RADAR WINDSHEAR DETECTION
During both takeoff and landing, microbursts have been the cause of numerous transport aircraft accidents.
WINDSHEAR/MICROBURST DESCRIPTION
A microburst is a cool shaft of air, like a cylinder, between 1/2 and 1 1/2 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 knots.
Two types of microbursts exist; wet and dry. A wet microburst is characterized by the rain droplets within the air shaft falling all the way to the earth’s surface largely intact. This type of event is typical of humid areas like the southeastern part of the United States. A dry microburst is characterized by virga, rain that exits from the cloud base, but substantially 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 air shaft’s wind characteristics are identical.
When the downward moving air flow is translated to a horizontal flow at the base of the air shaft, 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 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 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-4000 has the ability to detect the presence of microbursts up to 5 nm ahead of the aircraft when below 1800 feet AGL.
WINDSHEAR AVOIDANCE FLYING
The air shaft of a microburst creates problems for aircraft for two reasons. The first problem is due to the downward air movement. Since the aircraft is flying within the air mass, as the air mass plummets earthward, so does the aircraft. Second, the lift that is generated by the wing is related to the relative velocity of air traveling over the wing. If the air velocity suddenly changes, so does the lift. When the lift is reduced, the aircraft descends. As an aircraft enters a microburst, depending on the point of entry, it will experience at least one of these conditions and most probably both.
The key to surviving a microburst is to enter it at a high enough aircraft energy state (high altitude and fast airspeed). The RDR-4000 system provides a warning prior to encountering the windshear, significantly improving the chances of surviving the encounter.
MICROBURST ENCOUNTER EXAMPLE
SECTION 6: RDR-4000 TECHNICAL OPERATION
3D VOLUMETRIC MEMORY SCANNING/PROCESSING
The RDR-4000 collects a complete 3D volumetric scan of all the weather and terrain ahead of the aircraft. The RDR-4000 contains an internal worldwide terrain database, enabling it to extract ground clutter without the significant losses associated with signal-based ground clutter suppression techniques. The data in the memory is continuously updated and compensated for aircraft movement.
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本文链接地址：WX RADAR RDR4000-PILOT GUIDE 气象雷达 RDR4000飞行员指南(14)
