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Microbursts and downdrafts

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Strong outflow and low-level windshear are ever-present dangers near thunderstorms.


By Karsten Shein
Comm-Inst Climate Scientist

Aircraft flying through a downburst will first encounter strong headwinds, tempting a power reduction, followed by rapid shift to a tailwind and loss of altitude. Full-power go-around should be initiated immediately
on recognizing downdraft penetration.
After a long but uneventful flight, the 2 pilots were looking forward to a few days off. Settling into the glidepath above the final approach fix, the copilot noted the early evening thunderstorm activity popping on their radar scope.

Most of the active cells were well outside their course to the runway, leading the senior pilot to retort that they’d be inside the FBO before the first drops hit the aircraft. Because the frequency was busy and the last weather check had light and variable winds, the pilots didn’t bother asking for a wind check.

With flaps set, wheels down and the altimeter spooling toward field elevation, the aircraft suddenly gained speed and ballooned upward with a jolt.

The copilot, with his hand already on the throttles, began to draw them back and push the nose forward, but his crewmate quickly reached out and pushed the copilot’s hand and throttles forward to the stops, calmly stating “I’ve got the plane.”

The business jet responded briskly to the pilot’s inputs just as the airspeed dropped and altitude began bleeding away quickly. The GPWS was whooping in their ears as the jet struggled but began to regain altitude.

The copilot, realizing what had just happened, called in the missed approach and requested vectors to a second approach. By the time they had returned to short final, the storm that had been maturing just to the east of the runway had moved past, and the pilots made a smooth landing.

Unfortunately, not all microburst encounters end so benignly. Aircraft have been brought down by these thunderstorm byproducts since the early years of powered flight, and, despite recurrent training and technology capable of detecting and alerting pilots to them, aviators continue to encounter them – sometimes with fatal outcomes.

Adiabatic cooling

Ground-based Doppler weather radar signatures of a microburst near STL (Intl, St Louis MO). Radar can provide microburst detection, but not much lead time for alerts as downbursts form and dissipate quickly.

Whenever air becomes denser than its surroundings, it will descend. There are several reasons why this may occur, but the most common cause is air that has been rapidly cooled. This cooling happens when air rises, either mechanically or thermally.

A mechanical process is one in which air is forced to flow over obstacles, such as rising terrain, while a thermal process is one in which air is heated to become less dense, and it rises. In either case, rising air cools adiabatically, meaning it cools without transferring heat or matter to its surroundings.

Adiabatic cooling is at the heart of the meteorological concept of the air parcel – an isolated volume of air that moves up or down in the atmosphere. As air rises into altitudes where the environment is less dense, it expands.

In expanding, its volume increases, but its mass remains constant, meaning its density – and therefore pressure – decreases. This also means that its temperature decreases at a constant rate of -9.8° C per 1000 m (-5.38° F per 1000 ft) for unsaturated air.

As the air cools, humidity rises and water vapor condenses out, releasing heat into the parcel that slows the rate of cooling. Less constant, the saturated adiabatic lapse rate is around -3.6 to -9.2° C (-2 to -5° F) per 1000 ft.

Since the rate of cooling with altitude of the lower atmosphere is roughly -6.5° C per 1000 m, rising air can become denser than its surroundings quickly, especially if it remains unsaturated. The result is that the cold, dense air will descend through the warmer air beneath it.

Although it will also warm adiabatically as it descends (again at 9.8° C per 1000 m), if there is water or ice present, evaporation and sublimation will pull heat from the air, slowing the warming. In some instances, the descending air will remain colder and denser than the air through which it is falling, allowing it to flow back along the downsloping terrain, or reach the surface in the case of a thermal downdraft.

Lee flow

Wet downburst near PHX (Intl, Phoenix AZ) in July 2016 as seen from an approaching helo. Massive rain shafts frequently contain strong downdrafts and microbursts, and through-flight should be avoided.

Mechanical downdrafts are frequently found on the lee slopes of mountain ranges as Chinook (aka föhn or Santa Ana) katabatic winds and lee wave winds that generate low-level rotors which can easily upset aircraft departing or arriving at nearby airports.

These warming winds often accelerate as they descend, reaching speeds of 70 kts or more. The record Chinook gust is 93 kts near Spearfish SD, which also raised the surface temperature from -4° F (-20° C) to 45° F (7° C) in just 2 minutes.

Chinook and, to a lesser extent, all downslope winds – such as what occurs in a valley at night – are relatively shallow events. A pilot may not experience their effects if they are more than a few hundred feet above the round.

They will always come from the direction of rising terrain, and may create a sudden wind shift (directional shear) if the prevailing atmospheric flow is from a different direction. They will also likely create speed shear as they arrive, causing a sudden increase in speed and gustiness that may last for just a few minutes.

That low-level windshear will also appear as the katabatic wind dissipates, returning the flow to its previous state. Critically, however, pilots must exercise caution when approaching to land, as the shallow nature of the wind means that the aircraft will encounter it just before landing, when it is in a low-and-slow configuration and poorly responsive to control inputs that may be necessary to land safely or go around.

If a Chinook or valley downdraft is possible, maintaining a greater approach speed than normal is warranted – if the runway length will allow. Also, if available, request wind checks on approach if winds are blowing from higher elevation terrain.

Strong thermally-driven downdrafts are far more common, occurring in every thunderstorm. During formation, thunderstorms are driven primarily by updrafts with little downdraft. At maturity, however, both up and downdrafts are present.

In most thunderstorms, a lack of windshear aloft means that the downdraft falls through the updraft core, disorganizing both and limiting the strength of the downdraft as it exits the storm’s base. In its dissipating phase, the disorganized updrafts are incapable of sustaining the storm, and the downdraft dominates.

However, most downdrafts exiting a storm base produce only relatively weak radial outflow incapable of upsetting an aircraft. Frequently, we feel this as a mild gust front from the approaching storm.

Downbursts

Downbursts and their more compact variants, called microbursts, are strong downdrafts able to maintain coherence all the way to the ground, normally by avoiding interaction with updrafts.

Downbursts are not clearly defined, but in general are described as strong downdrafts that develop quickly and may last only a few minutes. As they reach the ground, they spread out radially, normally generating a horizontally rotating vortex of air around their core that can extend several hundred feet into the air.

Downbursts are differentiated into microbursts if their radius is under 2.5 mi (4 km), and macrobursts if the radius is larger. Downbursts can also be separated into dry and wet, depending on whether they are accompanied by rain.

Wet downbursts are often easily identifiable from the rain shaft extending from the cloud, and are prevalent in humid regions where there is ample moisture. In arid regions, the warming air of the downdraft and the dry air beneath the cloud may combine to evaporate the rain as a virga shaft, making it more difficult to “see” the downburst, except in the dust that may be kicked up in a radial pattern at the surface.

Any time pilots see virga beneath a cumulonimbus, they should assume the accompanying presence of a downburst. While the sudden onset and short-lived nature of downbursts makes them difficult to forecast, meteorologists can identify atmospheric conditions that support downbursts.

These factors include strong winds aloft, high low-altitude humidity, and dry mid-level air. Such conditions are frequently associated with late afternoon summertime thunderstorms in humid subtropical places such as the US Southeast.

Downbursts can also often be seen on Doppler radar, which measures the motion of reflectivity toward and away from the radar. In the same way that the radar can pick up the signature of a rotating tornado, divergent motion patterns on the scope will identify a possible downburst.

Downburst encounters

The dangers that downbursts pose to aviation are their violent strength and the way in which they act on aircraft passing through their domain. Downdrafts in strong thunderstorms can exceed 100 kts, and there is little to slow that airflow down as it reaches the surface and spreads out.

There are 2 ways in which an aircraft might encounter a downdraft. The first is that it flies directly through it, and the second is that the downdraft hits the aircraft from the side.

Since the leading edge of the expanding downburst on the ground is air that is pushing outward and rising into the leading edge vortex, the effect on the aircraft will most likely be to lift the upwind wing.

As the vortex passes over the aircraft, the lifting will shift to the downwind wing, while the upwind wing may be forced down, possibly initiating an uncommanded roll. The pass-through is more complex, and has been the cause of several high-profile commercial air crashes through the years.

In the pass-through scenario, the landing or departing aircraft encounters the leading edge of the roll vortex and the strong headwinds that accompany it. Initially, the aircraft balloons and appears to increase airspeed.

The instinctive response of pilots is to reduce power and lower the nose. A few seconds later, the aircraft enters the downburst core, where 100-kt winds push directly down on the aircraft, and any headwind advantage the aircraft may have had is lost.

Here, a rapid loss of altitude is experienced. The untrained response would be to pull back on the yoke and increase power to maintain altitude and airspeed. Unfortunately, as jets take a few moments to spool up, the sudden backpressure at a slow airspeed may create a stall scenario.

Finally, if the aircraft hasn’t yet impacted any terrain or obstacles, it exits the downburst core into a region of strong tailwinds that may reduce airspeed critically. Even if the aircraft is producing maximum thrust at this point, airspeed may not be sufficient to arrest a descent, and trying to force a climb may result in stalling out.

In the few seconds it took for the aircraft to transit the downburst, it may have experienced as much as 150-kt shear (headwind to tailwind change).

Avoidance and recovery

On August 2, 1985, a Lockheed L-1011 TriStar passed through a microburst on approach to DFW (Dallas–Fort Worth TX), killing 137 people, and drawing attention to the dangers of downbursts to aviation.

As a result of that accident, windshear monitoring systems were installed at dozens of major airports, pilots were instructed to report windshear whenever it was encountered, and ATC reporting of low-level windshear (LLWS) was standardized across ATC systems.

Also, many bizjets and all commercial jet aircraft now have an airborne windshear detection and alert system that is capable of detecting and alerting pilots rapidly to possible windshear situations. The system is now commonly integrated into airborne weather radar avionics.

Without a doubt, the best way to avoid encountering a downburst is never to fly directly beneath thunderstorms, and preferably not to fly beneath the base altitude of any thunderstorm within about 2 miles of your flightpath if you will be operating below 1000 ft AGL.

However, should you inadvertently encounter a downburst, timely recognition is essential. At the first indication of strong low-level windshear on approach, a full power go-around should be initiated. And, if it happens on departure, maintain full power.

Pitch should be set for optimal climb (just at stick shaker activation for those aircraft so equipped) and decreased slightly when the downburst core is penetrated to account for the loss of airspeed that will follow when exiting the core.

Conclusion

Downbursts, even if just suspected, are nothing to mess with, and likely will have dissipated by the time the aircraft is positioned for a second approach. Because the forces within a downburst can be severe, and the leading edge is a vortex that has the potential to upset the aircraft, wings should be kept as level as possible until clear of the downburst.

At busy airports, it can be tempting to take previous pilot reports as gospel, but you could inadvertently fly directly into a downburst, since downbursts are ephemeral and come on without warning.

The aircraft ahead of you on approach may pass through a rainshaft without incident, but when you reach that same position a minute later, a downburst may be waiting for you. Importantly, when pilots encounter windshear on approach or departure, they should report it immediately to ATC.

Such reports should be specific, such as “encountered windshear on final, loss of 15 kts at 300 ft,” while avoiding the terms “negative” or “positive,” as their meaning can be misinterpreted as a gain/loss in airspeed or altitude, or that windshear was/was not encountered.

If specifics are unknown, the effect on the aircraft will do (eg, “windshear on final, lost altitude, max power required”). Every bit of information will help your fellow pilots.


Karsten Shein is co­founder and science director at ExplorEiS. He was formerly an assistant professor at Shippensburg Univer­sity and a climatolo­gist with NOAA. Shein holds a commercial license with instrument rating.

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