durmg the period 1948-1957, forecasts are provided erght times daily) and at a spatial resolution of 2 5 degrees The most 1mportant observational data mcorporated ito the reanalysis data set for the purposes of our study are the upper-air wind data Upper-atr wid data are primarily derived from the world’s upper-atr rawinsonde network, although a considerable amount of anrcraft reconnaissance data was also incorporated mto the NCAR/NCEPreanalysis The US Aur Force prepared a global collection of atrcraft data that covered the time period 1948-1970 Arrcraft data were also contributed by the University of Hawanfrom locations mthe tropics for the time period 1960-1973 Early rawmsonde coverage m the US 1s farrly complete, though less data are available for other parts of the world Good coverage m the US began in 1948 while periods of good coverage m China, India, and Russia cannot be found earlter than the 1950's and 1960’s Wind profiles from several operational weather stations m the Marshall Islands were available in addition 2358 to upper-air wind measurements taken at test site locations by the US Army (DNA 1979), although the supporting literature does not unequivocally report that those observations were mcorporated into the NCAR/ NCEPreanalysis Precipitation observations im the reanalysis data set are also 1mportant, but appear to be muchless rehable than upper-air wind data In the NCAR/NCEPreanalysis model, observed precipitation data do not play a direct part m the reanalysis output Precipitation dataare solely driven by the reanalysis model and can exhubit regional biases (Kalnay et al 1996) NUCLEAR TEST SIMULATIONS AND MODEL EVALUATIONS General methods The HYSPLIT model wastested using a systematic approach for stmulatmg the transport and deposition of radioactive particles resultmg from a nuclear weapons test A range ofparticle sizes were released from a range of altitudes at a smgle source location and tracked over time The proportion of deposited particles and the location of deposition were then evaluated by comparing model-predicted deposition patterns and time of fallout arrival against known deposition patterns and best available estimates In order to simulate the deposition density of a specific radionuchde, such as ‘Cs, a crude model was developed to qualitatively relate HYSPLIT particle dep- osition to "Cs deposition as a function offission yield, debris cloud size, and stabilization altitude Ths model assumesa lognormaldistribution of activity as a function of particle diameter based on data from the NTS (Izrael 2002) In order to simulate *’Cs ground deposition density (Bq m~*) for comparison with the "Cs depos tion densities m the Marshall Islands reported by Beck et al (2010), the "Cs particle-size distribution was modi- fied to reflect the fact that '’Cs tends to be depleted on larger particles Much of the 'Cs 1s formed after the heavier particles have already deposited due to '’Cs havmg a gaseous precursor, 'Xe (Beck et al 2010) Here we assumethat ~80% ofthe total '’Cs activity 1s found on particles less than 50 2m m diameter This assumption 1s consistent with data and models for a coral surface shot, as reported by Freiling et al (1965) The apportionment of the "Cs on particle sizes less than 50 pin (ie, 80%) was chosen based on experience and Judgment since little actual data are available from the literature Theparticle-size distributionsfor ¥’Cs activity used for the Marshall Island stmulations are shown m Table 1 In order to estrmate the sensitivity of the simulated deposition density of "Cs, two otherparticle- size distributions were tested m selected simulations The