258 Health Physics the test site were in agreement with the meteorological input data used by the model, there is no guarantee that the model will predict correct depositions because the meteorology downwindcan also be in error. Significant differences in predicted fallout were frequently observed when precipitation processing was enabled as compared to when precipitation processing was disabled. As shown in Table 2, the differences in deposition were occasionally as great as a factor of three, although generally much less. Often, the amount of fallout was reduced at a given atoll when precipitation processing was enabled, apparently due to cloud depletion at upwind locations. At other times, predicted deposition was higher, suggesting local rainout or washout. The limitations in the HYSPLIT wet deposition model (in common with other meteorological models), and the lack of precipitation data on an event-specific basis, may have contributed, in some cases, to the quantitative differences seen between the measured fallout deposition and model simulated estimates at some locations at some times. Based on comparisons for tests where significant monitoring data were available, predicted depositions using the HYSPLIT three-dimensional particle model coupled with NCEP/NCAR reanalysis meteorological data agree with measured '°’Cs densities to within a factor of ten. This was almost always true when the initial wind speed and direction of the meteorological input data agreed reasonably well with that of the actual wind data at the test site. Table 2. Comparison of HYPSLIT simulationsof '*’Cs deposition density (Bq m7’) with and without wet deposition for selected tests and atolls. Wet deposition disabled Atoll/Test Fir (11 May 1958)* Kili Ebon Mejit Flathead (11 June 1956)* Namorik Wotho Nectar (13 May 1954)* Namorik Kili Jabat Lib Lae Dog (7 April 1951)* Kwajalein Ujelang Lae * All dates GMT. Wet deposition enabled Atoll domain 1 degree domain Atoll domain 1 degree domain 30 30 78 22 30 100 15 40 110 22 22 100 340 5,200 300 3,300 140 4,100 300 3,000 40 74 130 850 440 160 40 85 520 740 85 48 180 560 560 160 40 110 520 780 5.2 24 0 3.3 110 2.6 9.3 60 3.0 6.7 190 5.6 August 2010, Volume 99, Number 2 Inadequate meteorological data were generally the limiting factor in the HYSPLIT model’s ability to predict accurate arrival times of fallout for Marshall Islands tests, although high-quality data on actual arrival times were also often lacking due to few measurements having been made and someinconsistencies between the available measurements (Beck et al. 2010). These conditions made comparing model-based estimates with measurements a difficult exercise. A comparison of model-predicted fallout arrival times with reported best estimates (Beck etal. 2010) is provided in Table 3. As discussed, the HYSPLIT model uses a simple rainout and washout model that may not adequately simulate such complex processes, particularly for the relatively large amounts of debris in nuclear test clouds. Forthis reason and because of the normal high frequency of precipitation events in the Marshall Islands, most of which would not have been recorded in archival meteorological data sets, particularly in the southern atolls (Beck et al. 2010) where rainfall is the highest, the HYSPLIT simulations may not have predicted some actual deposition events. Moreover, it is possible that some of the differences in the HYSPLIT-predicted fallout arrival times as compared to the generally earlier arrival times observed could be a result of fallout deposited as a result of precipitation scavenging from unrecordedprecipitation events. Test-specific results. The results of simulations of fallout from five nuclear tests in the Marshall Islands are discussed here. Simulation results were compared against existing measurement data when possible, although anecdotal reports of fallout from test participants were considered as well. Further details on the characteristics and dates of the tests are given in an appendix of a companion paper (Becket al. 2010). Greenhouse Dog was a pure fission device which was detonated on Enewetak Atoll on 7 (GMT) April 1951. No radiological survey data are available for the Dog test. However, the HYSPLIT simulations suggest that small amounts of fallout could have occurred at several atolls in the Marshall Islands including Ujelang (Fig. 1), Wotho, Kwajalein, and Utrik. Modelpredicted wind speed and direction agree fairly well with the observed values reported in DNA (1979) (Table 4). Dog is an example where HYSPLIT simu- lations wereparticularly valuable because there are no historical monitoring data. Greenhouse Item was a fusion device detonated on Enewetak Atoll on 24 (GMT) May 1951. Similar to the situation for Dog, no radiological survey data exist for the Item test. The HYSPLIT simulation suggests there was significant fallout at Ujelang Atoll (Fig. 1). Because