Overview

Turbulence, particularly clear air turbulence, is a significant and often unexpected hazard to aircraft during flight.  Turbulent eddies with wavelengths of 10 m to 1 km are felt as bumpiness, say by aircraft flying at ~250 m s-1. Aircraft are more sensitive to vertical gusts than to lateral gusts.

Clear air turbulence is often caused by:

  • shear instabilities (Kelvin–Helmholtz instabilities).
  • upper-level fronto-genesis.
  • large-amplitude mountain waves.
  • breaking gravity waves generated from convection.

Eddy dissipation rate (EDR) is the International Civil Aviation Organization (ICAO) standard for aircraft reporting of turbulence and hence is also the standard measure for clear-air turbulence (CAT).  The units of EDR are m2/3 s-1.

In the IFS the energy dissipation rate is derived from:

  • vertical wind shear and the total deformation (Ellrod1 index).
  • subgrid contribution to gravity wave drag by breaking convectively generated gravity waves.
  • tendencies for horizontal momentum. This has contributions from
    • vertical diffusion scheme. This includes:
      • dissipation due to turbulent mixing.
      • orographic wave drag.
      • orographic blocking.
    • dissipation due to convective momentum transport.


Clear air turbulence corresponds approximately to the eddy diffusion rate.

CATEDR

ecChart colours

(Fig8.1.14-1)

Weak<0.05 m2/3 s-1

Light blue


Moderate0.05 – 0.3 m2/3 s-1

Blue, dark blue,

green, yellow 

Strong to severe >0.3 m2/3 s-1

Orange, dark orange,

red, dark brown, purple

Severe (observed example) >0.5 m2/3 s-1

Light purple and clear


Table8.1.14-1: Relationship between CAT descriptions and eddy dissipation rate.


An example of clear air turbulence

Fig8.1.14-1: ecChart of energy dissipation rate. DT 12UTC 2 March 2023, VT 03UTC 3 March 2023.  Colours given in Table81.14A.


Fig8.1.14-2: ecChart of 300 hPa contours (dam).  300hPa winds (kt) shown as arrows and isotachs (coloured). DT 12UTC 2 March 2023, VT 03UTC 3 March 2023. Severe CAT values of EDR to the east of the strongest jet and extend into the unshaded area enclosing winds below 50kt.  Pin shows Dallas, Texas.  Severe turbulence was encountered by an aeroplane in this system causing emergency diversion to an available airfield.


Fig8.1.14-3:  ecChart of forecast clear air turbulence.  DT 12UTC 2 March 2023, VT 03UTC 3 March 2023.   A strong anticyclonic jet  flow is often associated with clear air turbulence.  In this case the jet streak reached 265 mph on 250 hPa.  The short range HRES forecast verifies quite well, but note the strong model winds exceeded the colour range (ecChart wind speed colour scheme does not go above 100 m s-1).



An example of gravity wave turbulence

Gravity waves can occur above and in the outflow region of convection, including also Kelvin Helmholts instability.

Fig8.1.14-4: IFS forecast CAT chart (top) and satellite visible imagery (bottom) illustrating violent turbulence encountered by an inter-continental flight 21 May 2024. The aircraft suffered strong descents and ascents over a short period of time necessitating an emergency diversion. The aircraft might have encountered strong convective draughts, but there are numerous studies and literature about gravity waves above and in the outflow region of convection, including also Kelvin Helmholts instability.  The CAT forecast indicated local strong to severe turbulence (>0.3m2/3 s-1) but it is unclear which effects were the stronger factors.  Kelvin Helmholts instability above deep active convection is considered more likely.


 

Fig8.1.14-5: IFS forecast vertical profile within the red circle shown in Fig8.1.14-4, DT 00UTC 21 May 2024, VT 06UTC 21 May 2024.  This supports CAT above and around the outflow associated with deep convection.  There is weak stability in the upper troposphere and a wind maximum in the vertical which supports breaking strong gravity waves.  IFS CAT accounts for this which is an important contribution in the tropics.


Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

ECMWF Newsletter 168

ECMWF Technical Memorandum No874