WMOD00455

• Contract/Permit #: 14-06-D-6842
• Title: Research and Development Technique for Estimating Airflow and Diffussion Parameters Related to the Atmospheric Water Resources Program
• Date: 9/1/1969
• Country: United States
• Weather Modification - Doc Type: Report

II. LABORATORY SIMILITUDE AND DISPERSION MODELS

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Transport and Dispersion Over Irregular Terrain
The problem of atmospheric transport and dispersion of fine
particulate material (e.g., silver iodide) and gaseous material is most
easily approached by considering the factors that influences the trans-
port and spread of the material from the source. These factors are 1)
source characteristics, 2) depletion mechanisms, 3) atmospheric motions,
and 4) lower boundary conditions.
The source characteristics consider the strength, height, efflux
velocity and temperature, and dissemination time of the source. These
characteristics as well as the source location can be varied in an
effort to obtain specific operational objectives. The depletion
mechanisms enhance the fall in concentration of suspended matter with
downwind distance.
Atmospheric motions can be separated into two components, turbu-
lence and mean motion. An examination of the turbulent diffusion
equation indicates the manner which these two motions enter into the
transport mechanism. For material with negligible fall velocity, the
time averaged diffusion equation may be written as follows:

ac ac a2c a(u!c')
U. k 1 2-1
-+ =
at 1 ax. m ax? ax.
1 1
~ 1
The various terms may be interpreted as follows:

- ac
u -
i ax.
1

convective transport by mean flow

k a2c
m ax?
1

molecular diffusion

a(u~c')
. 1
ax.
1

turbulent diffusion.

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In the field, molecular diffusion is neglected since it is small in
comparison to the turbulent diffusion term on the time scale considered.
Turbulence governs the diffusion of airborne material and is a function
of atmospheric stability, directional and speed wind-shear, and surface
roughness. The large-scale mean motions govern the direction in which
the diffusing cloud of airborne material will be transported. Motions
of the convective (advective) transport scale are governed by general
synoptic flow-patterns, mesoscale circulations, and the nature of the
terrain.
The lower boundary conditions, especially if they are irregular,
may act to enhance the effects of depletion of a plume and increase the
effects of turbulence and mean motion. Irregular terrain is difficult
to generalize but nevertheless three types can be recognized: 1) block-
ing ridge or mountain range, 2) valley channeling, and 3) isolated
mountain. The only avenue for the oncoming airflow approaching a

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