the speedometer, therefore, speeds of 0-49 mph areunambiguous. Radar speeds that exceed the maximumunambiguous velocity are said to be aliased, andreferred to as aliased velocities.Velocity aliasing occurs when frequencies too highto be analyzed with the given sampling interval appearas a frequency less than the Nyquist frequency (thehighest frequency that can be determined in data that hasbeen sampled). In other words, wind speed greater thanthe unambiguous velocity (Nyquist co-interval [theentire range of detectable velocities]) for the currentpulse repetition frequency (PRF) are wrapped aroundinto the incorrect Nyquist co-interval. A sophisticatedvelocity-dealiasing technique is implemented in theWSR-88D (referenced in part D of the FMH-11),However, it is expected that improperly dealiased datawill occasionally occur.Recognition of Velocity-Aliased DataData that are incorrectly dealiased can be difficultto ascertain. However, understanding the limitations ofthe algorithm should help to recognize improperlydealiased data.If the suspected data is in an area isolated from otherdata and there is a large variation in the subsynoptic ormesoscale wind field, the algorithm may not be able toinitially assign the correct velocity. Other instances ofincorrect dealiasing may occur when there are shifts inthe inward- and outward-bound velocities along theradials of data that do not fit those allowed by thealgorithm. In these cases, the actual values maybe offby a factor of twice the unambiguous velocity of the PRFin use at the time. Typical unambiguous velocities forthe WSR-88D, in the Precipitation mode, range from 40to 60 kts. Occasionally, groupings of data appear alonga small set of ranges that could not be successfullydealiased. These should be obvious.Assessing Impacts of Velocity AliasingIncorrectly dealiased velocity data can seriouslyimpact certain WSR-88D algorithms and products.Mean Radial Velocity products will be difficult toanalyze when contaminated with incorrectly dealiaseddata. To determine the extent of the dealiasing problem,it is recommended that earlier displays of these productsbe examined to determine if there is time or spacecontinuity. In addition, other elevation angles of theMean Radial Velocity products may be used todetermine if there is vertical continuity.Algorithms and products that ingest mean radialvelocity data can output incorrect results when such dataare used. In the case of the mesocyclone detectionalgorithm, there will likely be a lack of verticalcontinuity of incorrectly dealiased data. Consequently,only uncorrelated shears should result from usingaliased data. In the rare event of a tornadic vortexsignature being output in the vicinity of an identifiedmesocyclone because of vertical continuity ofincorrectly dealiased data, other products should beexamined to verify the existence of a severethunderstorm.It is expected that incorrectly dealiased data will nothave a large impact on Combined Shear productsbecause of the amount of averaging of data done by thealgorithm.RANGE-FOLDED DATASecond trip echoes (range folding) occurs when theradar hears a previous pulse, while listening for the mostrecent pulse.Due to the sensitivity and narrow beam width of theWSR-88D, precipitation echoes beyond 250 nmi willoccasional y appear in closer range due to range folding.However, far more significant are the range ambiguitiesin the doppler velocity and width fields caused by theWSR-88D’S pulsed doppler sampling interval(referenced in part B of the FMH-11). For any pulseddoppler system, the product of the unambiguous rangeand the doppler Nyquist interval is a constant functionof 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 lowerPRF creates a problem in determining radial velocities.THE DOPPLER DILEMMAHigh PRFs are required for high velocitymeasurements and low PRFs are required for longranges. The solution to this dilemma lies in finding abalance between the effects of velocity aliasing andrange folding. This dilemma is caused by physicalrestrictions based on the laws of nature. To solve thisdilemma, the WSR-88D will use several methods towork around these restrictions. One method is tooperate at variable PRFs; the second is to collectrefractivity information at low PRFs and velocityinformation at high PRFs. The two sets of informationcollected are compared, then processed to estimate trueradial velocities and ranges.12-5

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