Wind Shear

[Ref ISBN978-1-56027-901-3, Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 12-11, 12-24]

Sudden, drastic change in wind speed and/or direction over a very small area

Wind shear (WS) can occur at any altitude and it is dangerous to an aircraft. WS can rapidly change the performance of the aircraft and disrupt the normal flight attitude.

 

Low-level wind shear (LLWS)

Low-level wind shear is commonly associated with following WX or phenomena:

  1. passing frontal system
  2. thunderstorms
  3. temperature inversions
  4. strong upper level wind (> 25KT)

 

LLWS is common behind a fast-moving cold front, where has gusty, turbulent winds and colder temperatures.

In another cases, when two high pressure systems are adjacent, the winds are almost in direct opposition to each other at the point of contact, and therefore, wind shifts are continually occurring within a front.

 

LLWS can be associated with thunderstorm. For example:

  1. “Roll cloud”
  2. “Gust fronts”

Roll cloud” on the leading edge of a storm marks the top of the eddies in this shear, and it signifies an extremely turbulent zone.

Gust fronts” often move far ahead (up to 15 miles) of the thunderstorm and it has associated precipitation.

What’s “Mountain Waves”?

 

 

The most severe type of low-level wind shear is microburst.

Microburst

Microburst is associated with convective precipitation into dry air at cloud base. Microburst activity may be indicated by an intense rain shaft at the surface but virga at cloud base and a ring of blowing dust is often the only visible clue.

Features of microburst –

  • horizontal diameter: 1-2 miles
  • nominal depth: 1000 feet

From the above diagram, during a takeoff into a microburst, the plane may first experience (1) increasing headwind, followed by (2) downdrafts and (3) a rapidly increasing tailwind. This is literally low-level wind shear. This can result in terrain impact or flight dangerously close to the ground.

Indeed, the downdraft could be up to 6000 fpm and the loss of headwind could be a range of 30 to 90 KT.

 

Detection of LLWS

Nowadays, there are various weather radar systems installed at major airports for LLWS and microburst alerting purposes – e.g LLWAS-NE (Network Expansion), TDWR, ASR-9 WSP.

 

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