(Weather) List the categories of turbulence, the specific types of turbulence within these categories and the root causes of each.
Low-level turbulence (LLT)
- 1. Low-Level Turbulence (LLT)
- 2. Turbulence in and Near Thunderstorms (TNT)
- 3. Clear-Air Turbulence (CAT)
- 4. Mountain Wave Turbulence (MWT)
: Low-level turbulence is defined as that turbulence which occurs primarily within the atmospheric boundary layer. LLT includes mechanical turbulence
, thermal turbulence
, and turbulence in fronts
. Although wake turbulence
may be encountered at any altitude, it is particularly hazardous near the ground, so it is also considered with LLT.
: Over flat ground, significant LLT occurs when surface winds are strong, this is called mechanical turbulence. It occurs because friction slows the wind in the lowest layers causing the air to turn over in turbulent eddies. These eddies cause fluctuations (gusts) in winds and vertical velocities. As they become stronger, the mechanical turbulence extends to greater heights above the ground. The presence of obstructions such as buildings and stands of trees increase the effect of surface roughness and strengthen LLT and wind shear.
Thermal turbulence is LLT produced by dry convection in the boundary layer. Solar radiation heats the ground generating convection at the bottom of the boundary layer. During the morning and early afternoon, the convection intensifies and deepens. It reaches a maximum in the afternoon, and then gradually dies out as the earth’s surface cools. In contrast, where cool air moves over a warm surface, thermal turbulence can occur any time, day or night. Thermals are the basic elements of thermal turbulence and are an important source of LLT. They develop just above the ground because of uneven heating. Patterns of rising bubbles of warm air are initially somewhat chaotic, but soon take on distinct patterns. Thermal plumes, narrow curtains of rising air, and dust devils are common close to the ground, especially when the ground is very hot. All of these are LLT sources.
Turbulence in Fronts
: Fronts produce moderate or greater turbulence. In the boundary layer fast-moving cold fronts are usually steeper than at higher levels and updrafts may reach 1000 f.p.m in a narrow zone just ahead of the front. The intensity of the turbulence depends on the strength and speed of those fronts and any associated convection.
: Wake turbulence is created by an aircraft in flight and is considered as a form of mechanical turbulence. However, rather than the air blowing past an obstacle, the obstacle (in this case the wing of the aircraft) is moving through the air. The result is still the same; a turbulent wake is produced behind the obstacle. The term wake turbulence is applied to the vortices that form behind and aircraft that is generating lift.
Turbulence in and Near Thunderstorms (TNT):
TNT is that turbulence that occurs within developing convective clouds and thunderstorms, in the vicinity of thunderstorm tops and wakes, in downbursts, and in gust fronts. Turbulence within the thunderstorm cloud boundaries is caused by the strong updrafts and downdrafts. The most frequent and, typically, the most intense TNT is found within the cloud.
Clear-Air Turbulence (CAT)
: CAT is that turbulence which occurs in the free atmosphere away from any visible convective activity. CAT includes high level frontal and jet stream turbulence, typically above 15,000 feet MSL.
Mountain Wave Turbulence (MWT)
: MWT is that turbulence produced in connection with mountain lee waves. Wave action occurs throughout the depth of the lee wave system, but the worst turbulence occurs mainly in two well-defined layers: near the tropopause in the lee wave region and throughout the lower turbulent zone. Lee wave region: Lee waves are more often smooth than turbulent, but if turbulence does occur in the lee wave region, it is most likely to occur within 5,000 feet of the tropopause. This happens because the winds reach maximum speeds near the tropopause, with vertical shears above and below that level. Mountain wave activity strengthens the shear, promoting the development of shearing-gravity waves, especially near stable layers. Lower turbulent zone: The lower turbulent zone is the boundary layer in the lee wave system. Strong winds and wind shears produce widespread turbulence there. In the typical case the worst turbulence occurs along the lee slopes of the mountain, below the first lee wave trough, and in the rotor.