Dispersion and deposition of fallout from nuclear testmg @B E Moroz er at APPLICATION OF HYSPLIT TO MARSHALL ISLANDS FALLOUT ASSESSMENT HYSPLIT model simulations were used to support the analysis of deposition of fallout m the Marshall Islands and related dose assessments (Beck et al 2010, Bouville et al 2010, Simon et al 2010a, 2010b) This 1956 and 1958 tests compared to the 1954 Castle tests For this reason, the uncertamty contnbution of the HYSPLIT stmulations to the overall uncertamty of the estimated external and mternal doses (Bouville et al 2010, Simon et al 2010a) was also small CONCLUSION application 1s described below Simulations of air mass trajectories and deposition patterns were used to help make '’Cs deposition density 267 A well-established meteorological model, HYSPLIT, time of arrival, and to fill out the estimated deposition wastested for its ability to predict dispersion and deposition of nuclear test-related fallout at varying distances downwind The model was evaluated by comparmg modelpredicted deposition patterns and arrival times agamst measured deposition density Particles of varymg sizes were released from a range of startmg heights to represent a stabihzed radioactive debris cloud Deposition domams were definedto track the locations of deposited particles so as to test the model’s abihty to predict downwind deposttions of differently sized particles at specific locations in agreement with known patterns Because of the general hmited availability of ground-based radiological measurement data,it 1s very difficult to separate the relative contributions of different factors to the overall predictve ability of HYSPLIT Results from our stmulations suggest that the accuracy and spatial resolution of the meteorological inputdata 1s one of the most important factors im modeling fallout with the HYSPLIT model since the advection and dispersion calculations directly depend on the meteorological data The simplification of physical processes and the particle distribution that 1s assumed m the debris few (4 to 6) atolls were momtored For example, for the 1956 Flathead test, model predictions were used in implemented m HYSPLIT, all may have had an impact on the quantitative predictions of fallout at a particular estimates for specific locations m the Marshall Islands for tests m which fallout momtoring data were either sparse or nonexistent The results were used to assist m interpolations of deposition at atolls where no momtonng data were available usmg measurements of deposition at nearby atolls (Beck et al 2010) Despite the uncertamty found after testmg the model under conditions of questionable mput data and lmited measurement data, the model-based deposition estimates were useful m estimating fallout deposited m the Marshall Islands (Beck et al 2010) im certain specific cases For example, the HYSPLIT predictions were the only source of information on fallout deposited and often supported anecdotal reports of sigmficantfallout prior to 1952 when there were no momtormg data This 1s particularly true at Ujelang Atoll where anecdotal reports imdicated fallout resultmg from the 1951 Greenhouse Dog and Item tests but no actual measurements were reported Additionally, the HYSPLIT model simulations were used to support imterpolations of deposition and patterns from the 1956 and 1958 tests for which only a conjunction with GF measurements at Kwayjalem and survey measurements at Ujelang, Wotho, Rongelap, and Utnk, to estimate fallout at atolls south and east of Kwayalem where no actual measurements were made Only very low levels of fallout deposition were predicted m other areas Similarly, for the 1958 Fir test, the HYSPLIT simulations were used to aid m estimating the relatively low levels of deposition at atolls not momtored As discussed m Beck et al (2010), a high uncer- tainty estimate (a probability distribution function with a geometric standard deviation of 30) was apphed to the HYSPLIT-based deposition density estimates Although uncertam, the HYSPLITresults had rela- tively little impact on estimates of total fallout m the Marshall Islands presented m Beck et al (2010) because the model-predicted fallout estimates used were almost always small compared to fallout levels from tests with momitoring data This was the case, for example, for the cloud model, as well as m the wet deposition model atoll When relatively accurate wmd data were used, however, we confirmed that the model-predicted deposition is reasonably consistent with available ground measurement data In our simulations, meteorological reanalysis data were used Although methods im data assimilation and reanalysis have greatly improved, reanalyses prior to the geophysical year (1957-1958)still suffer from a lack of satisfactory observations However, the HYSPLIT model was able to predict reasonably accurate fallout arrival timesfor simulations m whichthe meteorological reanalysis data were consistent with observed data at several altitudes within the cap ofthe stabilized debris cloud at the test site Under those conditions, model-predicted arrival mes were often within several hours of those reported Conversely, when the reanalysis data did not agree with the local observed wind measurements, fallout arrival trmes and depositions deviated, largely m some cases, from reported values im the literature