Ch 11 – Wind Shear

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

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

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.

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.

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

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

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

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

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.

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.

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

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

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.

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

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

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.

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.

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.

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.

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

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.

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.

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.

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).