Mountain Waves
- Waves or ripples which form in the atmosphere caused by air encountering an obstruction or barrier
- large scale disturbance in the horizontal flow
Mountain wave terminology
- Orographic waves
- Standing Waves
- Lee waves
- Gravity waves
4 factors that affect the evolution of standing waves
- Obstruction
- Stable layer
- Strong wind
- Weak directional shear
4 factors that affect the evolution of standing waves
Obstruction
- disturbs horizontal air flow
- can extend 80 to 240 km from obstruction
- wave pattern can be produced along entire troposphere and into the stratosphere
- wavelengths between 5 and 25 km
- amplitude and strength are affected by leeside fall
- waves are strongest closest to obstruction and get weaker downstream
4 factors that affect the evolution of standing waves
Stable layer
- Stable layer must be present just above the crest of the obstruction
- Stable air resists the vertical motion of the wave
4 factors that affect the evolution of standing waves
Strong Wind
- minimum of 25 kts at the height of the peak, perpendicular to the peak
- 30 degrees to each side of perpendicular is the limit of deviation
4 factors that affect the evolution of standing waves
Weak directional shear
Three cloud types associated with mountain waves when sufficient moisture is present
- Cap
- Rotor
- Lenticular (ACSL)
Cap cloud
- orographic lift forms cap cloud on the windward side of the obstruction through adiabatic cooling
- dissipates by adiabatic heating
- can have appearance of waterfall
Rotor cloud
- form past obstruction as a result of turbulent mixing
- appear as a long line of SC parallel to the ridge, lying stationary
- base below peaks, tops extend above
- extremely turbulent, CBs TS may form
Lenticulars (ACSL)
- form in the same way as cap clouds, but following the wave length instead of obstruction
- adiabatic cooling on the windward side, adiabatic warming on the leeward side
- excellent indicator of mountain waves
5 hazards to flying associated with mountain waves
- updrafts/downdrafts
- changes in pressure
- turbulence
- wind shear
- Obscured terrain
5 hazards to flying associated with mountain waves
Updrafts/downdrafts
- downdrafts are most severe near obstruction, about the same height as the ridge top
- smooth, strong downdraft on the downwind side of ridge
- associated with a change in pressure, can be fatal
5 hazards to flying associated with mountain waves
Changes in pressure
- Convergence leads to funnelling, which leads to acceleration
- bernoulli says pressure decreases
- altimeters can read higher than what the aircraft is at, by as much as 1000m
5 hazards to flying associated with mountain waves
turbulence
- most severe associated with the rotor cloud
- as severe as any turb in the atmosphere
- can extend outside of cloud
- upper wave turbulence due to speed differential of air around lenticulars, MDT to SVR
5 hazards to flying associated with mountain waves
- Vertical windshear
- Obscured terain
- interaction between mountain wave and the jetstream can cause local breakdown of airflow into turbulence
- Cap cloud can obscure higher terrain
