WX BRIEF

Bumpy rides

Understanding and avoiding turbulence


Banner clouds rest atop the mountains surrounding the Cook Inlet south of Homer AK. Banner clouds often indicate the presence of strong wind flow and the possibility of lee waves.

Also, these eddies can be generated anywhere along the shear boundary. In a thunderstorm, that means from the surface up to the tops of the up­draft—often around 40,000–50,000 ft—and larger eddies can extend many hundred feet from the boundary, meaning that severe turbulence can easily be encountered almost a mile from the thunderstorm core.

Mechanical thrill rides

At the surface, downdrafts can translate into the second type of turbulence. Mechanical turbulence defines any turbulence generated by windshear that is not the result of thermal lifting.

Basically, this means that any turbulence generated by interaction with a physical obstacle—including interaction between 2 nonthermal flows—is considered mechanical. When a downdraft encounters the surface, it has no place to go other than to spread out from the point of impact.

As it spreads out laterally, it is forced into the calmer surface air and, if strong enough, may become a roll vortex—a tubular eddy wrapped around the downdraft core. Such vortices are most common in downbursts and microbursts. When an aircraft encounters such a vortex, it is normally in takeoff or landing configuration and is ill equipped to recover from its effects.

Even without becoming a roll vortex, the outflow from a thunderstorm presents one of the more dangerous types of mechanical turbulence—low-level windshear (LLWS). This describes a sudden but consistent change in wind direction across an airport that is created as a thunderstorm gust front moves across the field.

Aircraft arriving or departing the airport are in the most danger from LLWS, as the sudden shift in wind direction while the aircraft is at low altitude, airspeed and maneuverability can limit a pilot’s ability to take effective corrective action. Wake vortices are another low-level turbulence threat to aircraft. These are the eddies generated as air sheds from the tips of the airfoils.

Such vortices are a function of the size of the wing, the angle of attack and the speed of the aircraft. Although nearly all airfoils generate tip vortices, the most dangerous vortices are produced by large aircraft and helicopters during landing and takeoff phases. The danger is due to 2 factors.

First, unlike most turbulent eddies in the atmosphere, wake vortices are relatively stable, lasting for up to 3 min. They sink at a relatively low rate and stabilize about 600– 700 ft below their generation point. The second danger is that aircraft encountering these vortices tend to also be in takeoff or landing configuration and can easily lose attitude control very close to the ground.

However, if there is sufficient crosswind, the vortices will likely migrate away from the runway—although they may wind up over the parallel runway you plan to use. Low-level mechanical turbulence is also generated when strong winds blow around obstacles, across rough surfaces or over low ridges.

These ob­stacles act the same as that boulder in the middle of a stream. If the stream is flowing gently, only minor ripples will form beyond the boulder, but if the river is flowing rapidly, the water will be significantly churned up behind the boulder.

When air is forced to flow around or over an obstacle, the obstacle itself both creates friction and changes the speed and direction of the flow immediately surrounding it. Pilots who fly in and out of canyon airports know full well how rough the ride can get when a strong pressure gradient brings surface winds across the ridges paralleling the runway.

High anxiety

However, just because an aircraft is flying above the surface layer does not mean it won’t encounter strong turbulence. In addition to thunderstorms, there exist 2 major forms of higher-altitude turbulence—mountain waves and clear air turbulence.

Mountain waves, such as the one that brought down BOAC Flight 911, form when high-speed air is forced to flow over or around a mountain or range. As it does so, the flow com­presses around the mountain and sets up into a lee wave, oscillating up and down several thousand feet and stretching for possibly hundreds of miles downwind.

Sometimes called standing waves because the wave crests and troughs remain stationary relative to the mountain, the waves also shed eddies beneath their crests. Because the waves do not migrate, neither do these eddies or rotors—which contain some of the most extreme turbulence an aircraft may encounter.

Lee waves may crest over 3 times higher than the mountain peak, although the greatest danger exists below mountaintop level, beneath the first few wave crests downwind. Unlike the other forms of turbulence, clear air turbulence (CAT) is generally found at high altitude—normally at jet stream levels.

CAT is primarily a function of 2 flows of air moving at different speeds or directions but adjacent to one another. This situation is commonly found along the edges of jet streams such as the circumpolar vortex.

Not only does the jet represent a narrow ribbon of fast-moving air moving through a comparatively calm atmosphere, it also exhibits a great deal of transience—shifting directions as troughs and ridges meander through it. Turbulence along these jets, especially where the jet is changing direction or is moving exceptionally quickly, can be very strong.

Although CAT is primarily associated with the jet stream, it can also occur above frontal zones or wherever there is a sharp density gradient in the atmosphere. In addition, CAT is not necessarily restricted to clear air—often wispy cirrus will give a hint of the rough airflow aloft by forming wave shapes or appearing to be violently torn apart.

Planning and managing turbulence

Anyone who has spent time on the aviation frequencies will agree that a lot of the chatter revolves around finding the smoothest ride. But how do we keep the boss in back from tossing his or her cookies? In the air, signposts for turbulence are sometimes visible in the form of clouds—thunderstorms, lenticulars, rotors or cirrus.

But, since turbulence is due to windshear, and such shear is not always accompanied by moisture, many turbulent regions remain invisible to aviators. The best way to keep from straying into these regions is to pay close attention to weather maps and forecast charts, as well as Pireps from your fellow pilots.

Turbulent Airmets and Sigmets are a first line of defense that will give you a general idea of where turbulence can be expected and how strong it might be. A look at the Pireps over these areas will help you refine the levels at which the strongest turbulence has been encountered so far.

If you then compare this information against current and forecast weather maps of the jet stream, fronts or convection, it is generally not too difficult for a pilot to make an educated guess as to the routes to avoid to keep clear of the worst bumps. In the air, if telltale clouds are present, it is best to avoid that airspace by several thousand feet and several hundred miles. The same thing goes for a pilot report of strong turbulence.

While turbulence near the ground tends to be highly localized, turbulence aloft is more often relatively widespread. In areas of convective activity, expect some turbulence, but steer clear of any active storm cells by at least 20 miles. When near mountains, it is best to avoid flying beneath their crests on the lee side. Near the jet stream, avoid routes that take you close to regions where the jet is changing direction or has a local speed maximum.

On takeoff and landing, simple rules should ensure a smooth ride. For one, it’s a good idea not to take off or land through, under or near an active thunderstorm of any size. It’s also wise not to land or start a takeoff roll immediately after a larger aircraft has landed or departed. ICAO mandates minimum time and distance separation for following aircraft of at least 2 min for departing aircraft and 4 miles for landing aircraft.

If the tower clears you sooner, it may be in your best interests to request the minimum clearance. Lastly, if you do encounter turbulence, file a Pirep. Even if you think the light shake you got is nothing significant, it may be an entirely different experience for a following aircraft, and at the very least it helps forecasters determine where the turbulent air resides.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.

 

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