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1

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

2

Mountain wave terminology

-Orographic waves
-Standing Waves
-Lee waves
-Gravity waves

3

4 factors that affect the evolution of standing waves

-Obstruction
-Stable layer
-Strong wind
-Weak directional shear

4

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

5

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

6

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

7

4 factors that affect the evolution of standing waves

Weak directional shear

-uniform wind direction maintains orginization and prevents dispersal
-

8

Three cloud types associated with mountain waves when sufficient moisture is present

-Cap
-Rotor
-Lenticular (ACSL)

9

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

10

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

11

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

12

5 hazards to flying associated with mountain waves

-updrafts/downdrafts
-changes in pressure
-turbulence
-wind shear
-Obscured terrain

13

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

14

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

15

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

16

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