D.R. Miller, T.E. Stoughton Agricultural and Forest Meteorology 100 2000 49–58 51
Fig. 1. Location map of the experimental site. Table 1
Hardware configuration of the lidar during the spray runs Wavelength
m
m 1.064
Energy per pulse mJ 125
Repetition rate Hz 50
Pulse width ns 15
Pulse to pulse stability ±
3 Detector type
Avalanche photodiode High quantum efficiency
40 Useful area mm
2
7
erational parameters and scan sequences used during the spray runs. Fig. 1 is a map of the site showing the
locations of the spray swaths, lidar, and micrometeo- rology tower.
3. Results
After each spray swath, most of the material sprayed was contained in droplets larger than 100
m
which
Table 2 Lidar scan sequences and operational parameters during the spray runs
Run Scan type
Elevation range
◦
Azimuth range
◦
Step size
◦
Number of scans 1
Horizontal –
35 0.5
3 Vertical
9 –
0.5 48
2, 3, 4 Horizontal
– 35
0.5 5
Vertical 9
– 0.5
80
deposited on the foliage and ground below where cov- erage was complete. In each case, however, plumes of
spray droplets containing an unquantified amount of material, remained suspended in the air and dispersed
into the atmosphere. The period of time that it took to disperse was primarily dependent on the stability of
the atmosphere. The mean wind controlled the direc- tion and speed of plume translocation away from the
target site.
Four swaths are shown, in Figs. 2, 3, 4 and 5, of nine available in the three day experiment. The four
represent the wide variation in stability and wind con- ditions encountered when aerial spraying is commonly
conducted from stable, calm conditions to unstable, windy conditions. The remaining 5 swaths were within
the range of atmospheric conditions represented by the four examples. Table 3 lists the atmospheric con-
ditions during the run periods. Data are presented for each spray swath as sequential two-dimensional maps
showing contours of equal lidar backscatter in a sin- gle vertical slice through the plume. The sequences
start 1 min after the spray plane passed which was enough time for all of the large drops to fall out onto
the vegetation surfaces below and immediately down- wind. Therefore, only the smaller suspended droplets
remained in the air. The aerosol clouds shown in the lidar scans could not be seen by eye. Corresponding
descriptions of the plume movement and dispersion are presented in the paragraphs below listed by swath
number and corresponding figure.
3.1. Swath 1 Fig. 2 This swath was made during an unstable period
with the wind blowing into the forest edge. The path of the airplane was outside the edge and the residual
suspended spray is shown being blown into the for- est. At 1 min after the pass, the plume had already
moved over the edge and spread out horizontally about
52 D.R. Miller, T.E. Stoughton Agricultural and Forest Meteorology 100 2000 49–58
Fig. 2. Vertical cross plume scans through the Run 1 plume showing the evolution of the plume at a single location. Each contour map shows the x, z spatial distribution of backscatter intensity indicating relative droplet density. The plume is shown at 1 min intervals after
the single pass of the spray plane.
350 m. It had dispersed vertically to about 170 m above ground. During the second minute after spraying the
plume continued to move at about 4 m s
− 1
and contin- ued to break up and dissipate. After three minutes the
plume had dispersed enough that it could no longer be detected with the lidar.
3.2. Swath 2 Fig. 3 This swath was made under unstable conditions and
wind speeds similar to swath 1, but in the afternoon rather than the early morning. In this case the ver-
tical spread of the plume is most striking with the plume spreading vertically over 175 m after 2 min and
the highest concentrations are elevated well above the release height. The plume remained about 100 m in
width, throughout. The plume did not break up and dissipate as fast as those during the earlier higher wind
conditions. The organized plume could still be de- tected after 3 min but not after 4 min.
3.3. Swath 3 Fig. 4 This swath was sprayed at dusk when the at-
mospheric surface layer was in transition from
D.R. Miller, T.E. Stoughton Agricultural and Forest Meteorology 100 2000 49–58 53
Fig. 3. As Fig. 2 for Run 2.
calmunstable to calmstable conditions. The air- craft sprayed the swath outside the forest edge
and the plume did not move. In fact, it remained near the ground and spread slowly. Therefore,
it was easily detected a full 5 min after spray- ing. There was no plume rise due to the stable
conditions. 3.4. Swath 4 Fig. 5
This early morning swath was made in stable, calm conditions. The plane sprayed the swath over the
canopy and the plume drifted out of the edge at a speed of about 0.4 m s
− 1
. Over the 5 min shown, the suspended plume spread slowly from 50 m to over
54 D.R. Miller, T.E. Stoughton Agricultural and Forest Meteorology 100 2000 49–58
Fig. 4. As Fig. 2 for Run 3.
100 m in width, extended vertically to 120 m above the ground and moved several hundred meters downwind.
The decay of lidar backscatter from the plume with time in nine swaths, two during stable atmospheric
conditions and seven during unstable conditions, is shown in Fig. 6 by graphing the maximum backscatter
intensity in the plumes at 1 min intervals after spray- ing each swath. The maximum backscatter generally
occurred at or near the center of the plume.
4. Discussion
