Ch 11 – Wind Shear

2. Ch 11 – Wind Shear

3. Ch 11 – Wind Shear • Section A – Wind Shear Defined • Section B – Causes of Wind Shear • Microbursts • Fronts and Shallow Lows • Airmass Wind Shear • Elevated Stable Layers • Jet Streams

4. Ch 11 – Wind Shear • Strong wind shear is a hazard to aviation because it can cause turbulence and large airspeed fluctuations and, therefore, serious control problems. • It is a threat especially to aircraft operations near the ground because of the limited altitude for maneuvering, particularly during the takeoff and landing phases of flight. • In this chapter, we examine wind shear and its causes

5. Ch 11 – Wind Shear • When you complete the chapter, you will know what wind shear is and what its critical values are. • You will also know how, why, and where it develops in the vicinity of thunderstorms, inversions, developing extratropical cyclones, fronts, and jet streams.

6. Ch 11 – Wind Shear • Section A:Wind Shear Defined • Wind shear – a gradient in wind velocity. • It is interpreted in the same sense as a pressure gradient or temperature gradient • that is, it is a change of wind velocity over a given distance.

7. Ch 11 – Wind Shear • Horizontal wind shear – it is convenient to visualize wind shear as being composed of two parts: a horizontal wind shear (a change in wind over a horizontal distance) being one part. • Vertical wind shear - a change in wind over a vertical distance

8. Ch 11 – Wind Shear • ***Wind shear is best described as a change in wind direction and / or speed within a very short distance • ***During departure under conditions of suspected low-level wind shear, a sudden decrease in headwind will cause a loss in airspeed equal to the decrease in wind velocity

9. Ch 11 – Wind Shear • Section B: Causes of Wind Shear • ***An important characteristic of wind shear is that it may be associated with a thunderstorm, a low-level temperature inversion, a jet stream, or a frontal zone

10. Ch 11 – Wind Shear • Downburst – Professor T. Fujita, an atmospheric scientist from the University of Chicago, coined the term downburst for a concentrated, severe downdraft that induces an outward burst of damaging winds at the ground

11. Ch 11 – Wind Shear • Micro bursts • Microburst - Professor T. Fujita, an atmospheric scientist from the University of Chicago, introduced the term microburst for a downburst with horizontal dimensions of 2.2 n.m. (4km) or less.

12. Ch 11 – Wind Shear • Vortex ring – The microburst is characterized by a strong core of cool, dense air descending from the base of a convective cloud. • As it reaches the ground, it spreads out laterally as a vortex ring which rolls upward as a vortex ring which rolls upward along its outer boundary.

13. Ch 11 – Wind Shear • ***An aircraft that encounters a headwind of 45 knots with a microburst may expect a total shear across the microburst of 90 knots • ***The duration of an individual microburst is seldom longer than 15 minutes from the time the burst strikes the ground until dissipation

14. Ch 11 – Wind Shear • ***When a shear from a headwind to a tailwind is encountered while making an approach on a prescribed glide slope, the pilot should expect airspeed and pitch attitude decrease with a tendency to go below glide slope • ***If there is thunderstorm activity in the vicinity of an airport at which you plan to land, you should expect wind shear and turbulence on approach

15. Ch 11 – Wind Shear • Low-level wind shear systems (LLWAS) – These alert systems have been installed at many large airports around the U.S. where thunderstorms are frequent. • Terminal Doppler Weather Radar (TDWR) – These systems are being installed across the U.S. at many vulnerable airports to provide more comprehensive wind shear monitoring.

16. Ch 11 – Wind Shear • Fronts and Shallow Lows • Frontal wind shear – a front is a zone between two different air masses and frontal wind shear is concentrated in that zone

17. Ch 11 – Wind Shear • ***With a warm front, the most critical period for LLWS is before the front passes

18. Ch 11 – Wind Shear • Air mass Wind Shear • Air mass wind shear – occurs at night under fair weather conditions in the absence of strong fronts and/or strong surface pressure gradients. • It develops when the ground becomes cooler than the overlying air mass as a result of radiational cooling. If the cooling is strong enough, a ground-based or surface inversion will result. • In this case, the temperature increases with altitude from the surface to an altitude of a few hundred feet.

19. Ch 11 – Wind Shear • Nocturnal inversion – low-level soundings taken throughout the day and night during fair weather conditions have revealed stable layers developing at night due to radiational cooling of the ground. • By sunrise the stability has increased to a maximum as indicated by the nocturnal inversion.

20. Ch 11 – Wind Shear • ***A pilot can expect a wind shear zone in a surface-based temperature inversion whenever the wind speed at 2,000 to 4,000 feet above the surface is at least 25 knots.

21. Ch 11 – Wind Shear • Elevated Stable Layers • Elevated stable layers – In addition to fronts and surface-based nocturnal inversions, wind shears may be found in the free atmosphere, in elevated stable layers. • These layers are frequently found over shallow, relatively cool air masses. Convection from the ground concentrates wind shear at the base of the stable layer.

22. Ch 11 – Wind Shear • ***When a climb or descent through a stable layer is being performed, the pilot should be alert for a sudden change in airspeed

23. Ch 11 – Wind Shear • Jet Streams – Certain patterns of upper level, short wave troughs and ridges produce significant wind shear. • The strongest shears are usually associated with sharply curved contours on constant pressure surfaces and / or strong winds. • Stable layers near jet streams and within a few thousand feet of the tropopause have the highest probabilities of strong shears. • Occasionally, the shear is strong enough to cause large airspeed fluctuations, especially during climb or descent.

24. Summary • Wind shear is one of the most serious low-level flight hazards in the atmosphere. • Significant wind shear not only occurs with microbursts, but also with fronts and nocturnal inversions. • Wind shear is also found in elevated stable layers in the free atmosphere, especially capping cold air masses and in the vicinity of jet streams and the tropopause.

25. Summary • Failure to be aware of all causes and weather conditions that produce wind shear can lead to catastrophic results. • An encounter with LLWS, in particular, is unforgiving because of the proximity of your aircraft to the ground. • You now have some useful conceptual models and basic rules of thumb to help you recognize and, where possible, avoid potential wind shear conditions.

26. Summary • In the next chapter, you will become aware of a number of situations where wind shear and turbulence are present at the same time in the same location (Lester, 2006).