|WHAT HAPPENS DURING DUCTING?
METEOROLOGIST JEFF HABY
Ducting is exceptional super-refraction. Super-refraction occurs when the trajectory of a radar beam bends
towards the earth's surface more than normal. In other words, the rate the elevation of the radar beam changes
with distance away from the radar is less than normal. The radar beam will tend to increase in height above the
earth's surface when moving away from the radar site because of the earth's curvature. In a super-refraction
situation, the radar beam could be increasing at a lesser rate with height than normal as the beam moves away
from the radar site or the beam could even be bending back down and getting closer to the earth's surface in
spite of the earth's curvature. It is ducting when the radar beam actually bends closer the earth's surface
with distance away from the radar. The bending could be strong enough for the radar beam to bounce off the
Ducting is caused by strong low level
inversions (temperature increases with height). Ducting can also occur
strong cap (EML) of warm and dry air exists in the lower troposphere above very moist air. Ducting
causes the radar to be able to sample much further distances than normal. Ducting increases ground clutter
also since the radar beam remains closer to the earth's surface for a greater distance and can even bend
into the earth's surface. Ducting is more common in the morning hours since this time of the day experiences
the strongest low-level inversions (due to cooling of earth's surface through longwave radiation emission)
but ducting can also occur anytime a strong cap exists in the lower troposphere.
The advantages of ducting are increased radar range, being able to sample storms further from radar, and
being able to sample lower elevations within storms further from radar. The disadvantages of ducting are
increased ground clutter and increased anomalous propagation (due to radar beams bouncing energy back from
hitting earth's surface or sampling storms beyond the radar's maximum unambiguous range).