The Wave Propagation Lab. participated with the U.S. Environmental Protection Agency in a series of experiments with the goal of developing and validating dispersion models that perform substantially better that models currently available. The lidar systems deployed and the data processing procedures used in these experiments are briefly described. Highlights are presented of conclusions drawn thus far from the lidar data

Eberhard, W. L.

Mcnice, G. T.

Troxel, S. W.

English

NASA Technical Reports Server

N87-10321
THREE ATMOSPHERIC DISPERSION EXPERIMENTS
INVOLVING OIL FOG PLUMES MEASURED BY LIDAR
W.L. Eberhard, G.T. McNice, and S.W. Troxel
National Oceanic and Atmospheric Administration
Wave Propagation Laboratory
Boulder, Colorado, U.S.A.
Introduction
The Wave Propagation Laboratory (WPL) participated with
the U.S. Environmental Protection Agency (EPA) in a series of
experiments with the goal of developing and validating dis-
persion models that perform substantially better than models
currently available. We briefly describe the lidar system
deployed and the data processing procedures used in these
experiments. Highlights are presented of conclusions drawn
thus far from the lidar data.
Equipment and Processinq
The lidar was a substantially upgraded version of the
one operated at the Cinder Cone Butte experiment (Eberhard
and McNice, 1982; Eberhard, 1983). The laser for the experi-
ments described here was a frequency-doubled Nd:YAG laser
with a pulse rate of i0 s-I. The data acquisition system was
modernized to handle the higher pulse rate and to monitor
system operation more fully. Automatic scan control by com-
puter was implemented; operators can modify the scan instruc-
tions without interrupting data acquisition.
Computer processing programs to refine the plume data
were also substantially improved over those used for the
Cinder Cone Butte experiment. It was necessary to compensate
the data for the attenuation by the plume and surrounding
ambient air, to subtract out the backscatter from the ambient
air, and to edit spurious signals such as those from the
ground or vegetation. Earlier we performed these tasks
through interactive graphics on a pulse-by-pulse basis; for
these three experiments the interaction was with an entire
scan, which was much more efficient. Other software inter-
polated the tracer distributions from lidar coordinates to
rectangular grids that are more conducive to dispersion
analysis. Concentration profiles and other plume parameters
were calculated for both individual and averaged scans. The
processing computer archived the results on digital magnetic
tape, printed tables, and hardcopy graphs to make analysis as
convenient as possible.
The tracer plume consisted of a polydispersion of oil
drops with a mode diameter of several micrometers. The lidar
performed a repeating sequence of vertical scans to obtain
cross sections of the plume at typically five distances
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The detailed derivations will be presented and discussed.
References
Cahen C. (1984): Lidar Measurements of Atmospheric Pollution. Procee-
dings of the 12th International Lidar Conference (ILRC),
Aix en Provence, France, p. 225.
Cohen, A., Kleiman M., (1978): On the Measurement of Atmospheric Aerosol
Concentration by Lidar. J. Appl. Meteor. 17, 234-235.
157
downwind of release. Tracer concentrations were inferred
from the elastic backscatter. We found that the backscatter
coefficient of the oil fog was not fully conserved with
distance downwind. We attribute the decline mainly to
partial evaporation of the drops.
Hoqback Ridqe Experiment
The 1982 plume diffusion experiment at The Hogback, a
ridge near Farmington, New Mexico, was the second field study
in the Complex Terrain Model Development (CTMD) project of
the EPA. The main objective was to study impaction of el-
evated plumes on a ridge during stable conditions. This
experiment was similar to the one at Cinder Cone Butte (Eber-
hard and McNice, 1982), where the topographical "target" was
a hill. Oil fog at The Hogback was released from a 150-m
meteorological tower or a crane upwind of the ridge. The
lidar scanned in vertical planes almost parallel with the
crest of the ridge.
Important information from the lidar includes the small
but significant amount of plume rise that was caused by the
heat from the oil fogger. The lidar data reveal whether the
released plume passed over the ridge, impacted its side, or
was embedded in the blocked region. The vertical deflection
of the plume relative to the surface is an important factor
in modeling surface concentrations. Strimaitis et al. (1985)
were able to substantiate the importance of the plume's
position with respect to the height of the dividing stream-
line. Flow above the dividing streamline has sufficient
kinetic energy to overcome the density gradient and pass
above the crest of the hill or ridge. Flow below the
dividing streamline must pass around the side of a hill or
suffer blocking in front of long ridge. Strimaitis et al.
found that the degree of stratification above the dividing
streamline had a secondary but significant effect on the
plume's standoff distance above the crest of the ridge, but
accounting for the amount of wind shear in the same zone did
not improve the prediction.
Processed data from this experiment are archived with
EPA, and a technical report on the lidar data is under prep-
aration. This data set includes 3079 individual and 304
averaged cross sections that span 84 hour-long analysis
periods.
TracM Power Plant Experiment
A 1984 experiment at Tracy Power Plant was the cul-
minating field study in the CTMD project. The plant, which
is located beside the Truckee River east of Reno, Nevada, is
surrounded by hills and mountains. Drainage winds were ex-
pected frequently to carry a plume from the stack toward
higher terrain near a bend in the river. The stack exhaust
159
was seeded with oil fog because the plume from the gas-fired
plant is essentially free of particulates. Lidar scans were
designed to monitor plume rise, to measure the plume's
behavior and growth as it approached terrain, and to observe
the initial encounter of the plume with the terrain.
The algorithm for vertical plume growth that was de-
veloped from the Cinder Cone Butte data substantially over-
predicted the observed growth of the plant's plume. Since
the plume at Cinder Cone Butte was within or near the top of
the surface-based mixed layer, whereas the plant's plume was
usually embedded in flow that was completely decoupled from
the surface, the algorithm may not be valid in the latter
setting. Work will continue on this and other problems that
the lidar data are expected to help solve.
The lidar data from this experiment that is archived at
EPA contains 4507 individual and 433 averaged cross sections
that span 84 experimental hours. A technical report on the
lidar data is in preparation.
CONDORS
The CONDORS (CONvective Dispersion Observed with Remote
Sensors) experiment took place during 1982 and 1983 at the
Boulder Atmospheric Observatory (BAO). Eberhard et al.
(1985) described the experiment and presented some prelim-
inary results. The goal was to determine the validity of the
tank model predictions of Willis and Deardorff (e.g., 1981).
Under highly convective conditions, when bouyancy dominates
shear as a source of turbulence, they found that the vertical
profile of a tracer deviated markedly from the form predicted
by a Gaussian model. For instance, their locus of maximum
concentrations from a nonbouyant, elevated release descended
to the surface instead of remaining level at source height.
The 300-m meteorological tower at the BAO was the plat-
form for elevated releases of oil fog, but several surface
releases were also performed. The lidar monitored the height
of the haze to infer the depth of the mixed layer, which is a
critical scaling parameter in convective conditions. The
shape of the vertical profiles of oil fog resembled the tank
results in a general way, but some of the differences ob-
served may prove to be significant. On the basis of an
empirical calibration of the lidar-oil tracer system, the
measured oil fog concentrations near the surface were in good
agreement with surface samples of a gaseous tracer that was
released from the same point as the oil fog. "Chaff" (alu-
minized filaments), which was tracked by a 3-cm-wavelength
Doppler radar, was another important tracer used during
CONDORS.
The tracer and meteorological data from CONDORS are
comprehensive and of high quality. A data report is near
completion, and analysis of results are in progress. They
160
will include 911 individual lidar scans that have been com-
bined into 81 averaged cross sections; these data span a
total of i0 experimental hours.
Summary
The WPL lidar was a key component of the Hogback, Tracy,
and CONDORS plume dispersion experiments. The results are
important for air pollution modeling because they address
situations in which high concentrations of a pollutant can
occur at the surface. We were pleased with the dependability
of the lidar, which performed satisfactorily for about 95% of
the scheduled operating time. The lidar acquired a volumi-
nous amount of data, from which about 2/3 was selected for
reduction. Processed lidar data have been archived and will
be made available on request. Some of the scientific results
are already available; additional interpretation of the lidar
data is in progress.
Acknowledqement
The U.S. Environmental Protection Agency provided a
major part of the funds for these projects. This abstract
has not been subjected to EPA review and does not necessarily
reflect the views or policies of EPA.
References
Eberhard, W. L., 1983: Eye-safe tracking of oil fog plumes by
UV lidar. ADD1. ODt., 22, 2282-2285.
Eberhard, W. L., and G. T. McNice, 1982: Plume dispersion
tracked by UV lidar. Eleventh International Laser Radar
Conference, NASA Conference Publication 2228, 145-148.
Eberhard, W. L., W. R. Moninger, T. Uttal, S. W. Troxel, J.
E. Gaynor, and G. A Briggs, 1985: Field measurements in three
dimensions of plume dispersion in the highly convective boun-
dary layer. Preprint Vol., Seventh Symposium on Turbulence
and Diffusion, Amer. Meteorol. Soc., Boston, 115-118.
Strimaitis, D. G., D. C. DiCristofaro, R. J. Yamartino, and
W. L. Eberhard, 1985: Modeling stratified flow over the crest
of a two-dimensional hill: Field evaluation of the "cut-off"
hill approach. Preprint Vol., Seventh Symposium on Tur-
bulence and Diffusion, Amer. Meteorol. Soc., Boston, 300-303.
Willis, G. E., and J. W. Deardorff, 1981: A laboratory study
of dispersion from a source in the middle of the convectively
mixed layer. Atmos. Environ., 15, 109-117.
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Three atmospheric dispersion experiments involving oil fog plumes measured by lidar

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