TURBULENCE
Turbulence Types
1.
2.
3.
4.
Clear Air Turbulence (CAT)
Lee Wave/Mountain Waves
Mechanical
Shear
5. Low Level Wind Shear
(LLWS)
6. Convective Instability
7. Micorbursts/Downbursts
FORECASTING GUIDE ON TURBULENCE INTENSITY
Intensity
Vertical wind
Convective
Surface wind
Mountain waves
shear
cloud
Light
3-5 knots/ 1000
Fair wx CU,
> 15 knots and colder than the
feet
AC
underlying surface
Moderate
6-9 knots per
TS, CU, Tcu
> 25 knots or unstable
~5000 ft thick, 501000 feet
100mi downstream
Severe
>10 kts per 1000 Mature TS, CB,
NOT SPECIFIED
Near Rotor clouds
feet
Tcu
extend to ground
Extreme
NOT
Severe TS
NOT SPECIFIED
SPECIFIED
 MDT or greater turbulence under 24,000 ft forecasted in the GFA
 If severe or extreme at any level, it warrants a SIGMET
 Severe or extreme turbulence below 1,500 ft in the vicinity of an aerodrome should be
put as a remark in the TAF
Forecast tools
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
Pireps
Hodographs (wind shears)
Tephigrams (stability)
surface analyses for surface
roughness and wind speed

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Satellite imagery (transverse bands,
darken slot, deformation zone, lee
wave, lee cirrus).
Knowledge of ATSC processes
NWP guidance (m reg turb; DVSI)
Forecasting TB
1. Currently PIREPS? Characteristic cloud patterns? Previous TB forecasts?
Assess the relevance for your forecast area and time, use as guidance.
2. Air unstable? Convection expected? Can there be dry convection?
Consider gusty winds and mechanical TB, look LLWS with showery pcpn downdrafts.
3. Strong winds in PBL? Can instability, or channeling and other local effects increase them at the
surface? Friction and obstacles? Look LLWS once the air stabilizes at night. Consider
orographic Tb if mountains are present.
4. What LLWS exists at the station and upstream? Due to an inversion, a front, a downdraft? Can
LLWS arise or increase because of this? Actual wind shear is normally greater than observed
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because of resolution limitations. Associate it with the proper cause. Shear does not imply TB the
stability is strong enough.
5. Mountains in the area? Favorable wind and stability profiles? Characteristic wave clouds?
Orographic Tb, the likely increase in mechanical and CAT
6. Is the area within 3 degrees of latitude of a jet axis? Are characteristic cloud or upper flow
patterns observed? CAT.
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1. Convective Turbulence
The magnitude of the up and down drafts is the best indicator of turbulence intensity, can be
inferred from the cloud type and from energy considerations: greater of ΔT between a lifted
parcel and the environment (width of the positive area), the greater the vertical accelerations.
2. Microburst/Downburst


A horizontal “gust front” with strong wind maxima and intense shears in both the
horizontal and vertical directions.
Most dangerous during takeoff and landing (near airport)
3. Low Level Wind Shear
Occurs in the presence of:

a low level jet;

very stable airmass, often at night;

more often in the vicinity of mountainous terrain and valleys, but can happen over flat
terrain.
Suggested windshear values for SIGMETs and inclusion in TAFs:

25 kts shear in the first 500 feet (5 kts / 100 ft);

40 kts shear in the first 1000 feet (4 kts / 100 ft);

50 kts shear in the first 1500 feet (3.3 kts / 100 ft).

loss of indicated air speed of 20 kts or more in the first 1500 feet.
4. Shear Turbulence
Shear turbulence occurs in the presence of:
 A low level jet stream,
 Frontal surface aloft or a front at the
surface,
 Vicinity of mountain and valleys.




Associated with directional wind
shear.
neutral or slightly unstable airmass
Limited vertical extent.
More common in winter.
2
Intensity
Vertical wind shear/1000’
LGT
3-5 kts
MDT
6-9kts
SVR
10+ kts
5. Mechanical turbulence
Occurs in PBL (due to friction and obstacles), intensity and depth are a function of:

strength of low level and surface winds

terrain roughness

airmass instability & availability of heat sources

more intense if there is little directional shear
WIND (kts)
SEA
FLAT
HILLY
15 - 35
> 35
LGT - MDT
MDT - SVR
MDT
SVR
SVR
XTRM
6. Orographic Turbulence
Stable layer to top of mountain, wind within 30 degree of the perpendicular to the ridge
15-30+ kts: MDT to SVR CAT in lenticular clouds; SVR mechanical turbulence in
rotor clouds
7. Clear Air Turbulence
High level (500-200 mb) wind shear turbulence, associated with:
 Relatively wide, thick, wave-like
 Jet Streams > 110 kts, within 3
latitude on either side of the jet axis
transverse bands of cirrus clouds
 Gravity waves
 Dry slot
 Sharp decelerating anti-cyclonic wind
 Deformation zones
(>50 kts within 4 degree latitude)
 Upper troughs (cold advection)
 Speed convergence in the upper flow
CAT associated with cold advection in an upper trough near the jet stream core

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T gradient at least 5°C/200 km at 250 mb
Movement of the trough (at least 15 m/s)
Horizontal wind shear at least 20 m/s per
200 km in region of closely packed
isotherms;
A wind component greater than 30 m/s
normal to the region of closely packed
isotherms;
A sharp wind shift (at least 75°) in region
of closely packed isotherms
CAT associated with an upper-level ridge
3
Criteria for forecasting MDT to SVR CAT
a) Strong vertical wind shear (> 5m/s per 300 m);
b) Strong wind speeds (>70 m/s) in a region of large anticyclonic curvature; or
c) A large latitudinal displacement of a strong jet stream core with wind speeds >60 m/s.
d) CAT should be forecast in the region of sharpest anticyclonic curvature.
CAT associated with surface cyclogenesis
CAT associated with confluent jet-stream cores
When two confluent jet-stream cores are within 500 km of each other, CAT is most likely to
occur in the confluent zone between the two jet-stream cores from a point where the jet-stream
cores approach to within 5° latitude of each other to where the jet-stream cores begin to diverge.
Rules of Thumb for the production of moderate or greater CAT:
1) wind speed deceleration ≥ 50 kt within 4° lat
2) vertical wind shear ≥ 6 kt / 1000 ft; ≥ 15 kt/1000 ft
3) Richardson number < 1
4) thermal gradient ≥ 5°C/120 nm oriented across the flow
5) strong negative vorticity advection (NVA)
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