Lesson 5: Windshear and Microbursts

Our first lecture notes come directly from Chapter 7 of the Aeronautical Information Manual. Where appropriate, we will be adding information in green.

7-1-24. Microbursts

a. Relatively recent meteorological studies have confirmed the existence of microburst phenomenon. Microbursts are small scale intense downdrafts which, on reaching the surface, spread outward in all directions from the downdraft center. This causes the presence of both vertical and horizontal wind shears that can be extremely hazardous to all types and categories of aircraft, especially at low altitudes. Due to their small size, short life span, and the fact that they can occur over areas without surface precipitation, microbursts are not easily detectable using conventional weather radar or wind shear alert systems.

b. Parent clouds producing microburst activity can be any of the low or middle layer convective cloud types. Note, however, that microbursts commonly occur within the heavy rain portion of thunderstorms, and in much weaker, benign appearing convective cells that have little or no precipitation reaching the ground.

c. The life cycle of a microburst as it descends in a convective rain shaft is relatively short. An important consideration for pilots is the fact that the microburst intensifies for about 5 minutes after it strikes the ground.

d. Characteristics of microbursts include:

1. Size. The microburst downdraft is typically less than 1 mile in diameter as it descends from the cloud base to about 1,000-3,000 feet above the ground. In the transition zone near the ground, the downdraft changes to a horizontal outflow that can extend to approximately 2 1/2 miles in diameter.

2. Intensity
. The downdrafts can be as strong as 6,000 feet per minute. Horizontal winds near the surface can be as strong as 45 knots resulting in a 90 knot shear (headwind to tailwind change for a traversing aircraft) across the microburst. These strong horizontal winds occur within a few hundred feet of the ground.

3. Visual Signs.
Microbursts can be found almost anywhere that there is convective activity. They may be embedded in heavy rain associated with a thunderstorm or in light rain in benign appearing virga. When there is little or no precipitation at the surface accompanying the microburst, a ring of blowing dust may be the only visual clue of its existence.

4. Duration.
An individual microburst will seldom last longer than 15 minutes from the time it strikes the ground until dissipation. The horizontal winds continue to increase during the first 5 minutes with the maximum intensity winds lasting approximately 2-4 minutes. Sometimes microbursts are concentrated into a line structure, and under these conditions, activity may continue for as long as an hour. Once microburst activity starts, multiple microbursts in the same general area are not uncommon and should be expected.

e. Microburst wind shear may create a severe hazard for aircraft within 1,000 feet of the ground, particularly during the approach to landing and landing and take-off phases. The impact of a microburst on aircraft which have the unfortunate experience of penetrating one is characterized in the lesson from the University of Wisconsin. The aircraft may encounter a headwind (performance increasing) followed by a downdraft and tailwind (both performance decreasing), possibly resulting in terrain impact.
Last lesson we asked you to read the NOAA "Microbursts: Visual Identification Handbook." Now is a good time to read it if you didn't get the chance.

f. Detection of Microbursts, Wind Shear and Gust Fronts.

1. FAA's Integrated Wind Shear Detection Plan.

(a) The FAA currently employs an integrated plan for wind shear detection that will significantly improve both the safety and capacity of the majority of the airports currently served by the air carriers. This plan integrates several programs, such as the Integrated Terminal Weather System (ITWS), Terminal Doppler Weather Radar (TDWR), Weather System Processor (WSP), and Low Level Wind Shear Alert Systems (LLWAS) into a single strategic concept that significantly improves the aviation weather information in the terminal area.

(b) The wind shear/microburst information and warnings are displayed on the ribbon display terminals (RBDT) located in the tower cabs. They are identical (and standardized) in the LLWAS, TDWR and WSP systems, and so designed that the controller does not need to interpret the data, but simply read the displayed information to the pilot. The RBDT's are constantly monitored by the controller to ensure the rapid and timely dissemination of any hazardous event(s) to the pilot.

(c) The early detection of a wind shear/microburst event, and the subsequent warning(s) issued to an aircraft on approach or departure, will alert the pilot/crew to the potential of, and to be prepared for, a situation that could become very dangerous! Without these warnings, the aircraft may NOT be able to climb out of, or safely transition, the event, resulting in a catastrophe. The air carriers, working with the FAA, have developed specialized training programs using their simulators to train and prepare their pilots on the demanding aircraft procedures required to escape these very dangerous wind shear and/or microburst encounters.

It is CRITICAL for you to understand that at this time, there is no way for you to know which airport has which warning system, especially when it comes to the different LLWAS systems. Some approach charts do state on the back or front of the airport page (10-9) that there is a Low Level Wind Shear System or a Terminal Doppler Radar System available. There are significant differences.

2. Low Level Wind Shear Alert System (LLWAS).

(a) The LLWAS provides wind data and software processes to detect the presence of hazardous wind shear and microbursts in the vicinity of an airport. Wind sensors, mounted on poles sometimes as high as 150 feet, are (ideally) located 2,000 - 3,500 feet, but not more than 5,000 feet, from the centerline of the runway. Remember, an airport with only an LLWAS MUST wait until the wind shear/microburst is within 150 feet of the ground to be sensed and for you to be warned!

(b) LLWAS was fielded in 1988 at 110 airports across the nation. Many of these systems have been replaced by new TDWR and WSP technology. Eventually all LLWAS systems will be phased out; however, 39 airports will be upgraded to the LLWAS-NE (Network Expansion) system, which employs the very latest software and sensor technology. The new LLWAS-NE systems will not only provide the controller with wind shear warnings and alerts, including wind shear/microburst detection at the airport wind sensor location, but will also provide the location of the hazards relative to the airport runway(s). It will also have the flexibility and capability to grow with the airport as new runways are built. As many as 32 sensors, strategically located around the airport and in relationship to its runway configuration, can be accommodated by the LLWAS-NE network.

At this time (2003)the FAA still uses three different LLWAS systems even though the AIM only mentions one.

The LLWAS-2 has only six sensors and has the least amount of protection available. With only six sensors, it is NOT able to provide warnings in the same way as other systems. Microbursts, at a maximum diameter of 2.2 miles CAN descend on the airport and go undetected. You will know an airport has this system when the tower controller states that the following...

"American 123, Windshear Alert, NE boundary winds, 250 at 45 knots. Centerfield winds 200 at 10 knots."

The LLWAS-2 system will NEVER produce a "MICROBURST" warning from the tower controller.

The second system is the upgrade to the LLWAS-2, called the LLWAS Relocate and Sustain Program, or LLWAS-RS. Up to sixteen sensors are located around the airport. At this time the LLWAS-RS uses a warning phraseology that is similar to the LLWAS-NE mentioned below.
Besides the FAA websites, you can get the bigger picture from the Massachusetts Institute of Technology (MIT)-Lincoln Laboratory's weather website

3. Terminal Doppler Weather Radar (TDWR).

(a) TDWR's are being deployed at 45 locations across the U.S.. Optimum locations for TDWR's are 8 to 12 miles off of the airport proper, and designed to look at the airspace around and over the airport to detect microbursts, gust fronts, wind shifts and precipitation intensities. TDWR products advise the controller of wind shear and microburst events impacting all runways and the areas 1/2 mile on either side of the extended centerline of the runways out to 3 miles on final approach and 2 miles out on departure.

These warnings are displayed (as depicted in the examples in subparagraph 5) on the RBDT.

(b) It is very important to understand what TDWR does NOT DO:

It DOES NOT warn of wind shear outside of the alert boxes (on the arrival and departure ends of the runways); therefore you WON'T get an alert if you are on downwind, unless you listen up to tower frequency (NOT on a radar vector to final or the ILS). You won't hear it in IMC until you switch to tower frequency when you are only only several thousand feet above the ground and IT BECOMES CRITICAL!!! You won't hear a warning if you are on a different tower frequency for a NON-IMPACTED runway.

(The system is only correct about 80 percent of the time and ATC doesn't want to unnecessarily impact traffic flows into the busiest airports.)

It DOES NOT detect wind shear that is NOT a microburst or a gust front; but a PIREP will detect this kind and when reported to tower MANDATES that it is relayed to other aircraft!

It DOES NOT detect gusty or cross wind conditions; and

It DOES NOT detect turbulence.

However, research and development is continuing on these systems. Future improvements may include such areas as storm motion (movement), improved gust front detection, storm growth and decay, microburst prediction, and turbulence detection.

(c) TDWR also provides a geographical situation display (GSD) for supervisors and traffic management specialists for planning purposes. The GSD displays (in color) 6 levels of weather (precipitation), gust fronts and predicted storm movement(s). This data is used by the tower supervisor(s), traffic management specialists and controllers to plan for runway changes and arrival/departure route changes in order to both reduce aircraft delays and increase airport capacity.

4. Weather System Processor (WSP).

(a) The WSP provides the controller, supervisor, traffic management specialist, and ultimately the pilot, with the same products as the terminal doppler weather radar (TDWR) at a fraction of the cost of a TDWR. This is accomplished by utilizing new technologies to access the weather channel capabilities of the existing ASR-9 radar located on or near the airport, thus eliminating the requirements for a separate radar location, land acquisition, support facilities and the associated communication landlines and expenses.

(b) The WSP utilizes the same RBDT display as the TDWR and LLWAS, and, just like TDWR, also has a GSD for planning purposes by supervisors, traffic management specialists and controllers. The WSP GSD emulates the TDWR display, i.e., it also depicts 6 levels of precipitation, gust fronts and predicted storm movement, and like the TDWR GSD, is used to plan for runway changes and arrival/departure route changes in order to reduce aircraft delays and to increase airport capacity.

(c) This system is currently under development and is operating in a developmental test status at the Albuquerque, New Mexico, airport. When fielded, the WSP is expected to be installed at 34 airports across the nation, substantially increasing the safety of the American flying public.

The FAA is trying to upgrade the ATC radar to an ASR-11. This will also have the weather warning capability. Read about it on the FAA website.

The HIGHEST level of support is the Integrated Terminal Weather System or ITWS which is now found at Memphis, Dallas-Ft. Worth, Orlando, and several other airports. Read about the program on the FAA website or at this website.

5. Operational aspects of LLWAS, TDWR and WSP.

To demonstrate how this data is used by both the controller and the pilot, 3 ribbon display examples and their explanations are presented:

USAF and USArmy personnel may want to review this lesson from the Air Force Flight Standards Agency on weather warnings from ATC.



This is what the controller sees on his/her ribbon display in the tower cab.

27A MBA 35K- 2MF 250 20

This is what the controller will say when issuing the alert.



In plain language, the controller is telling the pilot that on approach to runway 27, there is a microburst alert on the approach lane to the runway, and to anticipate or expect a 35 knot loss of airspeed at approximately 2 miles out on final approach (where it will first encounter the phenomena). With that information, the aircrew is forewarned, and should be prepared to apply wind shear/microburst escape procedures should they decide to continue the approach. Additionally, the surface winds at the airport for landing runway 27 are reported as 250 degrees at 20 knots.

Threshold wind is at pilot's request or as deemed appropriate by the controller.

FAA Order 7110.65, Air Traffic Control, Low Level Wind Shear Advisories, Paragraph 3-1-8b2(a).



This is what the controller sees on his/her ribbon display in the tower cab.

27A WSA 20K- 3MF 200 15

This is what the controller will say when issuing the alert.



In plain language, the controller is advising the aircraft arriving on runway 27 that at about 3 miles out they can expect to encounter a wind shear condition that will decrease their airspeed by 20 knots and possibly encounter turbulence. Additionally, the airport surface winds for landing runway 27 are reported as 200 degrees at 15 knots.


Threshold wind is at pilot's request or as deemed appropriate by the controller.

FAA Order 7110.65, Air Traffic Control, Low Level Wind Shear Advisories, Paragraph 3-1-8b2(a).



This is what the controller sees on his/her ribbon display in the tower cab.

27A WSA 20K+ RWY 250 20

27D WSA 20K+ RWY 250 20

This is what the controller will say when issuing the alert.




In this example, the controller is advising arriving and departing aircraft that they could encounter a wind shear condition right on the runway due to a gust front (significant change of wind direction) with the possibility of a 20 knot gain in airspeed associated with the gust front. Additionally, the airport surface winds (for the runway in use) are reported as 250 degrees at 20 knots.

FAA Order 7110.65, Air Traffic Control, Low Level Wind Shear Advisories, Paragraph 3-1-8b2(d).

6. The Terminal Weather Information for Pilots System (TWIP).

(a) With the increase in the quantity and quality of terminal weather information available through TDWR, the next step is to provide this information directly to pilots rather than relying on voice communications from ATC. The National Airspace System has long been in need of a means of delivering terminal weather information to the cockpit more efficiently in terms of both speed and accuracy to enhance pilot awareness of weather hazards and reduce air traffic controller workload. With the TWIP capability, terminal weather information, both alphanumerically and graphically, is now available directly to the cockpit on a test basis at 9 locations.

(b) TWIP products are generated using weather data from the TDWR or the Integrated Terminal Weather System (ITWS) testbed. TWIP products are generated and stored in the form of text and character graphic messages. Software has been developed to allow TDWR or ITWS to format the data and send the TWIP products to a database resident at Aeronautical Radio, Inc. (ARINC). These products can then be accessed by pilots using the ARINC Aircraft Communications Addressing and Reporting System (ACARS) data link services. Airline dispatchers can also access this database and send messages to specific aircraft whenever wind shear activity begins or ends at an airport.

(c) TWIP products include descriptions and character graphics of microburst alerts, wind shear alerts, significant precipitation, convective activity within 30 NM surrounding the terminal area, and expected weather that will impact airport operations. During inclement weather, i.e., whenever a predetermined level of precipitation or wind shear is detected within 15 miles of the terminal area, TWIP products are updated once each minute for text messages and once every five minutes for character graphic messages. During good weather (below the predetermined precipitation or wind shear parameters) each message is updated every 10 minutes. These products are intended to improve the situational awareness of the pilot/flight crew, and to aid in flight planning prior to arriving or departing the terminal area. It is important to understand that, in the context of TWIP, the predetermined levels for inclement versus good weather has nothing to do with the criteria for VFR/MVFR/IFR/LIFR; it only deals with precipitation, wind shears and microbursts.

7. Medium-Intensity Airport Weather System (MIAWS)

This is a new test program now under study at Jackson, MS, airport. It uses the NEXRAD data and displays it to ATC so that the tower and radar facilities can describe the locations and intensities of the weather around the airport.
A critical point right now is that this system describes VIP levels 1 and 2 as "Light." It describes VIP levels 3 and 4 as "Moderate" which contradicts your aircraft radar which would show level 4 as red color. The controller describes VIP levels 5 and 6 as "severe."

Well, this concludes the fifth lesson of our course on thunderstorms and flying. The last lesson will concentrate on aircraft accidents that occurred in the vicinity of thunderstorms. It will be interesting.

Over these lessons, we've featured the biographies of some of the NWA members who have helped put this course together. As course director, it is important for me to acknowledge the contributions of our webmaster, Mr. Tim Oram, a meteorologist at the Johnson Space Center in Houston, Texas. "Thank you, Tim, for all your contributions to make this possible."
Study Questions:
1. At what altitude does a microburst change from downflow to a horizontal flow?
2. How rapidly does the air descend in a microburst?
3. What is the measurement of shearing in a microburst?
4. What weather produces a microburst?
5. How long is a microburst event?
6. Does the horizontal wind get stronger or weaker near the ground?
7. From the NOAA reference, what is a Double-Ring structure of a microburst?
8. How does a microburst curl near the ground?
9. What is an LLWAS?
10.. What is an LLWAS-2?
11. How low must an event get to the ground before an LLWAS will sense it?
12. Will an LLWAS-2 ever produce a Microburst Alert?
13. What is a TDWR?
14. Where will microburst or wind shear that is sensed by a TDWR, ITWS, WSP, or MIAWS produce an alert to tower?
15. How many sensors does a LLWAS-2, LLWAS-RS, LLWAS-NE have?
16. What is the WSP? Where is it now?
17. What will a tower controller tell you as a pilot to warn of a wind shear or microburst alert?
18. What is TWIP?
19. What is a MIAWS system?
20.. What terms will a controller use to describe the six different VIP levels to you as a pilot?
21. How does MIAWS contradict your radar?