Special Weather Phenomena
1- Thunderstorms, CB / TCU
Turbulence, hail, snow, lightning, sustained up- and downdrafts, icing conditions – are all present in thunderstorms. Up- and downdrafts extend far beyond the visible storm cloud. Severe turbulence can be expected up to some distance from large thunderstorms.
2- Downdraft and Microburst (MB)
A downdraft is a relative small scale downward current of air; often observed on the lee side of large objects restricting the smooth flow of the air or in precipitation areas in or near cumuliform clouds. Microburst are small-scale intense cold air downdrafts out of cumulus clouds or thunderstorm cells which, on reaching the surface, spread outward in all directions from the downdraft center. This causes the presence of vertical and horizontal wind shear, especially at low altitude within some 1000ft of the ground. Microburst either occur as wet microburst carrying precipitation to the ground or as dry microburst descending from cumulonimbi or towering cumuli with a high cloud base (around 10000ft), typically in desert regions.
The strong downdraft is typically less than 1NM in diameter; the horizontal outflow can extend to approx. 4NM in diameter.
The downdrafts can be as strong as 8000ft/MIN. Horizontal winds speeds near the surface can reach up to 100KT.
They may be embedded in heavy rain associated with convective clouds or little or no precipitation.
The main flight hazards of a microburst are:
• severe low level wind shear,
• the strong downdraft,
• severe turbulence at the gust front.
Other hazards found below thunderstorms may join the already critical situation, like heavy rain, hail, aquaplaning, lightning, and tornadoes. Wind shear may be horizontal, vertical wind shear, and up and downdraft shears. A flight through a microburst lasts only for seconds. The history of an aircraft encountering a microburst strongly depends on its path through the microburst and the phase of flight.
Microburst on final
A pilot penetrating a microburst during final approach will first experience a strong headwind, the aircraft first balloons above the glide slope due to air speed rise shear. Entering the downdraft there is air speed loss shear, downdraft shear, and the vertical displacement by the downdraft, which altogether leads to a dramatic loss of height. Finally a strong tailwind signifies another air speed drop shear. Additionally the aircraft may encounter turbulence at the gust front.
Microburst after Take-Off
An aircraft taking off in a microburst will first encounter a stronger headwind and thus a speed increase (1). After take-off, upon entering the tailwind area speed drops and the nose has to be lowered (2). In case of further downdraft activity the airplane may continue to sink and ground impact becomes imminent (3). Detection: Due to their small size, short lifespan and the fact that they often occur over areas without surface precipitation, microburst are not easily detectable using conventional weather radar or wind shear alert system. Look out for signs of convective weather, like CU, TCu, showers, and virga. Read (and issue) pilot reports, but do not rely on them too much, since the microburst changes by the minute. The bending of trees, dust, and sand in the air may be the only up to date information on the wind field around the aerodrome.
3- Mountain Waves (MTW)
Mountain waves occur when air is being blown over a mountain range. As the air hits the upwind side of the range, it starts to climb, thus creating a smooth updraft which turns into a turbulent downdraft as the air passes the crest of the ridge. From this point, for many miles downwind, there will be a series of down- and updrafts accompanied by moderate to severe turbulence.
4- Wind Shears
Wind shear conditions are normally associated with the following phenomena: • thunderstorms, microburst, tornado, and gusts. • frontal surfaces • mountain waves • strong wind coupled with local topography • see breeze fronts • low-level temperature inversions
An increase of temperature with height – a reversal of the normal decrease with height in the troposphere. In inversions windshear may be encountered.