1 Low Level Wind Shear (LLWS) By Bob Jackson, MIC Seattle CWSU
2 Introduction - 1 That which we don’t understand We explain to each other
3 Introduction - 2 Low Level Wind Shear (WS) occurs frequently Can be overlooked when composing a TAF
4 Goal of Presentation Refresher Maintain/Heighten awareness of WS Encourage application in aviation forecasting
5 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 When should a WS forecast be withdrawn?
6 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)
7 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
8 Define WS WS in this presentation refers to “Non-Convective” Low Level Wind Shear.
9 Define WS (continued) In order to be “Low Level”, it must occur at or below 2,000 ft AGL.
10 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.
11 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.
12 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.
13 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’
14 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.
15 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
16 NWP Products Surface Progs –‘Strong’ gradients are often ‘under forecast’ in prog runs.
17 NWP Products Numerical Guidance Products such as FRHTxx and FRHxx give –Surface pressure forecasts –Boundary layer wind forecasts –In need of Broad interpretation
18 Observations - These are NOT Forecasts! ROABs PIREPs ACARs Soundings Surface analysis –Not always timely METARs VAD profile
19 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.
20 WSR-88D VAD Wind Profiles METAR 141756Z …. 19009KT… 1,000 ft 2,000 ft
21 WSR-88D VAD Wind Profiles 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,
22 WSR-88D VAD Wind Profiles 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.) Generally good for SEA, however,
28 RAOBs Organized data also aids interpretation
29 Review of Main Points - 1 Become aware of WS and the need to include in TAFs. Use meteorology, FORECAST WS when conditions are favorable.
30 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.
31 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?
32 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?
33 Quiz Time ! ! ! ! How is WS encoded in the TAF? WSh ws h ws h ws /dddffKT WS = Non-Convective Wind Shear h ws h ws h ws = Ht of shear in 100’s of ft ddd = Direction ff = Speed
34 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.
35 Quiz Time ! ! ! ! In which groups can WS be included? –The initial forecast group and –The ‘FM’ group
36 Quiz Time ! ! ! ! In which groups can WS NOT be included? –BECMG –TEMPO –PROB
37 Quiz Time ! ! ! ! How is CONVECTIVE WS encoded in the TAF? You tell me…
38 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.
39 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,
40 They don’t Like Surprises !! What the * % ~ # ??