United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 2771 1
Research and Development
EPA-600/S3-84-111 Jan 1985
Project Summary
An Evaluation of Wind
Measurements by
Four Doppler Sodars
J C. Kaimal, J.E. Gaynor, P L. Fmkelstem, M.E. Graves, and T.J. Lockhart
Measurements by four Doppler sodars
of wind speed, wind direction, and the
vertical component of turbulence were
compared to similar measurements
made on the 300-m instrumented
tower at the National Oceanic and
Atmospheric Administration's Boulder
Atmospheric Observatory. The sodars
were manufactured and were operated
during the test by Aerovironment Inc.,
Radian Inc., Remtech et Cie (formerly
Berlin), and Xontech Inc. Sodar measure-
ments were compared to measurements
made by fast response sonic anemo-
meters at 100, 200, and 300 m. The
comparison ran continuously for 21 d in
September, 1982. Results of the
experiment indicated that all sodars
measured wind speed and direction
quite accurately and with reasonably
high precision. Comparison of the
measurements of the vertical compo-
nent of turbulence indicated that the
sodars tended to overestimate the
standard deviation of vertical velocity
at night and to underestimate it during
strongly convective situations. The
precision of measurement for the
vertical component of turbulence was
considerably poorer than for average
wind speed and direction. Comparison
also indicated some differences among
the various types of sodars. Analysis of
the spectra measured by the sodars
indicated that measurement inaccurac-
ies may have been due to a combination
of sampling volume and aliasing prob-
lems.
This Project Summary was developed
by EPA's A tmospheric Sciences Research
Laboratory. Research Triangle Park.
NC, to announce key findings of the
research project that is fully documented
in a separate report of the same title (see
Project Report ordering information at
back),
Introduction
During the first three weeks of Septem-
ber, 1982, an experiment was conducted
at the Boulder Atmospheric Observatory
(BAO), under the sponsorship of the
Environmental Protection Agency (EPA),
to assess the ability of tn situ and remote
sensors to measure the mean and
turbulent properties of the lower atmo-
sphere The experiment was conducted in
response to the need for comparative
data from which scientists could evaluate
the accuracy, field precision, and general
performance of some of the more
commonly used meteorological instru-
ments that measure atmospheric turbu-
lence
Recent advances in the modeling of
transport and diffusion of pollutants,
achieved largely through theoretical
insights gained from field experiments,
point to the site-specific nature of
turbulence Attention is, therefore, being
directed to better on-site characterization
of turbulence and to the development of
techniques for measuring the mean and
turbulent wind variables needed for input
into the models This experiment was
designed to provide information needed
to formulate a monitoring strategy for the
development of site-specific dispersion
meteorology
The BAO was chosen as the site for the
experiment because of the availability of
precise profile and turbulence data from
sensors on a 300-m tower Facilities for
launching rawmsondes and for processing
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the data received from them were added
benefits Four commercially-available
Doppler sodars with the capability to
measure variances in vertical wind
component in addition to the mean three-
dimensional wind field were used The
question addressed here is whether the
sodars can measure the mean and
turbulent properties of the flow at heights
100 to 300 m above the ground
Description of Instrumentation
Four Dopplersodar manufacturers who
currently market their products m the
United States were invited by EPA to
participate m the experiment Under
arrangements made through EPA's
principal contractor, Meteorology Research
Inc , the sodars were installed and
operated by personnel from the participat-
mg firms The four systems differ
significantly in their physical configuration
and approach to signal processing Two of
the systems used three-axis monostatic
arrays, one used a bistatic array and the
other used a colocated monostatic/bistatic
array The following four systems were
deployed
AeroVironment Three-Axis
Monostatic System (A V)
The system consisted of three acoustic
transceivers mounted on a trailer
Doppler shifts m the backscattered
signals received on each axis were
interpreted as wind components m the
radial directions Wind components thus
measured were transformed into compon-
ents along the N-S, E-W, and vertical
directions Because m this configuration
sampling volumes are separated by large
distances, the assumption of horizontal
homogeneity m the mean wind field is
essential to justify the use of wind
components measured along the different
axes for the coordinate transformation
The AV system transmitted a sound
pulse (1 50-200 W) at a frequency of 1500
Hz (duration 0-18 s) sequentially from
each of three adjacent pencil-beam
antennas One tilts south 30° from the
vertical to be sensitive to the N-S
component, one tilts west 30° from
vertical to be sensitive to the E-W
component, and one points straight up to
be sensitive to the vertical component
The receiver echo is heterodyned and
then passed through an electronic comb
filter with 31 teeth to yield the full
spectral distribution m the return signal
For each 33 3-m altitude range gate, the
spectrum is examined according to
several criteria to obtain a best estimate
of Doppler shift, along with an estimated
reliability factor The pulse repetition
interval was 8 s
Remtech Three-Axis
Monostatic System (REM)
The REM system (developed originally
at Bertm et Cie) also uses a trailer-
mounted array of three transceivers
They are operated m sequence as
monostatic systems, the same assumption
of horizontal homogeneity is invoked for
wind measurements In this system, the
horizontal wind sensing antennas are
tilted 18° from the vertical in the same
directions as are the AV's, the transmitted
pulse is a 1600-Hz signal of 0 08-s
duration The received signals are
digitized after appropriate bandpass
filtering, and the Doppler frequency shift
is extracted using Fast FounerTransforms
(FFT) techniques The pulse repetition
interval was 5 s
Radian Corporation Colocated
Monostatic/Bistatic System
(RAD)
The RAD's antenna configuration
permitted both monostatic and bistatic
operation Both systems shared the
central, vertically pointing, pencil-beam
transceiver The two tilted (18° from
vertical) monostatic transceivers were
not located close to the vertical transceiver
as on the AV and REM systems, but
were aimed to intersect at a height of
150-m In the bistatic mode, two fan-
beam transmitters (located 250 m to the
south and to the west) illuminated the
vertical beam of the central transceiver
The movement of the sound pulse up the
vertical beam was followed by time gating
of the receiver signal. Doppler frequency
shifts in each gated segment were
converted to wind velocity components to
produce a wind profile
In both configurations, the vertical
transceiver was operated in the monostatic
mode to measure the vertical wind
component Because the three monostatic
beams were not divergent, the assumption
of homogeneity is not as critical here The
RAD system transmitted 120-W pulses at
20 kHz (0 1-s duration) and computed
Doppler shifts using the Complex Covanance
method RAD operated in three modes
monostatic, bistatic, and multistatic(alter-
natmg one series of monostatic and one
series of bistatic pulses) Pulse repetition
interval for all systems was 5 s
Xontech Three-Axis Bistatic
System (XON)
This bistatic system consisted of a
vertical pencil-beam transceiver and two
fan-beam receivers aimed at a central
vertical common volume Therefore, the
geometry m this system was exactly the
reverse of the RAD bistatic system The
transmitted frequency was 2 OkHz(0 08-
and 016-s duration under computer
control) Its bistatic baseline was 350-m
long The fan-beam antennas received
signals scattered from the vertical
transceiver beam, so the winds were
computed along a vertical column above
the transceiver as in the RAD bistatic
system A microcomputer determined the
Doppler frequency shift with an FFT
detection scheme powerful enough to
sense small frequency variations in the
presence of high ambient noise levels
The wind data are, therefore, presented
without qualifiers, but if the program
could detect a consistent signal for the
entire averaging period, no data were
printed for the height range The pulse
repetition interval was 5 s
BAO Instrumented Tower
The 300-m tower at the BAO is
instrumented at 8 levels 10, 22, 50, 100,
150, 200, 250, and 300 m Sonic
anemometers installed at each of these
levels measured the three-dimensional
wind field R M Young propeller-vane
anemometers are mounted on the side of
the tower opposite from the sonic
anemometers to serve as back up wind
sensors when the tower is shadowing the
sonic anemometers For this experiment,
the sonic anemometers were mounted
on the booms pointing SSW and the
propeller-vane anemometers were
mounted on booms pointing NNE. These
booms also supported sensors for meas-
uring mean and fluctuating air tempera-
tures and the dewpoint temperature. Data
from the sonic anemometers and other
fast-response sensors were sampled ten
times per second, while the propeller-
vane anemometers, like other slow-
response sensors, were sampled only
once per second
Data from the BAO are recorded in one
of two modes In the "regular" mode, only
the 10-s averaged data points and 10-s
grab samples (last point m a 10-s data
block) of the time series are retained In
the "raw data" mode, all data points are
recorded In both modes, the software
computes and lists once every 20 mm the
means, variances, and fluxes for the
preceedmg 20-min period These listings
become the common reference for
comparing the performance of the
different sodars. The raw data mode is
employed only when the full time series is
needed for special analyses or for the
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details in the structure of the flow Such
recordings were made for three relatively
brief periods during this experiment for
the purpose of comparing the spectral
response of the sodars and examining the
limitations imposed by sampling volumes
and sampling rates
Procedures
The sodar measurements were centered
over an area 05x03 km, about 0 65 km
southwest of the BAO tower The bistatic
arrays were laid out to provide a height
range of at least 300 m, hence the
requirement for such a large test area
The electronic equipment associated
with the Doppler systems were housed in
trailers located within the visitors area A
larger trailer in the same area served as
the control center for the experiment
The terrain in the vicinity of the tower,
is reasonably flat Except for the trailers
and the fence surrounding the visitors
area, the site is free of small-scale
surface obstructions
Procedures for data collecting and
reporting were established to insure
against unfair bias for any of the
participants. All systems were assumed
to be capable of unattended continuous
operation All systems provided data in
the form of wind speeds, wind directions,
vertical wind components, and standard
deviations of vertical wind speed averaged
over 20-min periods coincident with the
BAO averaging periods The three compari-
son levels were 100, 200, and 300 m The
data collected over the previous 24-h
period were submitted to EPA personnel
directing the experiment every morning
at 0800-h MST in exchange for tower
data covering the same period
Concurrent operation of some of the
sodars was considered at one time, but
quickly ruled out because of cross-
contamination, even between systems
operating at different frequencies The
sodars were, therefore, operated m
sequence, with the switchover from one
system to another controlled by a central
timer switch The assigned observing
interval was normally one 20-mm period
each cycle The experiment covered the
period from September 1 to 21, 1982
The AV, REM, and RAD computed the
standard deviation of w (the vertical wind
component) from their time series
Missing data points were not filled in by
interpolation, but the number of points
missed (or accepted) was displayed The
REM used 4-pomt block averages instead
of the original time series. The XON
computed its standard deviation from the
width of its 2-mm w spectra, estimated
for each level Successive 2-mm standard
deviations were averaged to obtain the 20-
min values Each spectrum was automati-
cally examined for level and shape of
background noise and steps were taken
to remove their effects
No attempt is made m this report to
present the results of our standard
deviation of wind direction comparisons
The azimuth direction standard deviations
showed very large scatter The data are
withheld pending a better understanding
of the reasons for the scatter Meanwhile,
we can only suggest caution in using
standard deviation of wind direction for
diffusion predictions
The AV, REM, and XON maintained a
consistent operating pattern throughout
the experiment However, RAD changed
its operating mode every 24 h, switching
from multistage to bistatic and monostatic,
then back to multistatic, and so on
Results
Measurement of the Standard
Deviation of Vertical Wind
Velocity
Because the sonic calculations of the
standard deviation of vertical wind speed
provide reference values, the accuracy
and precision of each sodar system can
be determined from the collection of 20-
mm average differences The two input
variables for these computations are the
standard deviation of vertical wind
velocity calculated from sodar measure-
ments and the standard deviation of the
sonic vertical wind velocity The compara-
tive statistics used to estimate accuracy
and precision then become the average
difference (sample bias) and the standard
deviation of the differences In addition,
the root mean square difference, or com-
parability, is computed, this statistic
characterizes the repeatability of a
system Finally, the precision is also
represented as a percentage of the
average value, a coefficient of variation
Values for these statistics are presented
in Table 1 for the combined sodar
observations at each of three heights, as
well as for the individual vendor data
subsets The sample bias showed a large
range of values around nearly constant
composite values of 0 08 At 100 m the
spread was greatest, with the REM
having the only negative sample bias
value (i e , sodar < sonic) and the XON
having a sizable 0 23 m/s sample bias
The AV was well below the composite
sample bias value, the RAD was above it
At 200 m the REM value was slightly
negative, the AV remained small, the
XON was the same as the composite
value, but the RAD was in excess of 0 2
m/s At 300 m the RAD sample bias
continued to be relatively large, but the
other vendors were grouped between
003 and 007 m/s
From Table 1 it is clear that there is
much scatter m the comparability and
percentage of the average value about
the true value in all systems There was
no statistical difference between the
standard deviation of the differences and
Table 1. Standard Deviation of Sodar Vertical Wind Speed Compared to the Standard Deviation
of Sonic Vertical Wind Speed
s3 s4 AT
jVn/s; (%)
0 22
0 16
0 21
0 17
0 24
0 26
020
0 32
0 19
0 24
026
0 25
030
0 22
0 18
'b = sample bias (sodar measurement-sonic measurement) Estimates accuracy
2c - comparability
3s - standard deviation of differences Estimates precision
4s' - s expressed as a percentage of the average value of the sonic standard deviation
5N = number of observations
Height
(m)
WO
200
300
Vendor
Composite
AV
Ft AD
REM
XON
Composite
AV
RAD
REM
XON
Composite
AV
RAD
REM
XON
(m/s)
008
001
0 12
-005
0 23
008
003
022
-000
008
009
004
023
007
004
(m/s)
0 24
0 16
0 25
0 18
034
0 27
020
039
0 19
0 25
0 27
025
038
0 23
0 19
50
35
47
38
53
54
43
65
39
51
54
53
62
47
38
678
190
178
139
171
576
167
144
119
146
665
214
158
136
157
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the percentage of the average value for
the AV and the REM at any of the levels
Measurement of Wind Speed
To examine the accuracy and precision
of the sodars, simultaneous observations
of wind speed were recorded from sonic
anemometers at the same three heights
on a tower about 600 m from the sodar
systems The sonic systems had a
sampling rate of 10 Hz, and they were
regarded as the reference instruments m
the evaluation However, due to a wind
shadow zone created by the tower,
extending +40° from north for the sonic
instruments, reference data m this sector
were obtained by the propeller vane at the
BAO tower A comparison of sonic and
propeller wind speed measurements on
the tower showed that the instruments
were approximately equivalent
Values of sample bias, comparability,
standard deviation for the differences
between sodar and reference values are
presented m Table 2 for combined sodar
observations at each height as well asfor
the sodar record of each vendor Propeller
wind speeds were excluded when the wind
speed was less than 1 m/s
The estimates of sample bias m Table 2
show mostly negative values at 100 m,
with a composite value near -04 m/s
Because the difference was sodar minus
reference, this means that the sodar
systems tended to register too low An
exception was the RAD, which did not
have a significant sample bias at 100 m
At 200 m, the vendors all recorded too
high At 300 m the RAD and XON again
recorded too high, whereas, the AV and
REM were unbiased Sample biases were
also computed for day (0600-1800 h) and
night (1800-0600 h) values Most differences
between day and night were insignificant
The comparability of sodar wind speeds
with reference values is also given m
Table 2 Precision is represented by
standard deviation and percentage
deviations The percentage deviation
values ranged from about 15% to 35%
around composite values near 25%
Measurement of Wind
Direction
An investigation of the propeller-vane
data indicated that they could not be
substituted as reference values There
were unexplained disparities between
the propeller and sonic data that are still
under investigation Because sonic data
showed more consistent behavior and we
believe them to be more reliable, only the
sonic wind direction measurements are
used as reference data
Values of sample bias, comparability,
and standard deviation for the differences
between sodar and sonic reference
values are presented m Table 3 for
combined sodar observations at each
height as well as for the sodar record for
each vendor
The values of bias in Table 3 show
negative values at 100 m and 200 m for
all vendors, but positive values at 300 m
for all vendors except for RAD However,
most of these values were not significantly
different from zero, considering the
Table 2 Sodar Wind Speed Compared With Reference Wind Speed
Height
(m)
100
200
300
Vendor
Composite
AV
RAD
REM
XON
Composite
AV
RAD
REM
XON
Composite
AV
RAD
REM
XON
b'
(m/s)
-042
050
002
-012
-1 04
0 14
005
031
0 12
009
0 16
-0 10
029
002
0 44
c?
(m/s)
1 28
1 03
1 18
062
1 88
098
072
1 00
0 73
071
1 24
1 15
1 17
0 74
1 20
s3
(m/s)
1 21
090
1 18
060
1 56
096
0 72
1 47
072
0 70
1 23
1 15
1 69
0 74
1 12
s4
(%)
28
21
28
14
37
23
17
35
17
17
27
25
37
17
25
/V-"
1179
327
315
236
301
1019
298
258
194
269
1005
328
198
183
296
variability of the wind direction and the
number of cases included
The comparability of sodar wind
direction with sonic reference values is
also given in Table 3 Considering the
scatter in data, it can be assumed thatthe
vendors' measurements of wind directions
were equivalent to each other
The Project Report also gives informa-
tion on the sodar performance above 300
m It also compares vertical velocity
spectra determined from the sodar
measurements and from the sonic
anemometer measurements
Conclusions and
Recommendations
The wind measurements analyzed in
this report represent the state-of-the-art
in Doppler sodar wind sensing Consider-
ing the requirements that the sodar
operation be unattended, except for
maintenance and repair, and that the
data be subjected to no editing by the
vendors, the results obtained were
reassuringly good The scatter inthe wind
speed and direction data compared very
well with scatter in past experiments
when some of the same sodar systems
were compared under more controlled
conditions In these data, some vendors
showed more scatter than others, but
much of that could be attributed to factors
such as ram, high winds, cable damage,
and transducer failures If data from
these suspected periods are eliminated,
the scatter for the different systems
would appear more similar
The measurement of standard deviation
of vertical wind velocity with sodars
seems to show promise, at least for
daytime conditions Here too, vendor
performance showed variations that
could be attributed in some cases to
weather and equipment failure, but
individual differences in processing the
data might also have been a factor here
The predictable behavior of sodar vertical
velocity spectra m the convective boun-
dary layer leads one to believe that verti-
cal velocity variance measurements can
be made to within 10% of heights above
100 m However, the nighttime results
are not so encouraging More work is
needed to ascertain the reasons for the
large discrepancies in the vertical velocity
measurements at night
'b = sample bias (sodar measurement-sonic measurement) Estimates accuracy
2c - comparability
3s = standard deviation of differences Estimates precis/on
4s' = s expressed as a percentage of the average value of the sonic standard deviation
5/V = number of observations
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Table 3. Sodar Wind Direction Compared With Sonic Wind Direction
Height
fm)
WO
200
300
Vendor
Composite
AV
RAD
FtEM
XON
Composite
AV
RAD
REM
XON
Composite
AV
RAD
REM
XON
b'
(deg)
-441
-387
-676
-203
-449
-343
-079
-786
-389
-1 85
075
026
-3 25
062
405
c2
(deg)
2859
26 70
2706
1851
3785
2322
1947
25 67
2467
2380
29 59
2856
2998
19 70
3596
s?
(deg)
2825
2641
26 20
1840
37 58
2297
19 45
2443
2436
23 73
2958
28 56
29 80
19 69
3573
N"
667
187
177
137
166
523
155
128
no
130
697
227
131
142
197
'b = Sample bias (sodar measurement-sonic measurement) Estimates accuracy
2c = comparability
3s = standard deviation of differences Estimates precision
"N - number of observations
J. C. Kaimal and J. E. Gay nor are with the National Oceanic and Atmospheric
Administration's (NOAA) Wave Propagation Laboratory, Boulder, CO 80303,
the EPA author P. L. Finkelstein (also the EPA Project Officer, see below) is on
assignment to the Atmospheric Sciences Research Laboratory, Research
Triangle Park, NC 27711 from NOAA; M. E. Graves is with Northrop Services,
Inc., Research Triangle Park, NC 27709, and T. J. Lockhart (formerly with
Meteorological Research, Inc.) is with Meteorological Standards Institute, Fox
Island, WA 98333.
The complete report, entitled "An Evaluation of Wind Measurements by Four
Doppler Sodars," (Order No PB 85-115 301, Cost $13 00, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
. S. GOVERNMENT PRINTING OFFICE-1985/559-111/10780
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