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
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


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

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