Radar Windshear Detection; Windshear/Microburst Description; Windshear/Microburst Detection Process - Honeywell IntuVue RDR-4000 Pilot's Manual

IntuVue RDR-4000 Weather Radar Pilot's Guide

RADAR WINDSHEAR DETECTION

During both takeoff and landing, microbursts have been the result of
numerous transport 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 its edges.
The downward air velocities associated with these narrow air shafts
range from 20 to 40 kts.
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 kts.

WINDSHEAR/MICROBURST DETECTION PROCESS

When the airshaft of a microburst encounters the ground, it mushrooms
outward carrying with it a large number of raindrops. By measuring the
horizontal velocity of these water droplets the RDR-4000 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 the 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 proceeds to assess the severity of the event by
measuring how fast the droplets are moving. If the assessment of the
severity of the microburst exceeds a preset threshold value, a
windshear alert is issued on the radar display and through the flight
deck audio system.
The RDR-4000 has the ability to detect the presence of microbursts up
to 5 nm ahead of the aircraft when below 1,800 ft AGL.
Principles of Weather Radar Use 060-4492-000
50 Rev 7, February 2015