IntuVueRDR-7000 Weather Radar Pilot's Guide
D201911000094
Principles of Weather Radar Use
Rev 0, Feb 2020
55
RADAR WINDSHEAR DETECTION
During both takeoff and landing, microbursts have been the cause of
numerous aircraft accidents.
WINDSHEAR/MICROBURST DESCRIPTION
A microburst is a cool shaft of air, like a cylinder, between ½ and
1½ nm across that is moving downward. When it encounters the
ground, the air mass mushrooms in a horizontal direction curling
inward at the edges. The downward air velocities associated with
these narrow air shafts range from 20 to 40 knots.
Two types of microbursts exist; wet and dry. In a wet microburst, rain
droplets within the airshaft fall largely intact all the way to the earth’s
surface. This type of event is typical of humid areas like the southeast
United States. A dry microburst may contain virga, or rain that exits
from the cloud base, but mostly evaporates before reaching the
ground. Virga occurs in high-based rainstorms found in places like
the high plains and western United States. Regardless of whether the
microburst is wet or dry, the airshaft’s wind characteristics are
identical.
When the downward moving airflow becomes a horizontal flow at the
base of the airshaft, the outflow winds have front-to-back velocities
ranging from 20 to 80 knots.
WINDSHEAR/MICROBURST DETECTION PROCESS
When the airshaft of a microburst encounters the ground, it
mushrooms outward. By measuring the horizontal velocity of the
associated water droplets, the RDR-7000 is able to infer the
horizontal and vertical velocity of the winds carrying the raindrops.
The radar processor detects the Doppler frequency shift imparted
onto the reflected microwave pulses by a microburst. As the radar
scans across the windshear event, it will detect raindrops moving
toward it at one range and away from it at a slightly greater range.
The difference in range between the raindrops moving toward and
away is the width of the base of the microburst. After the radar
detects this condition, it then assesses the severity of the event by
measuring how fast the droplets are moving. If the assessment of the
severity of the micro-burst exceeds a preset threshold value, a
windshear alert is issued on the radar display and through the flight
deck audio system.
The RDR-7000 has the ability to detect the presence of microbursts
up to 5 nm ahead of the aircraft when below 1,800 feet AGL.
