Introducing Turbulence (from Meted)
Turbulence at all levels of the atmosphere is a major concern
for the aviation industry. It often goes undetected in cloud-free areas,
catching pilots off guard when they fly into it. Turbulence can injure
passengers and crew and cause structural damage to aircraft.
Turbulence is part of the Aeronautical Meteorological
Forecaster (AMF) competency standards, which require that warnings be issued in
a timely manner when hazardous conditions, such as turbulence, are expected to
occur or when parameters are expected to reach documented threshold values.
Warnings also need to be updated or cancelled according to documented warning
criteria. When turbulence (moderate or greater) is forecasted, its onset and
duration, intensity, spatial extent, and type (orographic, mechanical,
convective, and clear air) need to be indicated.
Pilots often do not report turbulence to aviation forecasting
centers. However, advances in satellite imaging now make it possible to detect
atmospheric signatures that can help identify the likely presence of turbulence
without waiting for it to be reported. This is particularly helpful for clear
air turbulence, the most challenging type of turbulence to forecast and
therefore the focus of this lesson.
Turbulence is caused by marked changes in wind speed and/or
direction, either vertically or horizontally. Turbulence encountered by
aircraft in flight results in bumpy, uncomfortable, and sometimes dangerous
flying conditions.
According to the UK Met Office Handbook of Aviation
Meteorology, turbulence is categorized as light, moderate, or severe with the
following thresholds:
·
Light turbulence: 2.6 to
8m/s (5 to 15kt) aircraft airspeed fluctuations occur with gust
velocities on the order of 1.5 to 6 m/s (5 to 20 feet/second). Passengers may
be required to use seatbelts but loose objects within the aircraft remain at
rest.
·
Moderate turbulence: 8
to 13 m/s (15 to 25kt) airspeed fluctuations occur with 6 to 11 m/s (20
to 35 feet/second) gust velocities lasting approximately 11 minutes. Passengers
require seatbelts and are occasionally thrown against them. Loose objects move
about. Frequent rolling of aircraft occurs and it is hard to walk about in the
aircraft.
·
Severe turbulence:
Airspeed fluctuations greater than 13 m/s (25kt) occur with 11 to 30 m/s
(35 to 50 feet/second) gust velocities lasting approximately 7 minutes. The
aircraft may lose control momentarily, and it is difficult to maintain flight
altitude. Passengers are thrown violently in their seats, and loose objects are
tossed about.
This lesson focuses on moderate and severe turbulence, since
light turbulence is not considered an aviation hazard according to the
International Civil Aviation Organisation or ICAO.
The rest of this section describes the various types of
turbulence, including convective, mountain wave or orographic, and clear air
turbulence.
Convective activity is a common cause of turbulence.
Localized areas of ascending and descending vertical air lead to wind shear.
According to the WMO Education and Training Programme ETR-20 (WMO/TD-No. 1390)
on Aviation Hazards, windshear consists of layers or columns of air flowing
with different velocities (i.e. speed and/or direction) from adjacent layers or
columns." Any sudden crossing of the layers or column boundaries may be sensed
as turbulence, with corresponding consequences.
The following table describes the intensity of turbulence
typically associated with the various types of convective motion.
Regime
|
Vertical
velocity
|
Vertical
velocity
|
Vertical
velocity
|
Turbulence
|
m/s
|
~kt
|
~ft/min
|
||
Small medium cumulus
|
1-3
|
2-6
|
200-600
|
Light (fbl)
|
Towering cumulus (TCU)
|
3-10
|
6-20
|
600-2000
|
Moderate
|
Cumulonimbus (CB)
|
10-25
|
20-50
|
2000-5000
|
Severe
|
Severe storms
|
20-65
|
40-130
|
4000-13000
|
Extreme
|
Dry thermals
|
1-5
|
2-10
|
200-1000
|
Light/Moderate
|
Downdrafts
|
3-15
|
6-30
|
600-3000
|
Moderate/Severe
|
Downdrafts
|
Up to 25
|
Up to 50
|
Up to 5000
|
Extreme
|
Depending on a variety of factors, mountain waves may develop
on the leeward side of mountain barriers, resulting in turbulence close to the
ridges of the mountains. Wave motions continue for hundreds of kilometers
downstream. Note that clear air turbulence associated with mountain waves is
more severe and has a greater extent on the lee of mountain ridges than the lee
of isolated peaks.
Mountain waves, which are also known as gravity waves, need a
sufficiently strong wind component perpendicular to the top of the mountain to
develop. There are two types of mountain waves:
·
Trapped waves: These form when wind speeds
increase with height and/or a less stable layer overlies a stable layer. Wave
energy is trapped within the respective horizontal layers at mostly low levels
and propagates downwind.
·
Un-trapped waves: These waves, also known as
vertically propagating waves, develop under stable conditions and/or with low
wind speeds, particularly around wide mountains. The wave energy is propagated
upwards, and the waves can often be seen in the upper levels of the troposphere
and even the stratosphere.
This graphic below shows streamlines associated with a
vertically propagating mountain wave:
Hopkins (1977) described CAT forecasting techniques for the
North America Rocky Mountains, but the same principles can hold for other
mountainous regions. Usually, good indicators for forecasting mountain
wave-induced turbulence are:
·
Wind speed ≥ 20kt at any level from the ground
up to 500 hPa
·
Wind crossing the ridge line at an angle within
30 degrees of normal
·
Rapidly falling pressure to the lee of the
mountain
·
Temperature gradient > 5°C/100 km across or
along the mountain barrier
Clear air turbulence (CAT) is
the term used to describe middle- or high-level turbulence produced in regions
of marked windshear. As the name suggests, CAT forms in regions absent of
clouds, making it difficult to detect visually. However, the presence of
particular cloud signatures can often signify that CAT is present in adjacent
areas. We will examine these cloud signatures in the case study.
CAT can occur at any time of the year depending on the
positioning and orientation of synoptic circulation systems in the lower,
middle, and upper levels of the atmosphere. CAT is less likely to occur during
the summer months over South Africa since the upper westerly trough systems and
associated upper jet streams are not as well developed and occur less
frequently than during the rest of the year.
The table below describes common synoptic patterns and
features that result in windshear and various tools for identifying it. The
information comes from WMO ETR Programme ETR-20 (WMO/TD-No. 1390) on Aviation
Hazards (Empirical Forecasting Techniques for Clear Air Turbulence (CAT), at http://www.wmo.int/pages/prog/dra/etrp/documents/new_AVIATIONHAZARDS.pdf)
and the South African Weather Service.
Common Synoptic Patterns and Features
Resulting in Windshear
|
||
Synoptic Patterns
|
Synoptic Features
|
Tools for Identifying Features
|
On the cold (poleward)
side of a jet stream
|
Near and below the core
where the wind shear is greatest
|
NWP geopotential height
and wind fields (200/300 hPa)
|
On the warm (equator
ward) side of a jet stream, above the core
|
The stronger the jet,
the more likely the CAT
|
NWP geopotential height
and wind fields (200/300 hPa)
|
In developing upper
ridges
|
Where the speed of wind
around the ridge approaches its limit due to curvature
|
NWP geopotential height
and wind fields (200/300 hPa)
|
In sharp upper troughs
|
Where wind direction
changes abruptly
|
NWP geopotential height
and wind fields (200/300 hPa)
|
Regions of confluence
and diffluence
|
In jet streams (60% of
CAT occurs near jet streams)
|
NWP geopotential height
and wind fields (200/300 hPa)
|
May occur or be
intensified over a region of convection
|
Especially embedded
frontal convection
|
Satellite
imagery/aerological diagrams
|
Can occur above towering
cumulus or cumulonimbus (thermals/microbursts)
|
Where air is forced up
at a rapid rate
|
Satellite
imagery/aerological diagrams
|
Can occur in upper col
areas (a col is a pass between two mounain peaks or a gap in a ridge)
|
Weak winds but marked
direction changes
|
NWP wind fields (200/300
hPa)
|
Characteristic wave-like
pattern of cirrus clouds (billows)
|
Indicates a breakdown in
the turbulent flow
|
Satellite imagery (water
vapour)
|
Normally occurs from
2000 ft above to 6000 ft below the tropopause
|
Aerological diagrams
(tephigrams) and NWP tropopause height fields
|
|
CAT is rare above a
well-defined tropopause
|
Aerological diagrams and
NWP tropopause height fields
|
|
The following table provides windshear criteria for the
various levels of CAT. (The information is also from WMO/TD-No. 1390.)
Wind Shear Criteria
|
|||
Moderate CAT
|
Moderate to Severe CAT
|
Severe CAT
|
|
Vertical wind shear
|
≥6kt/1000 ft
|
≥ 9kt/1000 ft
|
|
Horizontal wind shear
|
≥ 20kt/1 degrees
latitude
|
≥ 30kt/1 degree latitude
|
|
Wind speed deceleration
|
>40kt /10 degrees
latitude
|
>60kt /10 degrees
latitude
|
>125kt /10 degrees
latitude
|
Wind direction shear
|
75 degree wind shift
near to a temperature gradient
|
||
Jet streams are characterized by wind motions that generate
strong vertical and horizontal shearing. Jet streams are the main source of CAT
experienced by aircraft at cruising altitudes. They can be thousands of
kilometres long, hundreds of kilometres wide, and a few kilometres deep.
In a study done by Briggs (1961) over the British Isles using
73 severe turbulence events, it was found that most CAT occurred with
upper-level troughs below the jet axis and on the low pressure side of the jet.
CAT also occurred above the jet axis with 15 and 10 events on the high and low
pressure side respectively. CAT was mostly found within 200 km of the jet
stream and 1500 to 4500 meters (5000 to 15000 ft) below the tropopause.
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 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:
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