Low Level Wind Shear LLWS By Bob Jackson

Low Level Wind Shear (LLWS) By Bob Jackson, MIC Seattle CWSU 1

Introduction - 1 • That which we don’t understand We explain to each other 2

Introduction - 2 • Low Level Wind Shear (WS) occurs frequently • Can be overlooked when composing a TAF 3

Goal of Presentation • Refresher • Maintain/Heighten awareness of WS • Encourage application in aviation forecasting 4

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 When should a WS forecast be withdrawn? 5

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) 6

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 7

Define WS • WS in this presentation refers to “Non-Convective” Low Level Wind Shear. 8

Define WS (continued) • In order to be “Low Level”, it must occur at or below 2, 000 ft AGL. 9

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. 10

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. 11

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. 12

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’ 13

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. 14

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 15

NWP Products • Surface Progs – ‘Strong’ gradients are often ‘under forecast’ in prog runs. 16

NWP Products • Numerical Guidance • Products such as FRHTxx and FRHxx give – Surface pressure forecasts – Boundary layer wind forecasts – In need of Broad interpretation 17

Observations - These are NOT Forecasts! • • ROABs PIREPs ACARs Soundings Surface analysis – Not always timely • METARs • VAD profile 18

What are some SPECIFIC criteria for WS? • WSR-88 D 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. 19

WSR-88 D VAD Wind Profiles 2, 000 ft 1, 000 ft METAR 141756 Z …. 19009 KT… 20

WSR-88 D VAD Wind Profiles 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. 21

WSR-88 D VAD Wind Profiles Generally good for SEA, however, • Often usable for ATX, but not all sites. – (MSX’s WSR-88 D is on mountain and VAD winds begin at 9, 000, so not usable for low level winds. ) 22

ACARS Soundings 23

ACARS Wind Barbs 24

RAOBs • Not always representative of desired airport • Considered as a “Snap Shot” of conditions • Not always timely 25

RAOBs • Rapid interpretation is Difficult ZCZC SEASGLUIL TTAA 00 KUIL 061200 72797 TTBB 56120 72797 33859 00005 44853 01694 88729 00064 99610 11556 44315 46363 55277 51758 99100 58775 31313 05102 PPBB 24513 23538 28053 ƒ NNNN 56120 90789 23540 28535 72797 23514 92015 938// 90012 22515 25539 27548 00016 07205 55843 05095 11500 20756 66226 59359 81117= 11000 66837 22381 77186 17004 22517 26536 9503/ 19023 90346 19523 19521 24529 25536 25537 9167/ 926// 24547 93057 26043 32045= 18523 91234 24544 31543 06403 05295 35150 58974 22862 77767 33338 88165 00704 03495 42356 54376 26

RAOBs Graphic display aids interpretation 27

RAOBs Organized data also aids interpretation 28

Review of Main Points - 1 • Become aware of WS and the need to include in TAFs. • Use meteorology, FORECAST WS when conditions are favorable. 29

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. 30

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? 31

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? 32

Quiz Time ! ! • • • How is WS encoded in the TAF? WShwshwshws/dddff. KT WS = Non-Convective Wind Shear hwshwshws = Ht of shear in 100’s of ft ddd = Direction ff = Speed 33

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. 34

Quiz Time ! ! • In which groups can WS be included? – The initial forecast group and – The ‘FM’ group 35

Quiz Time ! ! • In which groups can WS NOT be included? – BECMG – TEMPO – PROB 36

Quiz Time ! ! • How is CONVECTIVE WS encoded in the TAF? • You tell me… 37

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. 38

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, 39

They don’t Like Surprises !! What the * % ~ # ? ? 40

The End Thank You 41