wind-shear.pdf

Introduction

Wind shear is deﬁned as a wind direction and/or speed change over a vertical or

horizontal distance.

It is signiﬁcant when it causes changes to an aircraft’s headwind or tailwind such that

the aircraft is abruptly displaced from its intended ﬂight path and substantial control

action is required to correct it.

Although wind shear may be present at all levels of the atmosphere, its occurrence in

the lower levels is of particular importance to aircraft taking-off and landing. During the

climb-out and approach phases of ﬂight, aircraft airspeed and height are near critical

values, rendering the aircraft especially susceptible to the adverse effects of wind

shear. The response of aircraft to wind shear is extremely complex and depends on

many factors including the type of aircraft, the phase of ﬂight, the scale on which the

wind shear operates relative to the size of the aircraft, and the intensity and duration

of the wind shear encountered.

It should be noted that wind shear is always present in turbulent air, but windshear

can occur without turbulence being present.

Types of wind shear

Vertical wind shear is deﬁned as change of horizontal wind direction and/or speed

with height.

Horizontal wind shear is the change in wind speed and/or direction at the same level.

Updraft and downdraft wind shear is the change in vertical wind velocity across

adjacent columns of air. This type of shear is often encountered with convective

activity.

Aircraft taking-off may be signiﬁcantly affected by changes in headwind and tailwind

components which create changes in the amount of lift experienced. A decrease in the

vertical headwind component, or an increase in the tailwind component, will result in a

reduction in airspeed, and in extreme cases the resulting loss of lift may be enough to

cause the aircraft to stall or ﬂy into the ground.

For aircraft landing, a decrease in the headwind component (undershoot shear)

may cause it to drop below the target descent path and to land short of the runway

threshold. An increase in the headwind component (overshoot shear) may cause it to

ﬂy above the target descent path leading to a late touchdown and possible overrun.

The crosswind component of shear does not impact ﬂight to the same extent as

headwind/tailwind or vertical shear. In general it does not affect the airspeed and

angle of attack and hence does not alter the equilibrium of forces on the aircraft in the

vertical plane. It does, however, affect the drift and side-slip angles, which can cause

added complications for the pilot if ﬂying conditions are otherwise difﬁcult.

HAZARDOUS WEATHER PHENOMENA

Wind Shear

Bureau of Meteorology › Aviation Weather Services

Wind shear is a common

phenomenon within the

atmosphere, occurring at

any level where adjacent

layers or columns of air

have different velocities.

It can produce sudden

changes in aircraft

altitude and speed.

Updraft and downdraft wind shear.

Horizontal wind shear.

Vertical wind shear.

20 kts

50 kts

20 kts

23 kts

18 kts

15 kts

Wind Shear Associated with Thunderstorms

When thunderstorms (cumulonimbus clouds - CB) are forecast, pilots should be aware

of the potential for signiﬁcant wind shear. Towering cumulus clouds (TCU) may also

create similar wind shear effects.

Wind shear between adjacent updrafts and downdrafts within such clouds can

generate extreme turbulence. The danger is two-fold:

(a) Severe loadings may be imposed on the aircraft structure; and

(b) Violent changes in aircraft attitude may induce stall or other conditions in which an

attempted recovery may exceed the design limitations of the aircraft.

The outﬂow from the cloud’s downdraft can produce damaging winds on and near

the ground. The term microburst is used to describe a downburst which causes

damage over an area with horizontal dimensions of less than four kilometres.

The microburst is the most violent form of wind shear produced by CBs. If it occurs

over an airport, aircraft landing or taking-off may encounter strong headwinds, then

strong downdrafts followed by strong tailwinds. As microbursts can be symmetric or

asymmetric, pilots should not expect to always experience the standard sequence of

microburst encounter. Microbursts can be wet (i.e. occurring with precipitation) or dry.

A gust front is the leading edge of the cold air outﬂow from a CB after a downdraft

reaches the ground and spreads out in all directions, undercutting the surrounding

warmer air. In this respect it resembles a shallow cold front except that the associated

wind shear is generally far higher in the gust front. The gust front initially travels

along the ground equally in all directions. However if the CB itself is moving, as is

generally the case, the gust front advances furthest and fastest ahead of the CB in the

direction of the cloud’s movement. There is marked horizontal wind shear at ground

level following the passage of the leading edge of the front, and because the front

may be tens of kilometres ahead of the parent storm cell, such a sudden change in

the surface wind may take pilots completely by surprise. The change in the surface

wind direction is often as much as 180 degrees and the speed of the gusting winds

following passage of the front can exceed 50 knots. The gust front ahead of squall line

thunderstorms (a multicell thunderstorm where cells are arranged in a long line) can

be tens of kilometres wide, i.e. spanning the length of the multicell storm.

Wind Shear Associated with Frontal Systems

Frontal systems consist of air masses of different temperature separated by a narrow

zone called the frontal zone, the region where wind shear signiﬁcant to aviation is

most likely to occur. Vertical wind shear occurs at and behind the cold front, and since

the frontal zone usually slopes back with height, the height above a speciﬁc location

at which maximum vertical wind shear occurs will increase with time following the

passage of the cold front at that location. At ground level there is also horizontal wind

shear across the front although, given the usual speed of movement of fronts across

an aerodrome, this may be short-lived.

Wind Shear Associated with Sea Breezes

A sea breeze is essentially a shallow cold front because cooler air is replacing warmer

air. Wind shear occurs predominantly at the surface along the leading edge as the front

Causes of Wind Shear

Signiﬁcant wind shear is often

encountered with and in the

vicinity of:

• Thunderstorms

• Frontalsystems

• Seabreezes

• Frictionalshearing

• Temperatureinversions

• Obstacles

• Rotors

• Wakevortices

A wet microburst spreading

outward from the main rain shaft

with associated reverse ﬂow

evidenced by the rolling cloud/

dust (image © Jimmy Deguara,

sourced from Manual of Aviation

Meteorology 1st ed. 2003).

Microburst effects on an

aircraft glide path while

landing. The microburst

spreads out rapidly once

it contacts the surface.

The horizontal outﬂow

is a region of strong

vertical and horizontal

wind shear, which reaches

its maximum in the ﬁrst

vortex.

penetrates inland, although wind shear of lower magnitude exists at higher levels.

The extent of a sea breeze at any particular location is inﬂuenced considerably by the

surrounding topography and therefore may be of a localised nature. If the prevailing

wind is offshore, the sea breeze front may be marked by a line of convergence and

vigorous convection that in favourable circumstances gives rise to lines of showers or

even thunderstorms.

Wind Shear Associated with Frictional Shearing of Surface Winds

The lower atmosphere is sheared by frictional forces dragging airﬂow towards zero

at the surface. This can result in signiﬁcant differences between the surface wind

speed and that at higher levels. The resultant shear intensity can be greater over

ﬂat and open land than over rougher terrain because the rougher elements generate

turbulence to a greater depth, having the effect of mixing out layered velocity

ﬂuctuations.

Wind Shear Associated with Strong Temperature Inversions

Frictional shearing is enhanced when low-level winds are decoupled from upper level

winds due to overnight radiation inversions. Such inversions can almost completely

cut off the downward transfer of the horizontal wind momentum to the boundary

layer, resulting in large differences between surface ﬂow (which may be light or even

calm due to frictional effects) and the ﬂow above the inversion. An aircraft descending

through an inversion would pass through a zone of turbulence before experiencing a

dramatic loss or gain of lift and airspeed. Temperature inversions are more pronounced

in clear skies and wind shear is strongest around the inversion height.

When the inversions are very strong, low-level jets may form overnight, possibly

just a few hundred feet above a calm surface wind. The jet speed can be enhanced

ahead of an approaching cold front, and also windward of barriers such as hills and

escarpments. The inversion is effective in shielding the ﬂow above from surface

frictional effects, allowing the wind speed to increase in a narrow band near the top

of the inversion, with surface winds being very light or calm. A broad scale low-level

nocturnal jet can extend over southern Queensland and the Northern Territory during

the cooler months. The core of this jet is often located between Daly Waters and

Tennant Creek, with a maximum speed of about 50 knots occurring around 3000 feet.

It is usually strongest around dawn and dissipates by late morning. Funnelling of the

wind through valleys and ravines may produce similar effects on a local scale.

Wind Shear Associated With Obstacles

Strong surface winds ﬂowing over obstacles (such as large buildings, low hills or

close-planted stands of tall trees) upwind of an aerodrome can create localized areas

of horizontal wind shear on a runway. In these circumstances the shear is usually

accompanied by clear air turbulence (CAT). The effect that the obstacles have on the

prevailing wind ﬂow depends on a number of factors, the most important being the

speed of the wind and its orientation relative to the obstacle, and the scale of the

obstacle in relation to the runway dimensions.

Such wind shear, which is normally very localized, shallow and turbulent is of particular

concern to light aircraft operating into smaller aerodromes but has also been known to

affect larger aircraft.

Where a range of low hills lies alongside a runway, the height of the range may be

insufﬁcient to divert the airﬂow upstream of the range, but as a consequence of the

airﬂow being forced upwards over the range the airﬂow acquires a compensating

downwards vertical component downstream which, depending upon the proximity

of the hills to the runway, can cause localized low-level downdrafts along the runway.

Where the hills or mountains are sufﬁciently high to divert the low-level wind ﬂow, the

surface wind may be funnelled along the runway.

Nocturnal low-level jet.

Wind shear is caused by a

change in wind speed and/or

direction resulting in a change

in headwind or tailwind that can

displace an aircraft abruptly from

its intended ﬂight path, requiring

substantial control action to be

taken.

This compares with turbulence

which may disturb the aircraft’s

attitude about its major axis, and

cause rapid bumps or jolts to be

experienced, but in most cases

it does not signiﬁcantly alter the

aircraft’s ﬂight path.

Wind Shear Associated With Rotors in Lee Waves

Onalargerscale,whenthewindowisforcedoveramountainrangeaseriesof

standing waves may be formed in the wind ﬂow on the lee side of the mountains. The

meteorological conditions most suitable for the formation of lee waves include:

• astablelayerofairsandwichedbetweentwolessstablelayers,onenearthe

ground and the other at a higher level;

• awindinexcessof25knotsblowingwithin30degreeseithersideofaline

perpendicular to the ridge line;

• littleornodirectionalwindshearinthestablelayer;and

• amarkedmeansealevelpressuredifferentialacrossthemountainbarrier.

If the lee waves that develop are of sufﬁcient amplitude, a closed rotor eddy may be

formed beneath a wave crest. In extreme conditions, such a rotor can penetrate to

ground level and can reverse the prevailing surface wind directly below the rotor. Such

stationary wave systems produce marked downdrafts close to the mountain, and also

downdrafts of lesser magnitude at some considerable distance downwind from the

mountain in secondary and tertiary waves.

Wind Shear Associated with Wake Vortices

Wind shear is generated behind every aircraft in ﬂight, mainly as tip vortices forming

two counter-rotating cylindrical vortex tubes trailing behind the wing tips. Such vortices

are severe when generated by large, wide-bodied jet aircraft. The vortices generated

by aircraft taking off can pose a signiﬁcant hazard to aircraft following too closely

behind. Air Trafﬁc Control will apply appropriate separation minima to minimise the risk

of wake vortex encounters.

Detection & Monitoring

Recognitionofexternalmeteorologicalcluestothepossiblepresenceoflow-level

wind shear near an airport gives the pilot an opportunity to make an early decision

to avoid an encounter by going around or by delaying the approach or take-off until

conditions improve. External clues that may be directly visible to the pilot include:

(a) strong, gusty surface winds, especially where the aerodrome is located near hills

or where there are large buildings near the runway;

(b) virga from convective cloud, because downdrafts may still exist and reach the

ground even though the precipitation itself has evaporated;

cell, indicating the presence of a gust front;

(d) lenticular cloud (smooth lens-shaped altocumulus) indicating the presence of

standing waves, usually downwind from a mountain;

(e) areas of dust raised by wind, particularly when in the form of a ring below

convective clouds, indicating the presence of a downburst;

(f) wind socks indicating winds from different directions;

(g) smoke plumes, with upper and lower sections moving in different directions; and

(h) cumulonimbus clouds, which should always be assumed to have the capability of

producing hazardous wind shear.

TheBureaureliesonpilotreports,intheformofanAIREP,astheprimarymeans

ofdetectingwindshear.TheAIPBook,GEN3.6,section11.1.1states“Apilotin

commandshouldmakeaspecialAIREP….assoonaspracticableafterencountering

any….METconditionwhichislikelytoeffectthesafety....ofotheraircraft.”

Lenticular cloud, image courtesy,

David Miller.

Forecasts & Warnings

Wind shear is a very difﬁcult

phenomenon to forecast and

hence is only given in Wind

Shear Warnings, which are

issued for a limited number of

aerodromes. They are issued

when wind shear (that could

adversely affect aircraft on the

approach or take-off paths, or on

the runway during the landing

or takeoff phases, and during

circling approach) is observed,

reported or expected, between

runwayleveland1600feet

above that level.

Wind shear information will be

includedinSPECI,atairports

where manual observations are

provided, when reports of wind

shear are received from pilots

through ATC.

Airservices Australia is the ofﬁcial distributor of aviation forecasts, warnings and observations

issued by the Bureau of Meteorology. Airservices’ ﬂight brieﬁng services are available at

www.airservicesaustralia.com. Telephone contact details for elaborative brieﬁngs are contained

in Airservices’ Aeronautical Information Publication Australia (AIP), which is available online

through their website.

Other brochures produced by the Bureau of Meteorology’s aviation weather services program

can be found at www.bom.gov.au/aviation/knowledge-centre.