Low Level Atmospheric Wind Shear




DATE: Tuesday, July 23, 2019 (12:21 UTC )
WIND SHEAR DEFINITION

ICAO defines wind shear in Doc 9817 to be: Low-level wind shear, in the broadest sense, encompasses a family of air motions in the lower levels of the atmosphere, ranging from small-scale eddies and gustiness that may affect aircraft as turbulence, to the large-scale flow of one air mass layer past an adjacent layer. Included among the wide variety of phenomena that produce such air motions are thunderstorms, land/sea breezes, low-level jet streams, mountain waves and frontal systems. In order to understand, in this context, the common denominator linking such varied phenomena, it is necessary to explain the meaning of the term “wind shear”. The most generalized explanation of wind shear is “a change in wind speed and/or direction in space, including updrafts and downdrafts”. From this explanation it follows that any atmospheric phenomenon or any physical obstacle to the prevailing wind flow that produces a change in wind speed and/or direction, in effect, causes wind shear.

Or more simply: Wind shear is a change in the wind speed or direction, horizontally or vertically.

To be significant the wind shear should have a sustained change in speed or direction and the magnitude of the change measured along the direction of the runway and should be at least 15 knots.

Common types of Wind Shear in UAE (Please click here to view images).

  1. Sea Breeze Wind Shear
  2. Sea Breeze Front Wind Shear
  3. Shamal/Land Breeze Wind Shear
  4. Low Level Jet Wind Shear
  5. Very Low Level Dubai Shear
IDENTIFICATION OF WIND SHEAR

Forecast From Numerical Models:

Most wind shear is typically at a scale less than the lowest resolution of the meteorological atmospheric models. Sometimes it is possible to identify wind shear from the models and the synoptic situation, but not always.

Multiple methods exist to observe wind shear:

  1. Pilot Reports

    This is the most common way for wind shear to be observed. It is very important that when these observations are taken by the pilot and passed to ATC they are done in a structured/informative manner as defined by ICAO to allow the ATC to pass this information to other pilots and the NCM meteorological office. The GCAA has recently issue a safety bulletin on this subject.
  2. Radiosondes

    Once every 12 hours a balloon with measurement equipment attached is released over Abu Dhabi airport. These radiosondes measure the atmosphere with respect to wind, temperature and humidity up to 50,000 ft. The lowest measurement available is commonly 1000 ft so sometimes this technology can identify the low level wind shear and sometimes not. Only two single measurements in a day is not the best way to identify quickly changing wind shear conditions.
  3. Aircraft Data

    Most modern aircraft measure and record winds and temperature through their flight. The data from all arriving and departing Etihad A330s and A320 aircraft are automatically downloaded and sent to the NCM weather office at Abu Dhabi in near real time. This aircraft data is decoded and automatically analysed for shear at the weather office.
  4. Wind Profiler

    Wind Profilers are installed at Abu Dhabi and Dubai airports. These profilers actively measures the winds in the lower 5000 ft of the atmosphere, the lowest measurement being near 500 ft. The wind profile is updated every 5 minutes. The software associated with this system automatically identifies areas of shear in the atmosphere. This methodology allows for the most of lower atmospheric data to be sent to the meteorologists, however the data is only at the location directly over the profiler.
  5. Doppler Weather Radar

    Rain contaminated winds can be ‘seen’ by doppler weather radar. Wind Shear typically can be seen in thunderstorms by weather radars but not other wind shear producing phenomena as no rain is present.
  6. LiDAR

    Light Detection and Ranging.

Observations available per Abu Dhabi Emirate Airport:

( Abu Dhabi International )
  • Pilot Report
  • Radiosonde
  • Aircraft Data
  • Wind Profiler
  • Doppler Weather Radar
( Al Ain International )
  • Pilot Report
  • Doppler Weather Radar
( Al Bateen Executive )
  • Pilot Report
  • Doppler Weather Radar

Observations available per Dubai Emirate Airport:

( Dubai International )
  • Pilot Report
  • Radiosonde
  • Aircraft Data (later this year)
  • Wind Profiler
  • Doppler Weather Radar
( Al Maktoum International )
  • Pilot Report
  • Doppler Weather Radar
WIND SHEAR WARNINGS / MESSAGES:

Within ICAO Annex 3, Chapter 7 and ICAO document 9817, wind shear and associated warnings and alerts are defined. Clearly defined within the ICAO documents is that the warnings should cover wind shear between the surface and 1600 ft AGL, and that the significant wind shear that should be indicated is that parallel to the runway - gain/loss of airspeed. Although not clearly defined within ICAO docs, a gain/loss of airspeed of 15 knots is referenced within ICAO and is used as a minimum warning criteria at multiple airports including ours. In ICAO Annex 3, Appendix 6, a template exists that describes how a WS warning should be written.

An example of our current WS warning:

WS IN APCH AND CLIMB-OUT ALL RWY

FCST

SFC WIND 340/10KT

1500FT WIND 350/25KT

A template example of our proposed new WS warning:

OMAA

WS WRNG 1

Datetime VALID TL Datetime

WS [IN APCH][IN CLIMB-OUT] [DUE TO][SEA BREEZE][SEA BREEZE FRONT][LOW LEVEL JET][LAND BREEZE][TS]

[REP AT 0000 Airplane Type][OBS AT 0000][FCST]

[Speed in kts][LOSS][GAIN][RWY xx]

* Brackets in [RED] are non standard new information.

Example #1 proposed new WS Warning

OMAA

WS WRNG 1

211230 VALID TL 211330

WS DUE TO SEA BREEZE

OBS AT 1228

30 KT GAIN RWY 31

Example #2 proposed new WS Warning

OMDB

WS WRNG 1

211230 VALID TL 211330

WS IN APCH DUE TO SEA BREEZE FRONT

FCST

25 KT GAIN RWY 13

Example #3 proposed new WS Warning

OMAA

WS WRNG 1

211230 VALID TL 211330

WS DUE TO LAND BREEZE

REP AT 1225 B747

40 KT LOSS RWY 13

*Updated: 19/12/2018