Comparisons of radar-measured winds have been made with several types of measurements not only to verify radar data but also to seek a satisfactory comparison method. Three of the comparisons that have been made with Colorado Profiler radars are summarized. Radar measurements were compared with radiosonde measurements. Infrared lidar and 915 MHz radar were compared with radiosondes. A brief radar/radar comparison was made using the 50-MHz radar and a 3-cm wavelength meteorological Doppler radar during precipitation

Strauch, R. C.

English

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N87-10471 / .'Y 273
o
4.1.3 METHODS FOR VERIFYING THE ACCURACY OF WIND PROFILES
R. G. Strauch
Boulder, CO 80303 %2
Most of the verification tests of the accuracy of winds measured by UHF or
VHF radars have been made by comparing the radar data with radiosonde-measured
winds. The results usually "show general agreement" and differences are
attributed to the "lack of temporal and spatial simultaneity." There have not
been extensive or routine comparisons of radar-measured winds with radiosondes
or other wind measurements. One of the reasons that routine radlosonde/radar
comparisons have not been made is that the agreement that is obtained depends
on the particular wind field that is being observed. This is also true for
comparisons with other wind measurements such as those obtained by lasers
because the instruments are measuring the winds in fundamentally different
ways. For example, the Doppler radar and Doppler lidar do not measure the mean
radial velocity of the aemosphere in the resolution volume, but rather measure
the reflectivity-weighted average of the radial velocity. This is ignored when
the measured radial velocities are converted to horizontal winds. The
reflectivity weighting depends on the distribution of refractive turbulence for
the clear-air radar and depends on the distribution of aerosols for the lidar.
Also, the resolution volumes are usually very different. Another problem in
making comparisons is the lack of a standard or truth. Winds measured by
radiosonde are often regarded as the true wind; however, in tests where two
radiosonde packages were flown from the same balloon, the vector winds measured
by the two trackers had rms differences of 3.1 m/s (HO_NE, 1980). Winds
measured by radar or lidar will have much better precision. Therefore,
comparisons of radar-measured winds have been made with several types of
measurements not only to verify radar data but also to seek a satisfactory
comparison method.
Three of the comparisons that have been made with Colorado Profiler radars
are summarized below:
Radar/Radiosonde
The 915-MHz radar wind profiler at Denver's Stapleton Airport is located
adjacent to the radiosonde launch site of the National Weather Service. Hourly
averaged winds measured by the radar can be compared with the twice-daily
radiosonde winds. Figure 1 illustrates a comparison that "shows general
agreement." Note that in this case the 915-MHz radar did not have enough
sensitivity to measure the winds in the core of the jet and that above the jet
the radiosonde track was lost. If random cases are examined, poor agreement
can also result. However, if the time-helght cross section of radar-measured
winds shows spatial and temporal changes such as illustrated in Figure 2, poor
agreement should be expected since the instruments would be observing very
different wind fields. Data from radar and radiosondes are available for two
years, and these comparisons are continuing.
Radar/Lidar/Radiosonde
In March 1984, a series of comparisons were made with an infrared lidar
(10.6 micrometer wavelength), the 915-MHz radar, and radiosondes. The lidar
can measure winds with fixed telescope pointing, the same as the radar, or it
can do 360-degree VAD (velocity-azimuth-display) scans. Figure 3 illustrates
radar/lidar comparisons. Note that the data are plotted as radial velocities
at 75-degree elevation angle. Radar and lidar data are from 10-minute
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averages. The results of these comparisons (LAWR_qCE et al., 1985) show rms
differemces of about 3 m/s for horizontal wind comparisons for both lidar/
radiosonde and lidar/radar. The radar and lidar data sometimes show changes
in horizontal wind component speed of several meters per second between 10-rain
averaging periods.
Radar/Radar
A brief radar/radar comparison of wind measurements was made at
Platteville (STRAUCH et al., 1983) using the 50-M_z radar and a 3-cm wavelength
meteorological Doppler radar during precipitation. This comparison showed
"good agreement. " Since January 1985, there have been two profilers colocated
at Platteville; a 50-MHz and a 405-MHz system. Both operate in the same modes
and measure hourly-averaged winds. In one comparison, shown in Figure 4, the
rms difference of horizontal wind components measured at 6.8 km MSL was about I
m/s for a 5-day period. In general, the comparisons "show good agreement", but
the data from the 405-MHz radar are much noisier than from all other profilers.
This is believed to be caused by antenna sidelobes, and more of these compari-
sons will be made after the antenna performance is improved.
There are other comparisons that could be made to attempt to quantify the
accuracy of radar wind profiles. Some of these are:
Radar/Aircraft
Routine comparisons of radar-measured winds and winds measured by
commercial wide-body jet aircraft equipped with the Aircraft Communications and
Reporting System (ACARS) could be made. These aircraft have inertial
navigation systems and can derive wind data along the flight track when the
276
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Figure 3. Lidar and 915-MHz profiler radial velocity
profiles for 1700-1730 MST on April 12, 1984.
Upper: east, 75 ° elevation radial; Lower: north,
75 ° elevation radial. Also plotted are lidar
VAD data (75 ° elevation).
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Time (hrs)
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Platteville VHF HOR High Winds 6.79 km--MSL
10-.Jan to 15-Jan-85
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Correlation of winds from 405 mHz and 50 mHz __"
Both at Platteville
Regression Coeff. = 1.018162 ///
Y-Intercept = -0.121 m/s ,_'/
Correlation Coef. = 0.991
RMS Diff. fromRegr, Line = 1.071 _"//,
Total # Points= 118.0 ¥,_/;,,.
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405 mHz (UHF) (6.88 km-MSL)
North Comoonent (m/s)
10-Jan to 15-Jan 1985
Figure 4. Comparison of north wind component measured with
colocated 405-MHz and 50-MHz profilers. Left: 5-day
time series of measured values. Right: Scatter plot
of measured values.
277
35
track is a straight line at constant altitude. These data have not been used
for profiler comparisons; they are a potential source of data at altitudes
where wind speed is usually high. The aircraft-measured winds may be accurate
enough to calibrate the profiler or check the pointing angles of fixed-beam
profiles. Research aircraft could also be used for comparison but systematic
comparisons would be too costly.
Radar/Meteorological Inference
Wind data from a network of radar wind proffers can be used to calculate
divergence and vorticity; a case study was performed by ZAMORA and SHAPIRO
(1984), and these calculations were "in agreement" with divergence and
vorticity calculated from the radiosonde network. A comparison of winds or
quantities derived from wind data obtained by a network with radiosonde data
may yield a better accuracy determination than comparisons of individual
profiles because the differences in temporal and spatial averaging will be less
important when averaging over large areas. Data from a radar network could
also be tested for accuracy by initializing models with current radar-measured
winds and comparing model predictions with later data. This type of comparison
should be tried in simple terrain (such as Eastern Colorado, where two 50-MHz
profilers are collecting data routinely) and in simple flow regimes.
Radar/Scannin_ Radar
A fully scannable clear-air radar measuring wind profiles with VAD
analysis at various elevation angles may be the best standard available for
radar wind data. It would be especially valuable for calibrating fixed beam
systems that use temporal integration rather than spatial integration to obtain
representativeness and whose actual pointing andes are difficult to measure.
278
The antenna pointing angles of a scanning syst_n can be checked using the sun
as a source. The scanning system would operate at UHF to keep antenna size
manageable and so there would be no pointing uncertainty due to aspect
sensitivity. The effects of vertical motion and divergence on radar-measured
winds could be assessed. Plans are being made to convert a relatively
low-power 405-MHz radar to a scanning system that should be able to measure
winds in the clear air to 7-10 km. It is hoped that such a radar would provide
data that would be accepted as "true" wind data because it would have temporal
resolution of about 5 rain and height resolutions of less than i00 m as well as
continuous scanning. In addition to calibration of other profilers, such a
system would be a mesoscale wind profiler with many potential applications as a
stand-alone (as opposed to part of a network) wind profiler.
REFER_q CES
Hoehne, W. E. (1980), Precision of National Weather Service upper air measure-
ments, NOAA Tech. Memo. NWS T&ED-16, 23 pp.
Lawrence, T. R., B. F. Weber, M. J. Post, R. M. Hardesty, R. A. Richter,
N. L. Abshire, and F. F. Hall (1985), A comparison of Doppler lidar,
rawinsonde, and 915 MHz UHF wind profiler measurements of tropospheric
winds, NOAA Tech. Memo., in preSs.
Strauch, R. G., M. T. Decker, and D. C. Hogg (1983), Automated profiling of
the troposphere, J. Aircraft, 20, 359-362.
Zamora, R. J., and M. A. Shapiro (1984), Diagnostic divergence and vorticity
calculations using a network of mesoscale wind profilers, Preprints 10th
Conf. Weather Forecasting, Clearwater Beach, FL, Am. Meteorol. Soc., 386-
391.


Methods for verifying the accuracy of wind profiles

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