Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical. With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams. At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.
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:
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:
Clear Air Turbulence output includes non-orographic gravity-wave dissipation outside convection.
Clear air turbulence corresponds approximately to the eddy diffusion rate (EDR).
| CAT | EDR | ecChart colours (Fig8.1.14-1) |
|---|---|---|
| Weak | <0.05 m2/3 s-1 | Light blue |
| Moderate | 0.05 – 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.
Fig8.1.14-1: ecChart of energy dissipation rate. DT 12UTC 2 March 2023, VT 03UTC 3 March 2023. Colours given in Table81.14A.
Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical. With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams. At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.
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.
Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical. With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams. At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.
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/ensemble control 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).
Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical. With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams. At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.
Gravity waves can occur above and in the outflow region of convection, including also Kelvin Helmholtz 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 Helmholtz 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 Helmholtz instability above deep active convection is considered more likely.
Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical. With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams. At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.
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. The hodogram 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.
Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical. With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams. At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.
An Airbus A330 encountered severe turbulence while flying over the eastern coast of Greenland on 14 Nov 2024. The turbulence lay forward of a sharp upper trough and was likely driven by mountain waves triggered by Greenland’s rugged orography. Extreme vertical accelerations (decent -6800fpm and ascent 3800fpm) occurred and an in-flight shutdown of an engine was necessary followed by a return to Copenhagen. The IFS turbulence index forecast (colour shading) correctly identified the hazardous area with lead time of 16 hours.Also shown are IFS forecast wind trajectories for the flight level, also altitude and vertical speeds experienced by the aircraft.. Forecast VT16UTC 14 Nov 2024; DT00UTC 14 Nov 2024
Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical. With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.
(Note: In older material there may be references to issues that have subsequently been addressed)
Read more about forecasting clear air turbulence at ECMWF.
Read more about developing a clear air turbulence index at ECMWF Technical Memorandum No874
(FUG Associated with Cy49r1)