Check list mechanical turbulence

1. Check list  mechanical turbulence

Eirik Mikal Samuelsen, Steinar Skare, Rita Moi

Criteria for SEV TURB:

Ø SEV TURB is strong mechanical turbulence and not covered by SEV MTW.

Ø Typical in situations with unstable air masses and strong wind passing over terrain

Horizontal extension:

Ø Wind in 925/850 hPa >= 60KT (SEV)

Ø WIND in 925/850 hPa >= 40KT (MOD)

Ø Wind direction with strong wind over hilly terrain

Vertical extension:

Ø Upper level: About 1,5 x mountain height, typical FL080/120

Ø Lower level: SFC

NB. The criteria are just a first guess, an indication.

2. Check list CAT (not mountain waves)

Ø Issued as SEV TURB

Ø If CAT is because of mountain waves, then publish as SEV MTW (ref check list SEV MTW)

Ø Strong jet with significant wind shear.

Criteria for horizontal extension for SEV CAT:

Ø More than 60 m/s (120KT) in the jet  

AND

Ø Horizontal wind shear of minimum 30 m/s pr. 100km  (~60KT/latitude)

Ø Use quick menu CAT

Criteria for vertical extension:

Ø Upper limit: Tropopause in the warm air

Ø Lower limit: Tropopause in the cold air

Ø Please also check UK Met Office/Washington  significant weather charts for comparison/guidance

NB. The criteria are just a first guess, an indication.

Thresholds for MOD, MOD/SEV AND SEV CAT (first guess):

CAT	Horizontal wind shear	Wind speed in Jet maximum
MOD	> 20-25 m/s pr 100km	~ 80 - 100 knots
MOD/SEV	> 25-30 m/s pr 100 km	~ 100 - 120 knots
SEV	> 30 m/s pr 100 km	> 120 knots

 Also be aware of: 

Ø The strongest turbulence we find at the cold air side just above and just below the jet axis, meaning just close, but to the left of the jet axis relative to the wind direction.

Ø In a wavy jet, we find the most significant turbulence in the troughs, especially where we find the strongest changes in wind direction.

Ø It is not the wind speed itself which is responsible for the turbulence, but the wind shear.

Horizontal wind shear is defined as follows: