Windshear
Definitions Windshear is defined as: Low Level Windshear is: 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: Horizontal 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 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
VERTICAL WINDSHEAR
VERTICAL WINDSHEAR
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 (Handout)
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.