Aeronautic wind shear and turbulence. A review for forecasts.

Typ: Rapport
Serie: Meteorologi 74
Författare: Andersson, Tage


'Turbulence' in flight operations means all inhomogeneities in the wind field which cause unexpected deviations from the aircraft's intended flight path. It utters itself as bumpiness, which in extreme cases also may cause severe discomfort to passengers and crew, and even be dangerous because persons and loose objects may be tossed around. Even more serious is that in extreme cases the pilot may lose his control of the aircraft, and/or the aircraft may be damaged.

This is most serious when an aircraft is heavy loaded and flying at a speed just above its stalling speed, as in take-off.

'Turbulence' is related to the shear of the wind. A sharp wind shear by itself affects the aircraft; due to its inertia the aircraft tries to keep its speed relative to the ground also when the surrounding air has changed it. This affects the lifting forece, causing an acceleration and a change in the aircraft's attitude.

lf the air flow is laminar, shear does not produce turbulence. However, even if turbulence needs not to be present in strong shear at low altitudes ( NCR, 1983, after Lee and Beckwith, 1981 ) laminary flow belongs to the laboratory, not the atmosphere. In this manual we will mainly talk about wind shear, since this is a meteorological parameter that at least in principle is possible to measure. It has to be understood that wind shear is nearly always accompanied by turbulence (or bumpiness).

Some forecasting hints will be given in the text. The most important one will, however, be given already here: