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FLIGHT HAZARDS OF MOUNTAIN WAVES AND WIND EVENTS. Stan Rose National Weather Service, Pueblo, Colorado. Roughly one-half of the air accidents in vicinity of the Rocky Mountains are Weather Related. Mountain Turbulence: Convection. Stan Rose 2008. Convection: Thermally Generated flows.

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flight hazards of mountain waves and wind events

FLIGHT HAZARDS OFMOUNTAIN WAVES ANDWIND EVENTS

Stan Rose National Weather Service, Pueblo, Colorado

roughly one half of the air accidents in vicinity of the rocky mountains are weather related
Roughly one-half of the airaccidents in vicinity of the Rocky Mountains are Weather Related.
gap winds
Gap Winds
  • Strong winds flowing through gaps or passes.
  • Strongest winds found in the exit of the gap.
  • Pressure difference from one side to another is an indicator.
  • A reverse in expected pressure difference can indicate reversed flow, a possible hazard.
what determines how wind behaves when it encounters terrain
What determines how wind behaves when it encounters terrain?
  • The Nature of the obstacle (height, width)
  • The direction and strength of the wind (kinetic energy)
  • The Stability of the atmosphere (potential energy)
stability and turbulence unstable

Stability and turbulence: UNSTABLE

Only moderate cross-barrier flow needed.

Unstable air : if unstable air crosses a mountain barrier, turbulence is almost certain.

Cumulus clouds are good indicators.

Leeside downdrafts can be strong, but generally don’t extend far beyond the range.

stability and turbulence stable

Stability and turbulence: STABLE

Strong cross-barrier flow needed.

Cloud rows and lenticular clouds are indicators.

Rotor clouds can form in the lee, downstream, below ridge-top.

Turbulence can be found well downstream.

a common meteorological index the froude number
A common meteorological index:The Froude Number
  • A Ratio, Similar to KE/PE or Velocity/Stability
  • If F > 1, Flow moves over the obstacle (strong flow, unstable conditions)
  • If F < 1, Flow is blocked. (weak flow, stratified atmosphere).
  • F = 1, a resonance (favorable for windstorms)
lenticular clouds
Lenticular clouds
  • Indicative of high winds aloft (jet stream)

Dry air

Moist air

severe down slope winds

Severe Down-slope Winds

Most common in Winter Months.

Can occur night or day: Often night

Downdrafts of 50+ fps

severe down slope winds significant indicators

Severe Down-slope Winds:Significant Indicators

Winds at mountain top generally > 30 Kts.

Jet stream in the vicinity.

Frontal passage can precede SDW.

windstorms
WINDSTORMS
  • Most often occur in the presence of a CRITICAL LAYER.
critical layer reverse shear
CRITICAL LAYER: Reverse Shear
  • Winds decrease with height (Shear is zero)
critical layer change in stability
CRITICAL LAYER: Change in Stability
  • A Mountain top Stable Layer.
  • Decreasing stability with height.
model forecast data
Model Forecast Data

NAM 12km 27h forecast; 10/10/2008 18Z valid 21Z 10/11/2008

Sea Level Pressure or 850mb Charts

observational data
Observational Data

(adds.aviationweather.gov)

observational data1
Observational Data

Satellite imagery (wave clouds/cloud streets, lee drying)

observational data2
Observational Data

Satellite imagery (wave clouds/cloud streets, lee drying)

observational data3
Observational Data

Satellite imagery (wave clouds/cloud streets, lee drying)

crossing mountains
Crossing Mountains
  • Climb to 3,000+ feet above ridge-tops.
  • Begin ascent 100+ miles from the mountains.
  • Be prepared to avoid the obstacle entirely.
  • Approach ridges/passes at 45 degree angle.
  • Don’t fly when there are widespread lenticulars!