The radar sends a pulse of energy out, and then waits for a return echo. The length of time that the radar "waits" is based upon the "range" of the radar to detect useful echoes. This is known as Pulse Repetition Time. The number of pulses in a unit time is known as the Pulse Repetition Frequency (PRF). The range of the current WSR-88D (NEXRAD) radars is about 230 km. The radars can "see" beyond that range, but the echo resolution is very poor beyond that radius.

So the radar "waits" for the length of time it would take for the return pulse to return (all at about the speed of light) before sending out another pulse. The trouble is, the first pulse will have traveled out beyond 230 km and could have encountered another scatterer (although poorly). That scatterer will return a pulse of energy to the radar that will arrive slightly after the radar has sent out a second pulse. The radar will interpret this return energy as an echo from a source at a very nearby range. This is a false echo and the process by which false echoes appear in this manner is called "range folding".

This is the way it would work. Suppose the maximum unambiguous range is 225 kilometers. If a storm is located at 250 kilometers from the radar, the radar will detect the storm as being 25 kilometers from the radar instead. Any energy returned to radar beyond 225 kilometers would be range folded. This occurs because the radar energy bounced off the distant storm is returning to the radar after the radar has already sent out another pulse and is "listening" for return echoes.

To some extent, range folded data can be "unfolded" by the software. In areas in which this cannot be done, the color designation will indicate that range folding is an issue (RF) and the echoes in that area should not be accepted.

For more elaborate reading on this topic, I like to suggest you to refer to:

http://tornado.sfsu.edu/Geos ciences/classes/m815/rangefolding.h tml

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Answered by Karthick AR , an ibibo Master, at 11:55 AM on October 31, 2008

A range folded echo is one that is detected beyond the maximum unambiguous range. These echoes will show up at a distance from the radar equal to R - Rmax, where R is the distance from the radar to the actual echo returns and Rmax is the maximum unambiguous range. For example if a storm is 300 miles from the radar and the maximum unambiguous range is 270 miles, the storm will be shown on radar at a range of only 30 miles. This is because the reflectivity echoes from this storm arrive after the radar has sent out another pulse. The radar assumes it gets reflectivity only from the pulse it has most recently sent.

The following information covers how a radar operator can distinguish between real reflectivity and range folder reflectivity:

1. Look outside to visually verify the precipitation

2. Range folded echoes are often long and thin. The range gates are skinnier closer to the radar. Thus, precipitation that is far from the radar will be compacted into skinnier bands when it is brought closer to the radar.

3. Range folded echoes generally have anomalous low cloud tops. This is because the radar beam generally increases in altitude further away from the radar. Thus when a storm top far from the radar is brought closer to the radar the height of that echo will decrease.

4. Range folded echoes generally do not have a high reflectivity. Since storms at the outer edge of the radar are sampled at a very high altitude, the reflectivity from this precipitation will generally be low. Thus, range folded echoes often show in the low reflectivity colors such as green near the radar.

5. Range folded echoes will change location when the Pulse Repetition Frequency (PRF) is changed. As the PRF is decreased, the range folded echoes will eventually go away.

6. Check other nearby radars to see if the reflectivity in question shows up on those radars also.

7. Use multiple tilt angles. Range folded echoes if they show on a low tilt angle may not show on a higher tilt angle.

Answered by Anju Singh , an ibibo Master, at 8:03 PM on October 30, 2008