Transcript Turbulence - meteonaa.esy.es

Clear Air
Turbulence
Clear Air Turbulence
• Frictional stress caused by rapid changes of windspeed or wind
direction (windshear) causes three dimensional eddies.
– Near the tropopause.
– Close to jet streams, particularly on the cold polar airmass side.
– Jet stream CAT can be encountered above the tropopause boundary
– anywhere the vertical change in wind speed exceeds 5 knots/1000 ft or
the horizontal change exceeds 20 knots/60 nm.
– In upper troughs and ridges, particularly where they are rapidly
developing.
– In the lee of mountains, under certain conditions - standing waves.
CAT
• Defined as:
– Air turbulence of a type other than that
associated with airflow close to rough ground
or that encountered in or near convective cloud.
– It does not always occur in clear air and may be
associated with thin cirrus.
Incidence:
OCCASIONAL - less than 1/3 of the time
INTERMITTENT - 1/3 to 2/3
CONTINUOUS - more than 2/3
Intensity
Aircraft Reaction
(transport sized aircraft)
Turbulence that momentarily causes
slight, erratic changes in altitude and/or
attitude (pitch, roll, yaw). IAS fluctuates
5-15 kt. (<0.5 g at a/c C. of G.)
Report as 'Light Turbulence'
LIGHT
Reaction Inside Aircraft
Occupants may feel a slight strain against
seat belts or shoulder straps. Unsecured
objects may be displaced slightly. Food
service may be conducted and little or no
difficulty is encountered in walking
or
turbulence that causes slight, rapid and
somewhat rythmic bumpiness without
appreciable changes in altitude or
atttude. No IAS fluctuations.
Report as "Light Chop'.
Turbulence that is similar to Light
Turbulence but of a greater intensity.
Changes in altitude and/or attitude
occur but the aircraft remains in positive
control at all times. IAS fluctuates 15-25
kt. ( 0.5- 1.0 g at a/c C. of G.)
Report as 'Moderate Turbulence'
Occupants feel definite strains against seat
belts or shoulder straps. Unsecured
objects are dislodged. Food service and
walking are difficult.
MODERATE
or
turbulence that is similar to Light Chop
but of greater intensity. It causes rapid
bumps or jolts without appreciable
changes in aircraft altitude or attitude.
IAS may fluctuate slightly. Report as
'Moderate Chop'.
SEVERE
Turbulence that causes large, abrupt
changes in altitude and/or attitude.
Aircraft may be momentarily out of
control. IAS fluctuates more than 25 kt.
(>1.0 g at a/c C. of G.)
Report as 'Severe Turbulence'.
Occupants are forced violently against
seat belts or shoulder straps. Unsecured
objects are tossed about. Food service
and walking impossible.
Windshear
Definitions
• Windshear is defined as:
– variations in vector wind along flight path
which,
– causes abrupt displacement from the intended
flight path such that,
– substatial control action is required.
• Low Level Windshear is:
– windshear encountered during final approach,
– along the runway,
– along take-off flight path,
– along the initial climbout flight path.
Types of Windshear
• Vertical Windshear:
– change of horizontal vector wind with vertical
change of aircraft position.
• Horizontal Windshear:
– change of horizontal vector wind with
horizontal change of a/c position.
• Down-draught Windshear:
– change in the vertical component of the wind
with horizontal change in position.
Main Causes of Windshear
• CB/TS Activity:
– see Section 8 of Met Notes.
• Frontal Surfaces:
– Warning signs are:
• Sharp changes in wind direction.
• Temperature difference of 5°C or more across a
front.
• Frontal speed 30 kt or more.
– Cold fronts windshear is just behind surface
position of front.
– Warm front windshear is just ahead of
surface position of front.
Main Causes of Windshear (cont.)
• Inversions:
– A low level wind maximum occurs just below
the top of a strong night time or frontal
inversion.
– The effect is sometimes to cause a decoupling
of the surface wind from the prevailing
gradient wind.
– Thus air at the surface may be slow moving or
stagnant and fast moving aloft giving a
pronounced windshear across the interface.
• See MET AIP 0-9, 0-10 insert at back of
Met Practical notes.
Main Causes of Windshear (cont.)
• Turbulent Boundary Layer:
– Strong surface winds with gusts and lulls
produce horizontal wind shear.
– Thermal turbulence in hot countries due to
intense surface heating gives strong up/down
draughts.
• Topographical Windshear:
– “Wind shadow” caused my topographical or
man-made objects,
• ravine winds
• rotors
• lee side wind of hills, tree lines.
Windshear Warnings & Reporting.
• Warnings reported in following ways:
– Met warning,
– ATS warning
– Departure/ Approach ATIS,
– Pilot warning,
– On-board equipment.
• NOTE: Pilots encountering windshear on
approach or climb-out are required to
make a report to ATC as soon as possible
even if previously encountered or f/cast.
Windshear Alerts
• Issued when:
– A marked temperature inversion of 10° or
more exists between surface and 1000 ft. or
– Mean surface wind exceeds 20 kt. or
– Vector difference between mean surface wind
and gradient wind at 2000’ exceeds 40 kt. or
– TS or heavy showers are within 5 nm of
airport.