JDOP demonstrated the meteorological utility of operational doppler radars and also benchmarked the engineering requirements for the NEXRAD. The NEXRAD program formally began with the establishment of the Joint System Program Office (JSPO) at NWS headquarters in 1979. Since this time, NEXRAD has been officially renamed the (WSR-88D). A detailed discussion of the WSR-88D PUP and its capabilities is beyond the scope of this text. For a detailed discussion of capabilities and procedures refer to the technical manual, Operation Instructions Principal User Processor (PUP) Group/ Doppler Meteorological Radar WSR-88D, NAV EM400-AF-OPI-010/WSR-88D. We will now discuss velocity-aliased data, followed by a discussion on its recognition.

Doppler radar uses the change in frequency between the outgoing signal and the returning signal (doppler shift) to determine radar velocities. However, limitations in velocity measurements do exist. We now will take a look at an example to see what can go wrong. Trains are designed to go as fast forward as in reverse. The speedometer shows both forward and reverse speed. Refer to figure 12-4 throughout this discussion. Direct reading of speeds between 0-49 mph, whether forward or reverse is quite easy. As with a trains speedometer, radar has limits to speeds that it can measure without error. These speeds that can be measured without error are known as Unambiguous Velocities.

In part (a), the train is traveling at 40 mph in a forward direction. The speedometer indicates the correct speed. In part (b), the speed of the train has increased by 20 mph, so that the train is now traveling at 60 mph, but, the speedometer indicates 40 mph in reverse. The maximum forward speed was exceeded on

Figure 12-4.-Speedometer.

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.

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.

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