the speedometer, therefore, speeds of 0-49 mph are
unambiguous. Radar speeds that exceed the maximum
unambiguous velocity are said to be aliased, and
referred to as aliased velocities.
Velocity aliasing occurs when frequencies too high
to be analyzed with the given sampling interval appear
as a frequency less than the Nyquist frequency (the
highest frequency that can be determined in data that has
been sampled). In other words, wind speed greater than
the unambiguous velocity (Nyquist co-interval [the
entire range of detectable velocities]) for the current
pulse repetition frequency (PRF) are wrapped around
into the incorrect Nyquist co-interval. A sophisticated
velocity-dealiasing technique is implemented in the
WSR-88D (referenced in part D of the FMH-11),
However, it is expected that improperly dealiased data
will occasionally occur.
Recognition of Velocity-Aliased Data
Data that are incorrectly dealiased can be difficult
to ascertain. However, understanding the limitations of
the algorithm should help to recognize improperly
dealiased data.
If the suspected data is in an area isolated from other
data and there is a large variation in the subsynoptic or
mesoscale wind field, the algorithm may not be able to
initially assign the correct velocity. Other instances of
incorrect dealiasing may occur when there are shifts in
the inward- and outward-bound velocities along the
radials of data that do not fit those allowed by the
algorithm. In these cases, the actual values maybe off
by a factor of twice the unambiguous velocity of the PRF
in use at the time. Typical unambiguous velocities for
the WSR-88D, in the Precipitation mode, range from 40
to 60 kts. Occasionally, groupings of data appear along
a small set of ranges that could not be successfully
dealiased. These should be obvious.
Assessing Impacts of Velocity Aliasing
Incorrectly dealiased velocity data can seriously
impact certain WSR-88D algorithms and products.
Mean Radial Velocity products will be difficult to
analyze when contaminated with incorrectly dealiased
data. To determine the extent of the dealiasing problem,
it is recommended that earlier displays of these products
be examined to determine if there is time or space
continuity. In addition, other elevation angles of the
Mean Radial Velocity products may be used to
determine if there is vertical continuity.
Algorithms and products that ingest mean radial
velocity data can output incorrect results when such data
are used. In the case of the mesocyclone detection
algorithm, there will likely be a lack of vertical
continuity of incorrectly dealiased data. Consequently,
only uncorrelated shears should result from using
aliased data. In the rare event of a tornadic vortex
signature being output in the vicinity of an identified
mesocyclone because of vertical continuity of
incorrectly dealiased data, other products should be
examined to verify the existence of a severe
thunderstorm.
It is expected that incorrectly dealiased data will not
have a large impact on Combined Shear products
because of the amount of averaging of data done by the
algorithm.
RANGE-FOLDED DATA
Second trip echoes (range folding) occurs when the
radar hears a previous pulse, while listening for the most
recent pulse.
Due to the sensitivity and narrow beam width of the
WSR-88D, precipitation echoes beyond 250 nmi will
occasional y appear in closer range due to range folding.
However, far more significant are the range ambiguities
in the doppler velocity and width fields caused by the
WSR-88D’S pulsed doppler sampling interval
(referenced in part B of the FMH-11). For any pulsed
doppler system, the product of the unambiguous range
and the doppler Nyquist interval is a constant function
of the wavelength of the radar and the speed of light.
Decreasing the PRF allows for a longer listening time,
thus increasing the unambiguous range, but, this lower
PRF creates a problem in determining radial velocities.
THE DOPPLER DILEMMA
High PRFs are required for high velocity
measurements and low PRFs are required for long
ranges. The solution to this dilemma lies in finding a
balance between the effects of velocity aliasing and
range folding. This dilemma is caused by physical
restrictions based on the laws of nature. To solve this
dilemma, the WSR-88D will use several methods to
work around these restrictions. One method is to
operate at variable PRFs; the second is to collect
refractivity information at low PRFs and velocity
information at high PRFs. The two sets of information
collected are compared, then processed to estimate true
radial velocities and ranges.
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