WX BRIEF

Flying in turbulent skies

How to turn wind buffets into a breeze.


Wreckage of a USAF Lockheed C141 Starlifter in a UK field after being torn apart by extreme turbulence within a small thunderstorm on Aug 28, 1976. Thunderstorms contain some of the most concentrated and extreme turbulence there is.

The added release of energy stored in the water vapor can enhance the convection, making the updrafts and turbulence even stronger. What’s more, because the air density has decreased so much, it will rise to great heights—30,000 ft or more—and a corresponding cold, dense downdraft often develops adjacent to the updraft.

The result is a vertical shear zone that may extend from the surface to 30,000 or 40,000 ft, and have winds going upward at 1000 fpm next to winds going downward at 5000 fpm. The resulting 60-kt shear can generate severe turbulence extending several thousand feet away from the shear boundary.

The other type of turbulence is mechanically induced. Mechanical turbulence is the generic name for any turbulence not directly the result of a thermal process. This could be a turbulent rotor on the lee side of a mountain, or the eddies created as the jet stream twists or turns through the upper air.

Often, because the turbulence is not associated with the clouds common to convective turbulence, it is termed CAT. These days, however, the term is generally only applied to mechanical turbulence at high altitude.

Like thermally-induced turbulence, the strength of mechanical turbulence is directly related to the amount of available energy. This means that the greater the difference in flow, the stronger the resulting waves and eddies will be.

In the case of air flow around an obstacle, the speed of the obstacle is normally zero, so even a moderate wind can generate significant turbulence in the air downwind. Conversely, severe turbulence can be created as an obstacle, such as a wing, is forced at high speed through relatively calm air.

Turbulence from the wake vortices of landing aircraft is among the more common dangers faced by operators of lighter aircraft in low-altitude operations. However, 3 factors of the jet stream conspire to create strong turbulence at high altitudes.

The first is its mere existence. A jet stream is a narrow band of fast-moving air which is caused by a strong discontinuity in temperature and pressure over a very short distance.

For the most part, this ribbon of fast-moving air is flowing through relatively lighter winds, meaning the potential for turbulent wave development along the edges of the jet.

The second component that favors development of strong turbulence is that parts of the jet stream tend to move at speeds much faster than the rest of the jet or the air around it. These cores of fast-moving air, often reaching speeds in excess of 150 kts, are known as jet streaks.

They tend to occur in the bottom and downwind of sharp kinks in the jet stream. They’re more common in troughs but also occur in ridges. The extra speed translates into additional shear energy in this region.

Lastly, the jet itself is a series of longer waves. Any upper air map will show you the troughs and ridges of the jet stream. Large changes in wind direction over relatively short distances can increase the likelihood of generating turbulent waves and eddies, since shear is a function not just of speed but also of wind direction relative to the adjacent air.

This means the area surrounding a deep trough in the jet stream is a great place to find severe or extreme turbulence.

Not only is the jet changing direction rapidly—it is accelerating through the base in the form of a jet streak. Large, violent eddies can be created around the edges of this flow.

Forecasting and avoiding turbulence

Some turbulence can be anticipated without looking at any weather map Large cities produce abundant heat, which means perpetual light chop when approaching or departing an urban airport.

Transitions from land to water cause changes in the speed and direction of overflowing air, resulting in mechanical turbulence at lower levels. The same goes for airports in mountainous regions—especially those in narrow valleys. However, basic weather maps are an excellent resource for identifying regions of potential turbulence.

Thermal turbulence can be expected at low levels anywhere there are clear skies and dark spots on the landscape. So, sunny days during fallow periods in agricultural regions are often good for producing a bumpy climb-out. Most stronger turbulence is also not too difficult to anticipate.

Again, on any weather map, turbulence is likely to be found around frontal zones or other areas where strong winds and sudden wind shifts may be present. Forecasters generally have a pretty good idea where nonfrontal convection is possible.

Strong turbulence should always be expected in the vicinity of thunderstorms, whether associated with a front or not. Unfortunately, this is where the forecasting of turbulence begins to get a bit more tricky.

We’ve not yet reached the point where airmass thunderstorm formation can be pinpointed, but we have pretty good confidence when a particular area has the right conditions for thunderstorms.

So when forecasters issue a convective Sigmet, it generally applies to a pretty wide area—at least 3000 sq miles (7800 sq km), a line at least 60 miles long, or severe or embedded thunderstorms expected to last longer than half an hour.

So, in general, a pilot should assume that anywhere within the convective Sigmet region there is also the potential for moderate to extreme turbulence. A nonconvective Sigmet may also be issued specifically for turbulence, but only if the turbulence is observed or forecast to be severe or stronger over a 3000 sq mile area.

If forecasters think an area may have only moderate turbulence, or if the severe turbulence is over an area less than 3000 sq miles, it won’t be part of a turbulence Sigmet—although widespread moderate turbulence over an area of at least 3000 sq miles will merit an Airmet tango (turbulence Airmet), which will be valid for 6 hours.

Sigmets are only valid for 4 hours. Recalling the United 826 incident, wave turbulence was indeed forecast over the western Pacific at the time and altitude of the flight.

In fact, the forecast of turbulence was a primary factor leading the crew to select Pacific Ocean navigation track 12 during preflight planning. However, this decision was made largely on the fact that a turbulence Sigmet was covering some of the other tracks, and the crew felt track 12 would give them the least trouble.

The best bet when flying through an area with forecast turbulence aloft is to anticipate that it could be stronger than you think and that you may not get much, if any, warning before you get hit.

The greatest problem with turbulence is that it’s transitory and we can predict it only in very general terms based on what computer models and weather balloons say about conditions favoring turbulence.

Unless they’re flying in formation, no 2 aircraft are likely to experience the same turbulent eddy. An aircraft only a few miles in front of you might report light chop while you wind up getting banged about by severe jolts.

 

Karsten Shein is a climatologist with the National Climatic Data Center in Ashe­ville NC. He formerly served as an assistant professor at Shippens­­burg Uni­versity and was a scientist with NASA’s Global Change Master Directory. Shein holds a commercial license with instrument rating.

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