|RANGE FOLDING AND|
DETECTING RANGE FOLDED ECHOES
METEOROLOGIST JEFF HABY
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
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.