Chapter 15 Turbulence CAT and Wind shear Definition

Chapter 15 Turbulence CAT and Wind shear

Definition of Turbulence • Turbulence: . – thus may be defined as airflow causing random deviations from the desired flight path needing immediate corrective action.

• Gustiness: Gustiness – is a fluctuating wind situation where the horizontal wind component is subject to variations. – The wind velocity is subject to gusts and lulls about the mean wind velocity. – The ‘gustiness of the wind is expressed by the percentage ratio between maximum and minimum horizontal wind speeds to the mean wind speed recorded in a given period. – Gusts and lulls are of short term duration lasting seconds rather than minutes. Gustiness can vary from 25% to 100%.

Gust Factor • If the mean wind speed is 30 KT with gusts up to 45 KT and lulls down to 15 KT then the range is 30 KT and the factor is 100%.

Causes of Turbulence • Friction Layer Turbulence: – can vary from a few hundred feet on a cold night up to 4000 to 5000 feet under very turbulent windy conditions where both Mechanical and Thermal turbulence can occur. • Mechanical Turbulence: – is caused by physical obstruction to the air flow by the presence of hills, trees, mountains, buildings and coast lines and causes eddying and waves to develop downwind. .

Causes of Turbulence • Thermal Turbulence: – is caused when convection currents initiated by strong surface heating interact with the normal horizontal motion of the air as well as mechanical turbulence

Cloud Turbulence • Cloud Turbulence: – Is caused by the vertical motion present in convective clouds. – It is most severe in thunderstorm clouds where the large scale release of latent heat within the clouds adds thermal energy to the interior of the cloud generating strong updraughts. – Severe turbulence from CB clouds can affect the clear air in the vicinity of the cloud itself – May overshoot into the stratosphere.

Orographic Turbulence • Low hills and buildings cause downstream turbulence and eddies with reverse flow in the immediate downstream area of the obstruction.

Clear Air Turbulence • Can occur near the tropopause, in the vicinity of jetstreams • Standing (Mountain) waves • Upper level troughs and ridges • Caused by changes in wind speed/direction

Clear Air Turbulence • May result in: – Stalling at low level – Jet upset at high level, including structural damage

Clear Air Turbulence, cont’d Standing Waves

Formation of CAT • • The shear along the boundary increases, Waves begin to form which are unstable and which eventually ‘break, ’ forming a turbulent eddy. These may be small or large depending on the amount of shear. When large waves appear they are frequently referred to as gravity waves and aircraft encountering these my experience sudden and large vertical displacements known as ‘air pockets’. • Unsecured objects and people can be thrown violently against the cabin roof on occasions.

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

Clear Air Turbulence – 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.

Wake Turbulence

Mountain Waves • Substantial terrain such as mountains and the right conditions Standing or Mountain Waves develop with a characteristic regular downwind wave pattern in the upper atmosphere but at the height of the terrain downwind rotors form with severe turbulence which can cause structural damage.

Standing Waves

Windshear

Definitions • Windshear is defined as: – variations in vector wind along flight path which, – causes abrupt displacement from the intended flight path such that, – substantial control action is required.

Definitions • 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.

VERTICAL WINDSHEAR

VERTICAL WINDSHEAR

VERTICAL WIND SHEAR HERE DOWN DRAUGHT WINDSHEAR BELOW BASE OF STORM HORIZONTAL WIND SHEAR HERE AT GUST FRONT

DOWN DRAUGHT/MICRO-BURST WIND SHEAR

COMBINED EFFECT OF MICRO-BURST WIND SHEAR

Main Causes of Windshear • CB/TS Activity • 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.

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 by 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.

WIND SHEAR ALERTING SYSTEM

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. • More info available from CAA - AIC