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NEXRAD Aims to Stop Purple Haze

By Jesse Ferrell, Meteorologist/Community Director
3/06/2007, 9:34:41 AM

The NWS says in a PDF released last month that a new range-folding algorithm will soon be included in NEXRAD radar software that will significantly reduce range-folding in velocity data. Below is an example of the before and after shots. Impressive.


For non-radar-enthusiasts, PURPLE=BAD. What this breakthrough means is simply that we'll have much less missing wind data on radar maps in the future. When the radar sends out signals to detect winds, it waits to hear the signal come back. In some cases, it can't tell whether the answer back came from the original transmission or not. This data gets marked a purple color on NEXRAD displays, indicating that the radar can't determine which way the particle of precipitation is moving, or at what speed. Sometimes called "purple haze", the phenomenon can hide or obscure tornadic twisting of the winds.

They explain:

Addition of the SZ-2 (Sachidananda - Zrnic) Algorithm: Range ambiguity can occur when a returned signal may be associated with one of several pulses transmitted prior to the latest pulse. The SZ-2 Algorithm will provide a new range unfolding technique to reduce the effects of the range-velocity ambiguity that exists with Doppler weather radars and should result in fewer areas of range folded data, sometimes referred to as "purple haze." This algorithm will provide users a cleaner, more complete view of the Doppler data by correctly unfolding overlaid targets.

The American Meteorological Society defines range-folding, or range-aliasing, as follows.

A radar ordinarily computes range to targets by measuring the time interval between the transmission of a pulse and the receipt of the returned signal, assuming that the signal was associated with the pulse just transmitted. However, depending on the pulse repetition frequency, the returned signal may be associated with one of several pulses transmitted prior to the latest one.

The improvement is associated with "Build 9" of the software and will be distributed to NEXRADs across the nation during the latter half of 2007.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or


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