A Look at Low Level A Look at Low Level Wind Shear Wind Shear

Southwest Aviation Weather Southwest Aviation Weather Safety Workshop. Phoenix, AZSafety Workshop. Phoenix, AZ

Ken WidelskiKen WidelskiMeteorologistMeteorologistNWS: Lubbock, TXNWS: Lubbock, TX

ObjectivesObjectives

What is Low Level Wind Shear?What is Low Level Wind Shear? Speed Speed versus versus DirectionalDirectional Non-ConvectiveNon-Convective Wind ShearWind Shear ConvectiveConvective Wind ShearWind Shear Pre-Flight Planning/In-Flight PrecautionsPre-Flight Planning/In-Flight Precautions RecapRecap Questions??Questions??

What is Low Level Wind What is Low Level Wind Shear?Shear?

A change in the A change in the wind wind directiondirection or or speed speed within 2000 ft AGL. within 2000 ft AGL.

This condition can This condition can present danger to present danger to aircraft, aircraft, especially at especially at landinglanding, when a , when a sudden shift from sudden shift from headwindheadwind to to tailwindtailwind can cause a rapid can cause a rapid loss of airspeed and loss of airspeed and lift. lift.

Many Sources of LLWS Many Sources of LLWS

““Low Level Jet” Low Level Jet” (*)(*) -Strong winds above an inversion-Strong winds above an inversion Weather Fronts Weather Fronts (*)(*) Thunderstorms: Microburst (*)Thunderstorms: Microburst (*) Mountain wavesMountain waves Gap windsGap winds Sea and land breezesSea and land breezes

KEY: LLWS can be convective and KEY: LLWS can be convective and non- non- convectiveconvective

Speed versus DirectionalSpeed versus Directional

Speed ShearSpeed Shear: is : is the change in the change in wind speed with wind speed with height. In the height. In the illustration, the illustration, the wind is wind is increasing with increasing with height height

Directional Shear Directional Shear is a change in is a change in wind direction wind direction with height. with height.

Surface flow is Surface flow is southeast. Winds southeast. Winds veer (clockwise) veer (clockwise) turning to south-turning to south-southeast and southeast and south.south.

A Different PerspectiveA Different Perspective

Non-Convective LLWSNon-Convective LLWS

Change in wind Change in wind directiondirection or or speedspeed within 2000 ft AGL.within 2000 ft AGL.

The most common type of LLWSThe most common type of LLWS Occurs over a larger area Occurs over a larger area Longer periods of timeLonger periods of time Easier to detectEasier to detect Usually associated with a “Low Usually associated with a “Low

Level Jet”Level Jet”

Aspects of the “Low Level Aspects of the “Low Level Jet”Jet”

A narrow band of A narrow band of

strong winds in the strong winds in the

lower atmosphere.lower atmosphere.

Stronger winds Stronger winds

Typically above aTypically above a

Radiative inversionRadiative inversion

LLJ CharacteristicsLLJ Characteristics

Under the inversion topUnder the inversion top…… -Conditions are usually stable with-Conditions are usually stable with light winds. light winds. Above the inversion topAbove the inversion top…… -Winds can suddenly change up to -Winds can suddenly change up to

90 degrees in direction and up 90 degrees in direction and up to to 40 KTS in speed. 40 KTS in speed.

Can pose a serious hazard to aircraft.Can pose a serious hazard to aircraft.

LLJ Flight ImplicationsLLJ Flight ImplicationsCourtesy of Hong Kong Weather ServiceCourtesy of Hong Kong Weather Service

When an aircraft When an aircraft departing from the departing from the airport airport ascends and ascends and enters the “jet”,enters the “jet”, it it experiences increasing experiences increasing headwind and lift. headwind and lift.

As it As it departs the departs the “jet”,“jet”, however the however the headwind and lift headwind and lift decreasedecrease. .

Frontal PassagesFrontal Passages

Frontal passages can bring dramatic Frontal passages can bring dramatic shifts in…shifts in…

Surface wind strengthSurface wind strength Surface wind directionSurface wind direction

The more vigorous the frontThe more vigorous the front = = Stronger the Wind shiftStronger the Wind shift = = Better chance for Low Level Better chance for Low Level Wind ShearWind Shear

Frontal Type-Cold Frontal Type-Cold FrontFront Frontal Passages can be large Frontal Passages can be large

scalescale

-Dense cold air rapidly digs in -Dense cold air rapidly digs in (Back)(Back)

Frontal Type-Warm Frontal Type-Warm FrontFront

Warm air gently slopes over coolerWarm air gently slopes over cooler

surface air (Veer).surface air (Veer).

Small Scale Small Scale BoundariesBoundaries

Outflow boundaries form fromOutflow boundaries form from decaying thunderstormsdecaying thunderstorms..

Cooler and more stable Cooler and more stable air spreads out in all air spreads out in all directions.directions.

Convection is suppressed Convection is suppressed in this area.in this area.

Looks benign but can be Looks benign but can be the focus for LLWS and the focus for LLWS and new convectionnew convection

Convective Wind ShearConvective Wind Shear Affects a localized areaAffects a localized area Main form: Main form: MicroburstMicroburst

What is a Microburst?What is a Microburst?

A A microburstmicroburst is a very localized is a very localized column of sinking air, producing column of sinking air, producing damaging divergent and damaging divergent and straight-line winds at the surface. at the surface.

Microburst LifecycleMicroburst Lifecycle

Microburst SizeMicroburst Size:: -less than 1 mile in diameter as it descends -less than 1 mile in diameter as it descends

from the cloud base to about from the cloud base to about 1,000-3,000 1,000-3,000 feetfeet  above the ground. In the transition zone   above the ground. In the transition zone near the ground, the downdraft changes to a near the ground, the downdraft changes to a horizontal outflow that can extend to horizontal outflow that can extend to approximately 2 1/2 miles in diameter.  approximately 2 1/2 miles in diameter. 

Microburst IntensityMicroburst Intensity:: The downdrafts can be as strong as The downdrafts can be as strong as 6,0006,000

feet per minute. Horizontal winds near the feet per minute. Horizontal winds near the surface can be as strong as 45 knots surface can be as strong as 45 knots resulting in a resulting in a 90 knot90 knot shear shear (headwind to (headwind to tailwind change for a traversing aircraft)tailwind change for a traversing aircraft) across the microburst. These strong across the microburst. These strong horizontal winds occur within a few hundred horizontal winds occur within a few hundred feet of the ground.  feet of the ground. 

What are the dangers?What are the dangers?

The scale and suddenness of a The scale and suddenness of a microburst makes it a great microburst makes it a great danger to aircraft, particularly danger to aircraft, particularly those at low altitude which are those at low altitude which are taking off and landing. taking off and landing.

As the aircraft is coming in to As the aircraft is coming in to land, the pilots try to slow the land, the pilots try to slow the plane to an appropriate speed.plane to an appropriate speed.

When the microburst hits, pilots When the microburst hits, pilots will see a large spike in their will see a large spike in their airspeed, caused by the force of airspeed, caused by the force of the headwind created by the the headwind created by the microburst microburst

An initial response would be to An initial response would be to decrease the speed of the aircraft. decrease the speed of the aircraft.

The plane would then travel The plane would then travel through the microburst, and fly through the microburst, and fly into the tailwind, causing a into the tailwind, causing a sudden decrease in the amount of sudden decrease in the amount of air flowing across the wings air flowing across the wings

The sudden loss The sudden loss of air moving of air moving across the wings across the wings causes the causes the aircraft to aircraft to literally drop out literally drop out of the air. of the air.

Pre-Flight PlanningPre-Flight Planning

The NWS offers a host of data for The NWS offers a host of data for pre-flight planning.pre-flight planning.

Aviation Area Forecast DiscussionAviation Area Forecast Discussion::

-Generally at TAF issuance-Generally at TAF issuance

TAF: Terminal ForecastsTAF: Terminal Forecasts::

-4 routine issuances per day-4 routine issuances per day

TWEB: Transcribed Weather TWEB: Transcribed Weather BroadcastBroadcast

-4 routine issuances per day-4 routine issuances per day

Aviation discussionAviation discussion

More direct insight into the TAF Reasoning.More direct insight into the TAF Reasoning.

Example:Example: NATIONAL WEATHER SERVICENATIONAL WEATHER SERVICE 245 PM MDT WED APR 4 2007245 PM MDT WED APR 4 2007

WIDESPREAD MVFR CIGS WITH SOME LCL IFR WIDESPREAD MVFR CIGS WITH SOME LCL IFR NEAR THE BLKHLS WILL CONTINUE THROUGH THE NEAR THE BLKHLS WILL CONTINUE THROUGH THE AFTERNOON. CONDITIONS ARE EXPECTED TOAFTERNOON. CONDITIONS ARE EXPECTED TODETERIORATE LATER THIS EVENING FROM WEST DETERIORATE LATER THIS EVENING FROM WEST TO EAST AS AREAS OF SNOW START TO DEVELOP TO EAST AS AREAS OF SNOW START TO DEVELOP AND CIGS DROP TO IFR IN SOME LOCATIONSAND CIGS DROP TO IFR IN SOME LOCATIONS..

Experimental AFD PageExperimental AFD Pagehttp://aviationweather.gov/testbed/afd/http://aviationweather.gov/testbed/afd/

LLWS in the TAF LLWS in the TAF NWSI 10-813: Feb 2005NWSI 10-813: Feb 2005

Only refers to non-convective LLWSOnly refers to non-convective LLWS

from the surface up to 2000 FT AGL.from the surface up to 2000 FT AGL. Reason: LLWS is always assumed to Reason: LLWS is always assumed to

be present in convective activity.be present in convective activity. When LLWS conditions are expected When LLWS conditions are expected

the code the code WS WS will be added to the FM will be added to the FM prevailing group. prevailing group.

LLWS TAF FormatLLWS TAF Format

WSWS: Non-convective LLWS: Non-convective LLWS hwshwshwshwshwshws: Height of the top of : Height of the top of

the WS layer in hundreds of feet the WS layer in hundreds of feet AGLAGL

dddddd: True direction in ten degree : True direction in ten degree increments at the indicated heightincrements at the indicated height

ffff: Speed in knots of the forecast : Speed in knots of the forecast wind at the indicated height.wind at the indicated height.

KTKT: Unit indicator for wind: Unit indicator for wind

TAF ExampleTAF Example

KPUB 181122Z 181212 13012KT 5SM KPUB 181122Z 181212 13012KT 5SM

––RA SCT010 OVC035 RA SCT010 OVC035 WS020/27055KTWS020/27055KT

FM1400 32010KT P6SM FEW008 BKN045FM1400 32010KT P6SM FEW008 BKN045

WS020WS020=Top of the wind shear layer=Top of the wind shear layer

27055KT27055KT=Wind direction and speed at =Wind direction and speed at the top of the layer. the top of the layer. Not Not

a value for a value for the amount of shear.the amount of shear.

RAOBRAOB: Sounding data: 12z and : Sounding data: 12z and 00z00z

-Occasional 18z sounding for -Occasional 18z sounding for severesevere

weather.weather.

AOPA: Aircraft Owners and AOPA: Aircraft Owners and Pilots Association Pilots Association recommends…recommends…

If forecasts mention low level wind If forecasts mention low level wind shearshear, be prepared to go around and , be prepared to go around and carry carry extraextra airspeed on approach. airspeed on approach.

Usual recommendation is:Usual recommendation is: -Add -Add ½½ the gust factor to your usual the gust factor to your usual 1.3 Vso approach speed.1.3 Vso approach speed. EX: If the wind is 10 gusting to 20 KTSEX: If the wind is 10 gusting to 20 KTS Add 5 KTS to your handbook’s Add 5 KTS to your handbook’s target target

airspeed for final approach.airspeed for final approach.

Additional Help…Additional Help… Consider using Consider using

partial flaps for your partial flaps for your landing in gusty or landing in gusty or shear conditions. shear conditions.

--The extra lift and The extra lift and

drag caused by the drag caused by the full flaps may hinder full flaps may hinder your ability to your ability to preserve any preserve any semblance of a semblance of a stabilized approach stabilized approach and may lead to and may lead to control problems. control problems.

And Finally…And Finally…

Remember that Remember that you do have an you do have an out.out.

You can abandon You can abandon the approach at the approach at the first signs of the first signs of trouble. trouble.

RecapRecap LLWS can contain LLWS can contain speedspeed, , directiondirection

or a or a combination combination of both.of both. 2 types of LLWS: 2 types of LLWS: ConvectiveConvective

Non-ConvectiveNon-Convective Convective: Localized, small scaleConvective: Localized, small scale Non-Convective: Most common formNon-Convective: Most common form

Larger scaleLarger scale

More widespreadMore widespread

Easier to detectEasier to detect

Many NWS tools available for pre-flight Many NWS tools available for pre-flight planning. Main one’s include…planning. Main one’s include…

AVN DiscussionAVN Discussion TAFTAF TWEBTWEB RAOBRAOB

AOPA wind shear recommendationsAOPA wind shear recommendations

Still unsure: Still unsure: Contact your local NWS Contact your local NWS office for a personalized forecast office for a personalized forecast 24/7/365.24/7/365.

ReferencesReferences

NWSI 10-813 Aviation DirectiveNWSI 10-813 Aviation Directive AOPA: Aircraft Owners and Pilots AOPA: Aircraft Owners and Pilots

Association: Wind Shear Awareness and Association: Wind Shear Awareness and Coping StrategiesCoping Strategies

FAA: Flight Safety, Dangers of FAA: Flight Safety, Dangers of MicroburstsMicrobursts

Aviation Weather Center: AWCAviation Weather Center: AWC Weather Service, Hong KongWeather Service, Hong Kong Wikipedia.orgWikipedia.org

Thank you for Thank you for attending:attending:

Any Any Questions?Questions?