Low Level Wind Shear LLWS

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Low Level Wind Shear (LLWS)

• By
• Bob Jackson, MIC, ZSE CWSU

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Introduction - 1

• Low Level Wind Shear (WS) occurs frequently
• Can be overlooked when composing a TAF

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Introduction - 2

• That which we dont understand
• We explain to each other

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Goal of Presentation

• Refresher
• Maintain/Heighten awareness of WS
• Apply in aviation forecasting

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Some Questions Associated with WS

• What is WS?
• When should WS be the TAF?
• When should WS NOT be The TAF?
• When should a WS forecast be withdrawn?

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Define WS

• A change in horizontal (or vertical) wind speed
  and/or direction with distance (or height).
  (Badner)
• A Vector Difference composed of wind
  speed/direction (NWS)

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Two Types of WS

• Convective WS
• Could also be Micro-burst type of winds
• Can not be labeled as WS in TAF
• Non-Convective WS
• Mechanical
• Identified as WS in TAF

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Define WS

• WS in this presentation refers to
  Non-Convective Low Level Wind Shear.

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Define WS (continued)

• In order to be Low Level, it must occur at or
  below 2,000 ft AGL.

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When is it Low Level Turbc and not WS?

• When strong, low-level winds mix down to the
  surface
• The difference between surface and low-level
  winds minimal,
• Gives rise to Low Level Turbulence, not WS.

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How does WS impact airline flight operations?

• Compression is a term used to describe what
  happens when the aircraft in front begins to slow
  down.
• As it slows, following aircraft begin to catch
  up.
• Much like cars approaching a stop sign.

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How does WS impact airline flight operations?

• Compression occurs normally,
• but is enhanced by wind shear that slows landing
  traffic even more.
• This creates a ripple effect that impacts other
  incoming flights.
• Correct forecasts can help flight planning
• saving Time
• and Money.

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What are some SPECIFIC criteria for WS?

• Strong Pressure Gradients
• Expect WS If
• Pressure gradients support winds of 40 kts or
  more,
• And surface winds are light

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What are some SPECIFIC criteria for WS?

• Winds of 40 kts or more within 2,000 ft AGL, will
  give turbulence any time, but
• If surface wind is light, then expect WS.
• Need speed/direction differences to get WS
• Otherwise, it is low level turbulence.

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Examples of Strong Pressure Gradients

• A PDX-BLI gradient of 8 mbs or more (measured or
  forecast)
• South to North
• Twice that if North to South
• A SEA-EAT gradient of -11 mbs or more
• East to West

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NWP Products

• Surface progs
• Strong gradients are often under forecast in
  prog runs.
• Numerical guidance
• Products such as FRHTxx and FRHxx give
• Surface pressure forecasts
• Boundary layer wind forecasts
• In need of Broad interpretation

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Observations - These are NOT Forecasts!

• ROABs
• PIREPs
• ACARs Soundings
• Surface analysis
• Not always timely
• METARs
• VAD profile

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What are some SPECIFIC criteria for WS?

• VAD profile
• Light winds in METAR and 40 kts or more at VAD
  2,000 ft wind.
• Winds differences of 60 degrees or more between
  METAR and VAD 2,000 ft wind.

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VAD Wind Profiles
2,000 ft
1,000 ft
METAR 141756Z . 19009KT
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WSR-88D VAD Wind Profiles
Generally good for SEA, however,

• Lower level winds are affected by flow through
  the Strait of Juan De Fuca so dont always
  represent conditions over SEA.
• East winds over the airport are not always
  displayed in the VAD.

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WSR-88D VAD Wind Profiles
Generally good for SEA, however,

• Are not usable for all sites.
• MSXs WSR-88D is on mountain and VAD winds begin
  at 9,000, so not usable for LLWS.

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ACARS Soundings
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ACARS Wind Barbs
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RAOBs

• Rapid interpretation is Difficult

ZCZC SEASGLUIL TTAA00 KUIL 061200 72797 TTBB
56120 72797 00016 07205 11000 06403 22862 00704
33859 00005 44853 01694 55843 05095 66837 05295
77767 03495 88729 00064 99610 11556 11500 20756
22381 35150 33338 42356 44315 46363 55277 51758
66226 59359 77186 58974 88165 54376 99100 58775
31313 05102 81117 PPBB 56120 72797 90012 17004
18523 19023 90346 19523 19521 24513 90789 23514
22515 22517 91234 24529 25536 25537 9167/ 23538
23540 92015 25539 26536 24544 926// 24547 93057
26043 28053 28535 938// 27548 9503/ 31543
32045 NNNN
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RAOBs
Graphic display aids interpretation
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RAOBs
Organized data also aids interpretation
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RAOBs

• Not always near desired airport so not always
  representative
• Considered as a Snap Shot of conditions
• Not always timely

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Review of Main Points - 1

• Become aware of WS and the need to include in
  TAFs.
• Use meteorology, FORECAST WS when conditions are
  favorable.

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Review of Main Points - 2

• The TAF is a FORECAST not an Observation so,
• If conditions are favorable, dont wait for
  PIREPs of WS before putting WS in TAF.

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Quiz Time ! ! ! !

• If WS is mentioned in the AIRMET for your area of
  concern,
• Should you mention it in a TAF in that area when
  conditions are favorable?
• Should you mention it in a TWEB in that area?

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Quiz Time ! ! ! !

• If you have WS in a TAF, and need to amend for a
  different reason,
• What criteria would you use to drop WS from the
  amended TAF?

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Quiz Time ! ! ! !

• How is WS encoded in the TAF?

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Quiz Time ! ! ! !

• In which groups can/cannot WS be included?

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Quiz Time ! ! ! !

• How is CONVECTIVE WS encoded in the TAF?

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Quiz Time ! ! ! !

• In which groups can/cannot CONVECTIVE WS be
  included?
• How is it written?

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Summary

• WS has large impact on aviation
• Affects air traffic safety, flow, and comfort
• Occurs more often than is forecast
• Forecasters
• Must become more aware of WS
• Should not be reluctant to include WS in TAFs
  when conditions warrant.

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Bibliogrphy

• Julius Bander, Low-Level Wind Shear a Critical
  Review, NOAA Tech. Memo. NWS FCST-23, Apr 1979,
  reprinted Feb 1989
• NWS Weather Service Operations Manual , (WSOM)
  Chapter D-31, Issuance No. 97-5, June 6, 1997,

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They dont Like Surprises !!
What the ??
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Thank You

• The
• End