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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.

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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.

An example of gravity wave turbulence

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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 (>0.3m^2/3 s^-1) but it is unclear which effects were the stronger factors.  Kelvin Helmholts 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.  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. 

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.

Additional Sources of Information

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