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KNOX
Fig. 5—Simple wind hodograph assumed in the
model for wind specification.
hodograph.)
(Idealized wind
fis the standard deviation of the position of the particles (in the disk)
from the disk centroid. Since { is poorly known in nuclear-debris
clouds, it is usually set equal to R.» or 2R.,, if accelerated relative
diffusion is to be approximated in the fallout model.
Physical Processes Simulated in the Model
The principal physical or meteorological processes simulated by
the cratering fallout model are (1) the transport of the debris disks by
the mean wind in the layer through which the disks are falling, (2) the
relative advection of the debris disks by the horizontal wind field containing both a speed and a directional shear, and (3) the lateral eddy
diffusion, which expands the disks falling earthward,
The first two processes are simulated by calculating the transport
of the disk centroids by the ambient horizontal wind field during the
disk’s earthward fall until the disk centroid is on the ground surface
by using either the idealized wind hodograph or the arbitrary hodograph for the horizontal wind specification. The result of this calcula-
tion is the position vector from surface zero to the predicted ground
position of each disk centroid and the time of arrival at the ground
surface of each disk. The lateral eddy-diffusion process is modeled by
the expression for R,(t) which estimates the debris-disk radius at the
time of arrival of the disk centroid on the ground surface.