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Low-Altitud~e Wind Shear
The Nature of Low-Altitude Wind Shear
Wind variability is a perennial and inescapable problem for
aviation. Meteorological circulations or terrain-induced airflows can
on occasion induce large and rapidly changing variations in air
velocity over small distances. These variations produce
correspondingly sudden changes in the relative flow of air over an
aircraft's wings and other lifting surfaces, with attendant changes in
an aircraft's flight path. Thus, small-scale wind variations and
turbulence can pose hazards to aviation, particularly when they occur
in the lowest few hundred feet of the atmosphere, the zone that
aircraft must penetrate while landing or taking off. To eliminate--or
at least appreciably reduce--the hazards posed by low-altitude wind
variability, it is necessary to understand the sources of wind changes
and the risks they represent. It is also necessary to know how to
detect, measure, and predict them and how to communicate useful
information on wind variability to air traffic controllers and pilots
in a timely fashion.
The wind changes not only with distance but also with time. As a
result, the term ~ is sometimes used when considering
low-altitude flight hazards. Technically speaking, wind shear is the
local variation, at a particular time, of wind velocity with
distance. It is measured by dividing the velocity difference at two
points by the distance between them. Strong wind variations over
horizontal distances of 1 to 10 miles can cause particular
difficulties for aircraft. Most often, in this report, we refer to
wind variations as wind shears. In circumstances where time
variations are important, they will be identified.
The three-dimensional airflow in the lower atmosphere, and the
associated wind shears and turbulence, vary from place to place by
season and by meteorological conditions. Based on experience, rough
estimates can be made of the degree of hazard, frequency of occurrence,
and difficulty of detection of various types of wind shear.
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Atmospheric turbulence is generally defined statistically in terms
of scale and intensity. Its effects are seen in an aircraft's ride
and handling qualities and are taken into account in aerodynamic,
flight control, and structural design criteria. However, patchy
small-scale turbulence need not be present at low altitudes withi
layers of air with strong wind shear (Lee and Beckwith, 1981~.
The most serious ef fects of wind shear are those that cause an
aircraf t to lose lift and altitude. This is particularly hazardous
when an aircraft is close to the ground, either landing or taking of f,
when an aircraft unexpectedly flies from a region of headwinds into a
region of strong tailwinds, and especially ~ f the transition occurs in
a s bong downdraf t . Turbulence and heavy rain, when occurr ing in
association with wind shear, can contribute to flight hazards and
increase the chances of an accident. The following paragraphs
describe the types of wind-shear situations and the risks each poses
to aviat ion.
Convective Outflows
Thunderstorms and other convective clouds are critically important
sources of low-altitude wind variabil ity. Many produce strong
downdrafts that transport air downward, which then spreads out rapidly
over the ground. The size and strength of the downdraft depend on the
properties of the thunderstorm and on the humidity and temperature
structure of the atmosphere. As shown in Figure 1, thunderstorms
occur most frequently in Florida, along the Gulf of Mexico coast, and
over the central parts of the United States. Whenever there is a
thunderstorm or precipitating convective clouds, hazardous low-altitude
wind shear can be present. Some experts believe, however, that strong
downdrafts and associated flight hazards are more likely when the
thunderstorm cloud bases are high and the surface humidities are low.
Microbursts. Following the crash of Eastern Airlines Flight 66 at
New York City's Kennedy Airport on June 24, 1975, the term downburst
came into use to describe a strong downdraft that induces an outburst
of damaging winds on or near the ground. Subsequently, studies of the
EAL crash and of Continental Flight 426 at Denver on August 7, 1975,
and Allegheny Flight 121 at Philadelphia on June 23, 1976, concluded
that each of these accidents was related to downburst-induced wind
shear (Fu jita and Byers, 1977; Fu jita and Caracena, 1977) .
_
Results of the above studies indicated that the downbursts that
contributed to these accidents were of small size and short life, and
the term microbursts has been used to describe them. Microbursts are
small downbursts, less than 2.5 miles in outflow size, with the peak
winds lasting only 2 to 5 minutes. Some microbursts reach the ground,
while others dissipate in mid-air and are not detected by ground-based
anemometers. ~ critical point is that microbursts can come from
convective clouds that are not accompanied by lightning.
20
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The most recent wind-shear-related accident, the crash of Pan
American Airlines Flight 759 in New Orleans, was reported by the NTSB
(1983) to be a result of microburst-induced wind shear.
Climatology of Microbursts. Data on which to base averages of the
frequency and intensity of microbursts are very limited. During a
43-day period in May and June 1978, ground-based anemometers
associated with NCAR's portable automated mesonet (PAM) were deployed
as part of the NIMROD Project near Chicago. PAM automatically
measures wind speed and direction, dry- and wet-bulb temperatures, and
pressure and rainfall at 27 surface stations (wind measurement is at
12 feet above the ground). Fifty microbursts were detected, 32 with
rain (wet microbursts) and 18 without rain (dry microbursts) (see
Table 3~. Wind shear is apparently not related to rainfall intensity.
TABLE 3 Frequency of Microbursts Detected by PAM
NIMROD JAWS
-
Number of operational days 43 86
Number of microburst days 11 49
Number of wet microbursts* 32 31
Number of dry microbursts** 18 155
Total number of microbursts 50 186
Number per operational day
Number per microburst day
1.2 2.2
4.6 3.8
* Equal to or greater than O.O1 inches of rain
during the period of peak winds.
** Less than 0.01 inches of rain between both the
onset of high winds and the end of the
microburst winds including the calm period if
any.
The PAM deployed in the Joint Airport Weather Studies (JAWS)
Project near Denver detected 186 microbursts on 49 days of an 86-day
observation period from May to August 1982 (Fujita and Wakimoto,
1983~. This amounts to 2.2 per day, nearly twice the rate found in
the NIMROD Project. Of the total 186 microbursts, 155 were dry and
only 31 were wet. Most were not associated with active cumulonimbus
clouds but rather occurred under streaks of evaporating precipitation
(virga) from dissipating cumulonimbus or dissipating cumulus congestus
clouds. Because of their association with convective clouds,
microbursts tend to occur most often between noon and midnight, but,
as shown in Figure 2, the diurnal pattern differed at the NIMROD and
JAWS locations.
22
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40
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IS6 Microbursts
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FIGURE 2 Diurnal Variation of Surface Microbursts of All
Intensities Measured at the 27 PAM Stations.
~ Source: Fuj its and Wakimoto, 1983 ~ .
Aircraft passing through the center of a microburst will
experience a change in wind velocity that can be specified as the
vector difference of the headwind and the tailwind along the flight
path. The maximum wind difference of the JAWS microbursts, measured
by ground-based anemometers, exceeded 95 knots on one occasion (see
Figure 3~.
23
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~636 Surface Microbursts
REDRY
WET at 27 JAWS PAM Stations
MAXIMUM WIND SPEED DIFFERENCE
FIGURE 3 Frequency of Surface Microburats as a Function of the Maximum
Wind-Speed Difference Measured at the 27 PAM Stations.
(Source: Fujita and Wakimoto, 1983~.
The JAWS Project used Doppler radar primarily to observe
microbursts. With three wind-measuring Doppler radar systems,
three-dimensional pictures of microbursts were obtained. Figure 4
shows the horizontal and vertical velocity profiles of a particularly
strong microburst. The classical profile of headwind, downdraft, and
tailwind are clearly seen in this figure.
Because the Doppler radars were used to concentrate on specific
cases of interest and because they were not operated continuously,
their data cannot be used to establish a microburst climatology.
Nevertheless, the Doppler radars detected and observed 75 microbursts
on 33 of the 86 operational days (McCarthy et al., 1983~.
The Doppler radar data allowed for an examination of the time
history of JAWS microbursts observed near Denver. Figure 5 shows the
percentage of microbursts that reached maximum velocity differential
as a function of time from the detection of the initial velocity
divergence near the ground. Approximately half of those observed
reached maximum intensity within 5 minutes, and nearly all reached
maximum velocity within 10 minutes. This figure dramatically
illustrates the extremely short duration of these events.
24
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HORIZONTAL—16-33 AGL ~_~ 40 Knots
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DISTANCE EAST OF CP-2 (miles}
(CP-2, S-Band Doppler Radar Site)
FIGURE 4 Velocity Fields With Respect to the Ground,Based on a
Dual Doppler Analysis for a Microburst Occurring at
1452 MDT on July 14, 1982. Contours Are Radar
Reflectivity Factors(dBze). (Source: Wilson and
Roberts, 1983~.
25
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TIME FROM INITIAL DIVERGENCE (min)
FIGURE 5 Percent of Microbursts Reaching Maximum
Velocity Differential as a Function of Time
from Initial Divergence.
(Source: Wilson and Roberts, 1983~.
Gust Fronts
A gust front is the leading edge of a mass of cool air that has
recently descended from a thunderstorm or convective cloud. There is
a large amount of literature on this subject, which was examined for
the Thunderstorm Pro ject (Byers and graham, 1949 ) and has since been
much studied at the NSSL and by many researchers (Brandes, 1977;
Charba, 1974; Goff, 1976, 1977; Sasaki and Baxter, 1982~. As shown in
Figure 6, the cool air near the gust front, which may be up to 1 mile
in depth, is characterized by strong turbulent winds. The cool air
sinks, while the warm air rises. The depth of the gust front, the
associated wind shear and turbulence, and its speed of advance over
the ground depend on the nature of the parent cloud and the wind
distribution through the layer in which the cloud is Embedded.
Distortion
X Z= ~ 2.4
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8 7 6 5 4 3 2
l V' '' 1'"~- }~ I
13 12 11 10 9
DISTANCE FROM GUST FRONT (miles)
Gust Front 1
1 0 1 2 3
6,000
5,000
4,000 LLl
3,000 IL
2,000
1,000
FIGURE 6 Squall Line Thunderstorm Outf] ow ~ schematic) . (Source: Goff ~ 1980)
26
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the winds are light and change little with height, the gust front is
almost symmetrical around the storm that produced it. By the time the
storm has dissipated, the gust front may have moved tens of miles away
from the parent storm and weakened substantially.
When there is strong vertical wind shear through the atmosphere
and a severe long-lasting convective storm, the associated gust front
tends to be maintained at the leading edge of the parent storm. The
pattern of its advance can be very asymmetric with strong
outward-blowing winds in those sectors coinciding in direction with
the strongest winds aloft in the cloud layer.
Gust fronts, as they sweep over the ground, can usually be
detected by radar, by a suf f ic lent ly dense network of ground-based
anemometers (such as LLWSAS ), and by microbarographs ~ Once detected ,
it is possible--largely through extrapolation--to predict their
position anywhere from a few minutes to perhaps one-half hour in
advance. Also, radar echoes often indicate the presence of deep
convective clouds and the possibility of a gust front.
Gravity and/or Solitary Waves
As a gust front moves away from its parent s form, the temperature
contrast across it is gradually reduced. The resulting circulation,
when it exists in a shallow surface layer capped by a temperature
inversion, can persist as a strong clear-air circulation that usually
moves at speeds of 15 to 40 knots over long distances. Known
variously as gravity waves or solitary waves, these motions are caused
not only by gust fronts but also by dowxlslope winds and by sea-breeze
fronts. Solitary waves occur most often during the night and early
morning. Over northern Australia, where they have been studied
extens ively, they occur throughout the year but are mos t frequent
during the late winter and early spring (August to November).
Corresponding climatic information for the United States is almost
totally lacking. In vertical cross section, a typical solitary wave
can have a horizontal dimension of several hundred feet to 6 miles
with a 15-knot updraft at the leading section and a downdraft of
similar strength on the trailing part. Because of the relatively
long-lasting nature of solitary waves, their distinctive wind shifts,
and associated pressure patterns at the ground, it should be possible
to detect, track, and predict their arrival over an airport, given a
surface-observing network of sufficient density.
Sea-Breeze Fronts
A sea breeze is a local wind that blows from sea to land. It is
caused by the temperature differences that occur daily between the sea
surface and the adjacent land. It usually occurs on relatively calm,
sunny summer days. Often, the onset of a sea breeze occurs suddenly
as a sea-breeze front, separating the cool air from the warm air,
27
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moves inland. Sea-breeze fronts cause a sudden change in wind
velocity, from near calm to a brisk cool breeze. At the onset, the
sea breeze flows across the coastline, but as time goes on it turns to
its right (in the northern hemisphere) and has a component along the
coastline.
Wind shear associated with sea breezes can prove to be a hazard at
airports that are located along coastlines, such as Logan
International Airport in Boston and the John F. Kennedy International
Airport in New York City. For example, the occurrence of thunderstorm
downdrafts and outflows when a sea breeze could have been expected
caused some confusion about the wind velocity over the approach to
runway 22 at Kennedy Airport when EAL Flight 66 crashed on
June 24, 1975.
In some areas, sea breezes occur regularly during the summer and
can be predicted with a fair degree of accuracy. Ground-based
anemometers and airborne detectors should be able to detect wind
shears, so that pilots can take appropriate action.
Air-Mass Fronts
Separate air masses do not mix readily when they come into contact
if they have different temperatures and humidities. Instead, the
colder, more dense air mass passes under the warmer, less-dense air
mass. The zone of transition between the two air masses is called a
front. When the cold air advances, forcing the warm air to retreat
and pass over the wedge of cold air, it is called a cold front. When
the warm air advances, the frontal boundary moves toward the cold air
and a warm front is said to exist.
All fronts have some degree of wind shear across the zone of
transition between the air masses, but the narrower the zone the
stronger the wind shear is likely to be. When the transition zone is
Perhaps 300 or more feet deep, the wind-velocity change with height is
gradual and turbulence is weak or nonexistent. Nevertheless, an
aircraft taking off or landing through such a zone experiences changes
in wind velocity.
There usually is a sharp change of wind velocity across fronts.
Figure 7 shows a cross section of a warm front just south of O'Hare
International Airport in Chicago. The data were recorded aboard an
instrumented Boeing 747 during its approach to O'Hare's runway 32L
(Sowa, 1974~. The effective tailwind-to-headwind change was 22 knots
in this case. Had this landing been attempted on runway 04R (oriented
80° clockwise), the wind variability would have been 59 knots. The
front had an extremely narrow transition zone and was approximately
300 feet thick. The change of wind velocity was abrupt and had
moderate, or more severe, turbulence associated with it.
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Representative terms from entire chapter:
doppler radar
~-
_Warm Front
oPoo
/0~/
But most microbursts are not symmetrical, because air descending
toward the earth's surface carries the velocity characteristics of
winds aloft. In such cases an off-airport radar might easily miss a
hazardous case or overestimate its actual threat. In these cases it
would be best to have airport radar with beams parallel or nearly
parallel to the runways.
Alternatively, it is important that the space over the airport be
carefully monitored, since precursors of microbursts on and near the
ground are most 1 ikely to be found through the examination of Doppler
signatures aloft. These can most effectively be monitored from an
of f-airport s ite.
Next Generation Radar (NEXRAD ~
The FAA, NOAA, and DoD are working together to develop an advanced
pulsed Doppler weather radar. Referred to as the next generation
radar, or NEXRAD, it will address the common need among the principal
government users of meteorological data for information on the current
location, severity, and movement of such weather phenomena as
tornadoes, severe thunderstorms, heavy precipitation, tropical
cyclones, hail, high wind shears, and severe turbulence. Each NEXRAD
installation will provide weather information for ranges exceeding 200
miles and heights of 60,000 feet at a rate of once every 3.5 minutes.
Because of the large number of meteorological phenomena, the size of
the coverage volume, and the magnitude of the resultant data rate,
NEXRAD, whether manned or unmanned, will rely heavily on automation to
perform the required observations and to communicate the results to
users.
Several factors make the NEXRAD system an unlikely choice to
measure low-altitude wind shear over airport terminals. One serious
consideration is the minimum scan update time of 3.5 minutes for the
NEXRAD network. Such a rate is too slow to optimally detect
microbursts. Radar siting criteria also makes NEXRAD's use unlikely
in terminal areas. NEXRAD will require an unobstructed view for 200
miles. In addition, the height of the NEXRAD antenna and tower will
force some NEXRAD systems to be located sufficiently far away from
some airports as to eliminate any possibility for low-altitude
coverage of the terminal area.
However, because NEXRAD can outperform and suppress ground clutter
better than the radars used in research to detect microbursts, a
stand-alone NEXRAD devoted strictly to an airport's terminal area
might be useful. However, this would be difficult to accomplish
because of frequency overcrowding. NEXRAD uses a 2700- to 2900-MHz
frequency band, which is used by many other radar systems in many
areas of the continental United States. NEXRAD's frequency will
require additional design or operational measures to be employed in
some areas. Adding approximately 100 NEXRAD systems for terminal use
woul d, there f ore, appear impractical.
40
A Terminal Radar for Wind-Shear Detection
The FAA has proposed that an unmanned Doppler radar be dedicated
to the effective detection of low-altitude wind shear and the
reduction of hazards to aviation. The committee concludes that the
concept of a terminal radar can be justified and that FAA should
aggressively support its development. But such a program must not
interfere with the development of NEXRAD. To effectively reduce
flight hazards over airports, radar observations are necessary over
and in the vicinity of airports. Information from a terminal radar
and other sources must be communicated to both pilots and air traf fic
controllers in a timely, concise, reliable, and easily understood
manner .
If a Doppler weather radar is to serve airport terminal
operations, it must detect and warn of the most serious aviation
hazards with a high reliability. An analysis of wind-shear-related
accidents shows that the turbulent airflows associated with convective
clouds and thunderstorms represent major hazards. The design of a
Doppler radar system must take into account the nature of microbursts,
specifically their small dimensions, short durations , and the fact
that the s tronges t winds may be at altitudes of a few hundred feet.
Furthermore, such a radar should also be able to detect the other
types of shears described earlier.
A terminal radar must be able to measure low-altitude wind shear.
It must be able to encompass a wide range of velocities and radar
reflectivities in both clear and cloudy air, to eliminate or greatly
reduce ground clutter, and to observe phenomena located very near the
ground. It will also need a resolution capability of approximately
500 feet to enable it to identify a downdraft and an antenna that
scans the airspace surrounding the terminal every 1 to 2 minutes.
Although the scan strategy depends on the location of the radar
relative to the airport, scans at approximately 6 to 8 elevation
angles are necessary to properly monitor the airspace.
Since each Doppler radar scan produces three fields of
data-ore fleet ivity, mean velocity, and spec trum variance--the average
time available to scan and analyze one field varies from 2.5 to 6.7
seconds. Microbursts, downbursts, and gust fronts can be reliably
detected in research programs us ing manual techniques and highly
skilled observers. There is considerable uncertainty, however, that
it would be possible in day-to-day operations to extract and
communicate wind-shear informal ion in a timely fashion us ing manual
detection techniques at the very high scan rates that appear to be
necess ary .
A high degree of automation is needed if a terminal radar is to be
effective given the very short lifetimes of some microbursts and the
even shorter time available for making decisions. At present,
41
however, there are no automated techniques for detecting lo~altitude
wind shear. For an unmanned Doppler weather radar to be viable, a
systematic s tudy must be made of algorithms needed to detect wind
shear for representative climatically different areas of the country.
System Design Considerations. Ground clutter presents a
significant problem to using data from radars used to observe
meteorological targets near the ground. All available measures will
be needed to mitigate clutter in an airport's vicinity. The
combination of a low sidelobe antenna design and electronic
cancellation should be able to reduce clutter by at least 40 dB
compared with existing research radars .
A microwave Doppler radar operating at a 10-centimeter wavelength
proposed for NEXRAD can ef fictively measure wind shear around an
airport. What is not known is whether a 10-centimeter wavelength is
best for Doppler radars used at airport terminals. A 10-centimeter
wavelength is needed for NEXRAD in order to "see through" severe
storms and heavy prec ip itat ion to measure re f lee t ivi ty at long
ranges. This is not necessary for terminal radars that need to detect
s ignals out to distances of only 20-30 mi les .
A narrow beam (approximately 1°) requires an antenna about 25
feet in diameter at a 10-centimeter wavelength. The antenna diameter
needed for a fixed beamwidth is directly proportional to wavelength.
There fore, a 1° beamwidth at 5- or 3-centimeter wave lengths can be
achieved with antenna diameters of 13 and 8 feet, respectively. A
small antenna costs less and is more easily sited on a busy airport
than a large one. On the other hand, a larger antenna and shorter
wavelength would provide a narrower beam and improved ground-clutter
red ect ion .
There is a strong possibility that the signal-to-ground clutter
ratio can be increased further by using shorter wavelengths. This is
true when the objects that scatter the radar beam are water droplets
or insects in the clear air. In this case the power backscattered by
small particles is inversely proportional to the fourth power of
wavelength, while the ground-clutter return is only weakly dependent
on wavelength.
In determining the most appropriate wavelength for detecting
low-altitude wind shear, it is necessary to consider inhomogeneities
in the atmosphere that may be caused by water vapor and temperature
variations, because observing dry microburs ts depends on detecting
such radar targets.
Range and Velocity Ambiguities. Because there is very little time
to make decisions, the processing and signaling scheme must eliminate
range and velocity ambiguities for airport radars. Many techniques
42
address this problem, but none will be described in detail here.
Shorter wavelengths tend to make this problem more severe; however,
the re latively short range of the radar somewhat counteracts this
effect. Therefore, range and velocity ambiguity considerations alone
should not rule out the use of 5- or 3-centimeter wavelengths.
Siting of Doppler Radar Antenna.
Ideally, a Doppler radar antenna
should be located such that it can measure wind velocities along the
takeof f and departure corridors, including curved approach paths that
will be permitted by the microwave landing system (MLS) in the near
future. For a variety of reasons, such a goal is virtually impossible
to achieve because major airports usually have a number of runways
with different orientations. Airport buildings complicate this
problem. The question is further complicated by the nature of wind
shear. Some experts have concluded that radar antenna should be
installed on airport grounds, while others think it would be best to
have it off the airport. Studies are needed at each airport where a
terminal weather radar is to be installed to ascertain the best
location f or the antenna.
Remote Sens ing by Means of FM-COO Radar
Some experts have suggested that a frequency-modulated,
continuous-wave (FM-OW) Doppler radar might be used instead of a
pulsed Doppler radar to detect low-altitude wind shear. FM-COO Doppler
radar has many of the characteristics of the pulsed Doppler radars
described in the preceding section. For the same average power,
antenna aperture, and bandwidth, both types have the same sensitivity
and resolut ion. The key di f ference between them is the t a pulsed
Doppler radar transmits high-power, low duty-cycle pulses, while an
FM-COO Doppler radar transmits a low-power, full duty-cycle signal with
linear frequency modulation. Because of this, an FM-COO Doppler radar
can transmit high average power over a broad signal bandwidth and
there fore achieve high sensitivity and resolution at short ranges .
The high resolution contributes to significant ground-clutter
rejection. Wind measurements have been made at ranges as short as
40 feet. An FM-COO clear-air radar might also locate wingtip vortices,
another type of low altitude wind variability that is potentially
hazardous to small aircraft. Research should be continued on the use
of FM-COO radar for wind shear and turbulence detection.
Longer-Wave leng th Profiling Radar
Vertically pointing Doppler radars can measure a vertical profile
of the wind above the radar throughout the troposphere when operating
at long wavelengths of 1-10 meters. The measurements can be made at a
rapid rate. These radars can measure winds above the earth's boundary
layer better than microwave systems because of the nature of clear-air
turbulence. On the other hand, a VHF or UHF band radar cannot
~3
effectively detect microburs ts because ache beam is not eas fly scanned
and the beamwidth will tend to be wide. With adequate clutter
cancellation, however, such a radar could be a superb tool for
measuring vertical shear of the horizontal wind in all parts of th
country and during al 1 seasons . The NWS is inves ligating the use of
VHF or UHF wind profilers to supplement its rawinsonde network. Such
systems, if equipped with ground-clutter cancellers designed to
measure the lowest 3,000 feet of the atmosphere, should yield useful
data on the vertical shear of the horizontal wind at or near
airports . It does not appear, however, that such systems are
necessary components of a low-altitude wind-shear detection system.
Doppl er Lidar
Lidar (light detection and ranging) is the optical equivalent of
microwave radar. A suitably designed pulsed Doppler lidar operating
at a wavelength of about 10 micrometers can measure winds in optically
clear air during al 1 seasons by detecting the backscattered power from
small atmospheric particles. The principal advantage of lidar is its
ability to make clear-air measurements that are virtually free from
ground-clutter contamination. A disadvantage is that the laser signal
can be absorbed by rain, clouds, and fog, which can strongly attenuate
the signal over ranges of 1 to 2 miles. Doppler lidar should be
investigated as a technique for detecting wind shear, especially in
the western United States, where many clear-air microbursts occur. In
the JAWS Project, an infrared Doppler lidar detected and measured some
microbursts more clearly than did the radar.
Acous tic Sounding
Doppler acoustic sounders can measure profiles of the horizontal
wind in the boundary layer. The beams cannot be steered easily,
however, and the signals can be severely contaminated by natural and
man-made audio noises Using such a system at airports Is, therefore,
most appropriate for detecting areawide wind shear, such as that
associated with low-level jet streams. Such sounders are already in
use at the Hong Kong and Calgary, Canada, airports .
Airborne Remote Sensing of Wind Shear
Microwave Doppl er Radar
An airborne Doppler radar would be an effective tool for detecting
and warning of hazardous wind shears if it had capabilities equivalent
to those available in ground-based systems. Unfortunately, because of
size, space, and power limitations, the Doppler radars now under
development for airborne commercial use appear to be insuf ficiently
sensitive and provide inadequate spatial resolution to address the
low-altitude wind-shear problem.
44
Some airborne Doppler radars used in research programs where space
is not a problem (e.g., that operated by NOAA on its Lockheed P-3
aircraft and NASA on its Skyvan aircraft) offer capabilities similar
to those of ground-based systems. It is beyond the scope of this
study to determine whether a suitable airborne system can be
developed. However, a thorough study should be conducted to determine
the feasibility of such a development. If the results are positive, a
prototype radar should be developed and tested. Such a prototype
might have to use a wavelength between 1.5 and 3 centimeters, a flat
plate antenna to achieve very low antenna sidelobes, a ground-clutter
canceller, and substantial transmitter power (probably between 25 and
100 kilowatts of peak power).
Doppl er L idar
The British Royal Aircraft Establishment and the Royal Signals and
Radar Es tab lishment have collaborated to develop an airborne
cant inuous-wave (COO) Doppler lidar that is f town on a smal 1 je t
aircraft (HS 125 ~ and was tested during the JAWS Pro ject. The system
is focused to measure the wind at a range of approximately 1000 feet
ahead of the aircraft. The system has been flown reliably for extended
periods of time, is rugged and lightweight, and requires minimal main-
tenance and calibrations. Pulsed lidar technology should be explored
to see if a practical system can be developed to sense wind shear an
order of magnitude farther ahead, say to 2 miles .
The CW system can measure velocities accurately but only in the
region o f focus . Combined with the true airspeed measurement on the
aircraft, the system can warn of impending shear about 4 seconds in
advance o f an encounter during approach or departure. These short CW
lidar warnings are not unambiguous because of the lack of spatially
contiguous data but are certainly of value i f for no other purpose
than to alert pilots when to abort landings . On takeof f, ~ warning
after the aircraft is airborne may be of little value. As with other
optical systems, airborne lidar is severely attenuated by clouds and
precipitation. Such systems deserve careful scrutiny for use by the
aviation industry but at the present time remain in the research
ca tegory .
Passive Infrared Radiometry
NASA has been conducting test flights with a passive IR
radiometer. Devices of this type detect the IR radiation emitted by
gases and particles and thus detect temperature differences in the
field o f view. Gus t fronts are accompanied by a decrease in
temperature, and according to NASA an IR radiometer can give as much
as a 70-second warning of the arrival of a gus t front. An IR
radiometer is inexpensive, small, lightweight, and easily installed
and flown on aircraft.
45
Unfortunately, there is little correlation between measured
temperature differences and the occurrence of wind shear caused by
microbursts. In some instances, temperature increases; in others, it
decreases; and in yet others, no temperature change occurs. Even for
gust fronts there is no s bong correlation between the degree of
temperature change and the intensity of the wi nd shear resulting from
it. The IR radar beam can also be absorbed in clouds, fog, and rain,
causing ambiguous interpretations. Thus, IR radiometry cannot be
recommended as a primary sensor for wind shear, but it deserves
add it tonal re s earch .
Interpretation and Communication to
Air Traffic Controllers and Pilots
The previous sections have described various types of wind shear
and a variety of sensing techniques be used to detect them. Equally
if not more important to the overall aviation system is the need to
successfully interpret and effectively communicate wind-shear data to
air traffic controllers and pilots. The detection of low-altitude
wind shear requires application of sophisticated technology. Unless
this technology produces a useful product, its ultimate value will
fall short of expectations. A useful product is one that is easily
interpreted without ambiguity, provides definitive and quantitative
information on hazards, and does not have a large number of false
alarms. Any detection system must meet these criteria or its
usefulness will be limited at best. An effective warning system has
several essential elements: (1) sensing, (2) computation, (3)
display, (4) interpretation, and (5) dissemination.
Microbursts impose particularly severe requirements on such
systems because of their short lifetimes and small size. All too
frequently, plans for detection and warning systems place too much
emphasis on basic sensors and too little on the other ingredients,
which are equally important. Indeed, if any of the links is missing,
the system will be ineffective. For example, a Doppler radar is a
sensor, an essential ingredient for detecting wind shears.
Computation is also essential and in a Doppler radar system would
provide the signal and data processor to compute the fields of
velocity and various other derived information. In our era of
high-speed integrated circuits and rapidly advancing technology in
computers, computation is the strongest link in this chain. Today' s
computational systems are fast, inexpensive, and accessible. The
principal ques Lion is what computations are necessary to accomplish
the desired result.
46
Display and interpretation do not necessarily occur in that
order. Automated sys tems perform the interpretation in machines and
then display the result. Interpretation by humans requires, first,
that the data fields be displayed in a fashion suitable for viewing.
Rapid and reliable interpretation is the most challenging task in a
microburst detection and warning system. Automation is highly
desirable because machines are fast, vigilant, and promise low
operational costs. Unfortunately, machines are far inferior to humans
in pattern recognition, which is crucial to an ef fective system.
Machines also do only what they are programmed to do.
An effective detection and warning system must make the best use
of machines and people. Machines mus t be used for rapid real-time
calculations and image interpretation. Display of hazardous
situations mus t be provided in relatively simple and unambiguous
formats for controllers and other users. Meteorologists in Center
Weather Service Units (CWSUs ~ should also be able to view the data
f ields from which automated interpretat ions have been made in order to
resolve uncertainties that surely will occur in real time. With such
a process, false alarms will be minimized while retaining a high
probability for accurate detection of hazardous conditions.
Dissemination is the last link. Dissemination of information in
various forms is obvious ly involved throughout the chain, but mos t
important are the links from controllers to pilots or from machines to
controllers and pilots. Techniques exist to transmit virtually
limitless data to users, but this capability alone is no more than a
tool. Effective dissemination will occur only if technologists,
meteorologists, controllers, and pilots work together to design the
final products that are to be disseminated. Air traffic controllers
and pilots are very busy and cannot be burdened with excess ive data.
Neither can they afford to work with data that are insuff icient for
them to take effective corrective action in real time. The system
must provide just enough and no more.
There also will be a good deal of training needed. Users must be
well schooled in what the phenomenon is, what the system is trying to
do, why it was designed as it was, and what its limitations are. It
is only through a s bong education program for all operations and
flight staff that a system will realize its full potential.
In summary, the system's developers must give serious attention to
the interpretation and communication of information needed by the
user. In fact, this link is as important and as difficult as the
development of basic sensors.
47
Wind- She ar Pred i c t ion
The discussion to this point has concentrated on the nature of
low-altitude wind shear and its detection. I t should be clear, given
the highly localized and transitory nature of most wind shears and the
speed with which they develop, that effective monitoring techniques
must play a significant role in providing for aircraft safety. It is
also important to develop the capability to forecast weather
conditions and systems that are conducive to generating wind shear.
Such forecas ts can in turn be used to develop a corresponding
hierarchy of wind-shear alerts. Forecasting strategies need to be
developed us ing techniques that identi By precursor condit ions to
microbursts, rather than the more rigorous techniques of numerical
weather prediction.
As noted earlier, some types of low-altitude wind shear (e.g.,
shears caused by fronts, low-level jet streams, and sea breezes) can
be accurately predicted hours in advance, but this is not the case
with other types of shear. The hazardous shears caused by the
downdrafts associated with convective clouds cannot yet be predicted
successfully by either numerical or operational techniques. However,
in the latter area, some researchers have made limited progress in
using characteristic signatures of radiosonde temperature and humidity
profiles to identify days with a high potential for the occurrence of
microbursts (e.~., Caracena et al., 1983~. Others have developed a
model of downburst formation that, if verified, could provide some
basis for shorter-term microburst predictions (Emanuel, 1981~.
The primary objective of operational prediction is to make
possible the warning of conditions conducive to generating microbursts
or severe downdrafts, modeled after the severe thunderstorm or tornado
watch currently produced by the NWS. Such a wind-shear watch would
provide controllers and pilots with an early indication that
conditions are prevalent for hazardous low-altitude wind shears.
Because such conditions are relatively rare, such a watch would be
unusual and would heighten awareness in the several hours before a
possible encounter. Finally, the watch concept could be implemented
by the NWS.
National Weather Service Interaction
with the Federal Aviation Administration
The NWS provides a wide variety of forecast services, including
those provided to the FAA and to aviation community. It has often
been stated that the ability to forecast any meteorological phenomenon
is directly related to the ability to observe it. Development of an
adequate capability to observe low-altitude wind shear still requires
a lot of work .
48
Thunderstorms present the greatest danger to aircraft. Along with
the usual hazards to aircraft, thunderstorms also produce low-altitude
wind surges or gust fronts that move out ahead of the storm.
Dowobursts and microbursts are closely associated with the gust fronts
but on a smaller temporal and spatial scale. Radar and satellite
pictures are used mainly to detect thunderstorms . Unfortunately, we
are not yet able to observe with any degree of consistency which
thunderstorm cells have significant low-altitude wind shear associated
with them.
Meteorologists can predict wind shear associated with cold and
warm fronts and wi th low-level jet streams with greater accuracy than
those induced by convective activity. However, the low frequency and
wide spacing of upper-air observations inhibits the meteorological
monitoring of the parameters that cause the events. The turbulence
portion of the Aviation Area Forecast indicates the likelihood of
low-altitude wind shear. The forecast is prepared three times each
day by the National Aviation Weather Advisory Unit in Kansas. In
addition, 52 Weather Service Forecast offices issue, at the same
frequency, 498 site-specific terminal forecasts, including the
likelihood of low-altitude wind shear. These aviation/weather notices
are transmitted to FAA and NWS of fices as well as to those in the
aviation community having the appropriate communications equipment.
Progress has been slow in the development of techniques for
forecasting low-altitude wind shear associated with convective clouds
and thunderstorms. Thunderstorms are assumed to have the potential
for Great ing local titude wind shear, but only on some days do
convective clouds cause hazardous wind shears . Recent research at the
NOAA's ERLs indicates some skill in forecasting low-altitude wind
shear caused by convective clouds.
Potential hazards can also be identified by certain precipitation
radar echoes or satellite cloud images known to be associated with
low-altitude wind shear. These short-duration (less than 2 hours)
features are useful in specifying areas where hazards exist, but they
do not yield the type of specific indications of low-altitude wind
shear needed to plan flights.
The National Severe Storms Forecast Center in Kansas City issues
daily forecasts of thunderstorms that may approach severe limits.
Severe-thunderstorm watches are then issued as necessary. However, as
recent investigations in the JAWS Project and analyses of the Pan
American Airlines crash in New Orleans have shown, accidents can occur
in storms that are not considered severe.
Dissemination of Weather Advisories
While disseminating forecasts of the more persistent nonconnective
wind shears can be handled adequately, it is the convective
low-altitude shears that pose the greatest danger to aircraft. An
estimated 90 percent of significant operational low-altitude wind
49
shears are connectively induced (NWS, 1982 ~ .
frequently dealt with after the fact.
These ins Lances are
While weather forecasts and
in-flight advisories admonish that forecas ts of thunderstorms imply
the existence of low-altitude wind shears, pilot reports (PIREPs ~ are
the most effective warnings of the actual existence of shear. In
fact, until more definitive airborne and remote-sensing systems are
instituted, PIREPs may be the single most important safety item in
identifying most hazards to aircraft operations. Thus, aircraft
encounters with severe low-altitude wind shear should be reported in a
timely and accurate manner. Current reporting procedures are
considering how much
Current
cumbersome and time-consuming, particularly
pilots have to do during takeoff or landing.
_
-
Compounding the problem, many PIREPs are not disseminated beyond
the air traf fic controller receiving them. There are several
weaknesses in the pilot reporting system that mus t be improved before
pilots can depend on being consistently advised by other pilots of the
existence of low-altitude wind variations.
The NWS and the FAA currently have several arrangements for
communicating and disseminating weather information within the
National Airspace System. These arrangements need to be examined and
improved. One shortcoming Is that most ~ntormat~on ~ s communicated
via telephone. The FAA is currently developing equipment that will
enable meteorologic ts at its CWSUs to transmit weather messages
automatically to towers and control facilities within any center 's
area of responsibility.
50
