# Low Level Wind Shear (LLWS)

## Presentation on theme: "Low Level Wind Shear (LLWS)"— Presentation transcript:

Low Level Wind Shear (LLWS)
By Bob Jackson, MIC Seattle CWSU

We explain to each other
Introduction - 1 That which we don’t understand We explain to each other

Introduction - 2 Low Level Wind Shear (WS) occurs frequently
Can be overlooked when composing a TAF

Goal of Presentation Refresher Maintain/Heighten awareness of WS
Encourage application in aviation forecasting

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 When should a WS forecast be withdrawn?

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)

Two Types of WS: Convective WS Non-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

Define WS WS in this presentation refers to “Non-Convective” Low Level Wind Shear.

Define WS (continued) In order to be “Low Level”, it must occur at or below 2,000 ft AGL.

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.

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.

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.

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’

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.

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

NWP Products Surface Progs
‘Strong’ gradients are often ‘under forecast’ in prog runs.

NWP Products Numerical Guidance Products such as FRHTxx and FRHxx give
Surface pressure forecasts Boundary layer wind forecasts In need of Broad interpretation

Observations - These are NOT Forecasts!
ROABs PIREPs ACARs Soundings Surface analysis Not always timely METARs VAD profile

What are some SPECIFIC criteria for WS?
WSR-88D 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.

2,000 ft 1,000 ft METAR Z … KT…

Generally good for SEA, however, Lower level winds are affected by flow through the Strait of Juan De Fuca so don’t always represent conditions over SEA. East winds over the airport do not always displayed in VAD.

Generally good for SEA, however, Often usable for ATX, but not all sites. (MSX’s WSR-88D is on mountain and VAD winds begin at 9,000, so not usable for low level winds.)

ACARS Soundings

ACARS Wind Barbs

RAOBs Not always representative of desired airport
Considered as a “Snap Shot” of conditions Not always timely

RAOBs Rapid interpretation is Difficult ZCZC SEASGLUIL
TTAA00 KUIL 72797 TTBB = PPBB / // // / = ƒ NNNN

RAOBs Graphic display aids interpretation

RAOBs Organized data also aids interpretation

Review of Main Points - 1 Become aware of WS and the need to include in TAFs. Use meteorology, FORECAST WS when conditions are favorable.

Review of Main Points - 2 The TAF is a FORECAST not an Observation so,
If conditions are favorable, don’t wait for PIREPs of WS before putting WS in TAF.

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?

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?

Quiz Time ! ! ! ! How is WS encoded in the TAF? WShwshwshws/dddffKT
WS = Non-Convective Wind Shear hwshwshws = Ht of shear in 100’s of ft ddd = Direction ff = Speed

Quiz Time ! ! ! ! How is WS encoded in the TAF?
WS group follows the cloud forecast group. Remains in the forecast until the next ‘FM’ or the end of the forecast period if no ‘FM’ group follows.

Quiz Time ! ! ! ! In which groups can WS be included?
The initial forecast group and The ‘FM’ group

Quiz Time ! ! ! ! In which groups can WS NOT be included? BECMG TEMPO
PROB

Quiz Time ! ! ! ! How is CONVECTIVE WS encoded in the TAF?
You tell me…

Summary WS has large impact on aviation Forecasters
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.

Bibliography 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,

They don’t Like Surprises !!
What the * % ~ # ??

The End Thank You