Low-Level Low-Level Wind ShearWind ShearAnticipation, Avoidance & Forecasting

What is it?What is it?• “a change in horizontal wind speed and/or

direction, and/or vertical speed with distance, measured in a horizontal and/or vertical direction.”

• Could be speed shear, directional shear or a combination of both

• Occurs within 2000 ft AGL

What about low-level What about low-level turbulence?turbulence?

Turbulence Wind shear

• Fluctuations in airspeed (only momentary fluctuations in vertical speed)

• Prevailing winds at the top of the low level and the surface are similar

• Change in air speed accompanied by rapid change in vertical speed

• Wind speed and/or direction changes significantly from the top of the shear layer to the surface

What are the dangers?What are the dangers?• Headwind at low level to tailwind at surface

o With the loss of headwind, nose pitches down,o Pull nose up and further reduce air speedo Could lead to the plane landing short

• Tailwind at low level to headwind at surfaceo Once headwinds are encountered, nose pitches up, air speed increaseso Could lead to the plane landing long

• Headwind or Tailwind at low level to crosswind at the surface

• Much more of an impact to GA aircraft than larger commercial aircraft

Convective vs. non-Convective vs. non-ConvectiveConvective

Convective Non-Convective

• Generally short-lived• Radar is the best tool

for detection, though increasingly difficult further away from the radar and in complex terrain

• Can last for hours• Soundings, profilers,

and radar can all be used for detection

Convective LLWSConvective LLWS• Very hard to detect even with radar • Microburst, macroburst, and squall lines are most

often associated with convective LLWS, but almost any storm has the potential to produce an outflow boundary

• Not explicitly stated in the TAF, however, it is to be assumed whenever TS is forecastedo If storms are expected to strong outflow winds, a variable convective

wind group may be included (VRB25G40KT)

July 22, 2011 caseJuly 22, 2011 case

20:03Z – 20:54Z

July 22, 2011 caseJuly 22, 2011 case

Non-Convective LLWSNon-Convective LLWS• Causes:

o Frontal Passageo Inversiono Low-Level Jeto Sea breeze fronto Other local effects

• Explicitly stated in the TAF• Slightly easier to detect and forecast than

convective LLWS, but still one of the most difficult TAF elements

Non-Convective LLWS Non-Convective LLWS in a TAFin a TAF

• If it is included it will be after any cloud groups• Often the last group in a line (only PROB30 could

come after it)• Format

o WShhhhhh/dddssdddssKT

Height of the top of the WS layer (WS should be expected

below this altitude) in hundreds of ft. AGL to 020

Wind direction at top of the WS layer

Wind speed at top of the WS layer

Non-Convective LLWS Non-Convective LLWS in a TAFin a TAF

KAMA 102320Z 1100/1124 22015KT P6SM SCT050 FM110130 33025KT P6SM SKC WS010/15050KT FM110400 35011KT P6SM SCT030 FM111100 01015KT P6SM BKN050=

WS010/15050KT• Top of the wind shear layer is 1000 ft.

• The wind at 1000 ft. is southeasterly (150˚) at 50 KT

Frontal PassageFrontal Passage• Can occur with a

warm, cold, stationary, or occluded front

• The slower and more shallow the front, the more impact of LLWS

From: DLAC 2 Writing TAFs for Winds and Low-Level Wind Shear

http://www.meted.ucar.edu/dlac2/mod3/main.htm

Frontal Passage caseFrontal Passage case

Dec 31, 2010 Amarillo, Dec 31, 2010 Amarillo, TXTX

KAMA 312016Z 3120/0118 33010G19KT P6SM FEW200 WS015/28035KT FM312200 28020G26KT P6SM FEW200 FM010000 29010KT P6SM SCT100 FM010400 34008KT P6SM SKC=

METAR KAMA 311753Z 28026G32KT 10SM CLR 01/M17 A2972 RMK AO2 PK WND 27036/1656 SLP065 T00111172 10011 21050 50016 METAR KAMA 311853Z 31010G17KT 10SM CLR 01/M14 A2971 RMK AO2 PK WND 27036/1826 SLP065 T00061139 METAR KAMA 311953Z 01011G19KT 10SM CLR 01/M12 A2970 RMK AO2 SLP062 T00111122 SPECI KAMA 312042Z 26023G33KT 10SM CLR 03/M19 A2972 RMK AO2 PK WND 27033/2039 SPECI KAMA 312048Z 26020G33KT 10SM CLR 03/M19 A2970 RMK AO2 PK WND 27033/2039 WSHFT 2034 METAR KAMA 312049Z 26023G34KT 10SM CLR 03/M19 A2971 RMK AO2 PK WND 25034/2049 WSHFT 2034 SLP063 T00331194 55004

Inversion and Low-Inversion and Low-Level JetsLevel Jets

Other local effectsOther local effects• Generally terrain

influenced (Gap winds, valley, canyon and mountain effects)

• Knowledge of the surrounding terrain and climatology is key for understanding and anticipating

From: DLAC 2 Writing TAFs for Winds and Low-Level Wind Shear

http://www.meted.ucar.edu/dlac2/mod3/main.htm

Data Resources for Data Resources for LLWSLLWS

• Wind Profilers• VAD wind profiles• Soundings• Aircraft and Pilot observations

(ACARS/MDCRS/TAMDAR/PIREPS)• TAF and AIRMET

(http://www.aviationweather.gov/products/gairmet/ )

Wind profilersWind profilers• Available at:

http://www.profiler.noaa.gov/npn/profiler.jsp• Very few in the Southwestern region

Wind Profiler DataWind Profiler Data

VAD Wind ProfileVAD Wind Profile• Derived from WSR-

88D (weather radar) data

• More extensive coverage than wind profilers

• Data may be limited in areas of mountainous terrain or with few echoes on radar

• Data may be unrepresentative if a wind shift is present

Image courtesy of Radar Operations Centerhttp://www.roc.noaa.gov/WSR88D/Maps.aspx

VAD Wind ProfileVAD Wind Profile

SoundingsSoundings• Available at:

http://www.spc.noaa.gov/exper/soundings/

PIREPS: Help us help PIREPS: Help us help you!you!

• For most airports, PIREPs are the only direct verification of LLWS

• Often times, LLWS can occur an non-peak times, and thus you may be the first to experience the LLWS

LLWS forecastingLLWS forecasting• Arguably the least understood TAF element

among forecasters• So how do we increase forecaster situational

awareness of LLWS?o Trainingo In practice?

LLWS forecaster LLWS forecaster decision aiddecision aid

speed direction speed direction height AGLKT deg KT deg ft0 360 0 360 500

unitsKTdegKT/100 ft

Surface Wind

Speed Difference

TAF Code N/A

WS magnitudeType

0.00minimal WS

Direction Difference

Low Level Wind

00

ValueCategory

Instructions: In the top (blue) section, input the surface wind speed (KT) and direction (degrees), low level wind speed (KT) and direction (degrees), and the height of the top of the low level layer (between 200 and 2000 ft AGL, rounded to the nearest 100 ft).

In the output section, difference in speed (C8), direction (C9), and the magnitude of the vector difference between the two wind speeds (WS magnitude, C10) will be calculated. For each category, a color code will be assigned; green indicates little if any impact to aviation. Yellow is WS that is near criteria to impact air traffi c, and red indicates an impact to air traffi c. The criteria for speed and direction difference are based on old guidlines of speed shear of 40KT or more and directional shear of 60˚ or more. The criteria for WS magnitude comes from the definition of LLWS in NWS directive 10-813 which states that LLWS should be included when "non-convective vertical WS of 10 KTS or more per 100 ft...are expected" (p 36).

If the conditions meet a minimum of one criterion for LLWS, then the TAF code (C13) will be generated. Note: LLWS will only be reported in a FM group; it will not be included in TEMPO or PROB groups. VRB will not be used for direction in the LLWS group.

This decision aid only covers non-convectiveLLWS. Low level wind shear is always assumed to be present around convection, and would be noted in the TAF by a VRB convective group (i.e. VRB20G35KT) or assumed when ever TSRA or VCTS is forecast in the TAF. Non-convective LLWS could be associated with fronts, inversions, low-level jets, or local terrain effects.

Initial FeedbackInitial Feedback• Overwhelmingly positive feedback from forecasters• This, combined with feedback from local GA pilots,

has posed the question, are we over forecasting LLWS in low level jet scenarios?o Traditional “rules of thumb” may not accurately express LLWS conditionso “I had to put in some pretty extreme winds to get any response”

• Increased awareness of LLWS with fronts• Another question that has been raised is how to

handle LLWS associated with outflow boundaries of storms that are far away or have already dissipated

ReferencesReferences• Arkell, R. 2000: Differentiating Between Types of Wind

Shear in Aviation Forecasting. Natl. Wea. Dig., 24:3, 39-51.• National Weather Service, 2008: National Weather Service

Instruction 10-813. Available at http://www.weather.gov/directives/sym/pd01008013curr.pdf.

• National Weather Service, 1979: NOAA Technical Memorandum NWS FCST-23; “Low-Level Wind Shear: A critical review”

• Sinclair, P. and Kuhn, P., 1991: Aircraft Low Altitude Wind Shear Detection and Warning System. J. Appl. Meteor., 30, 3-16.

• University Corporation for Atmospheric Research, 2008: “Writing TAFs for Winds and Low-Level Wind Shear.” Available at http://www.meted.ucar.edu/dlac2/mod3/index.htm