Mountain Wave, Lee Turbulence and a Downslope Windstorm
You are planning a VFR crossing of a north-south mountain range with peaks at 9,000 ft. The 700 hPa wind is 270/45 kt, and the 500 hPa wind is 290/70 kt, nearly perpendicular to the ridge. The air is stable below 10,000 ft but there is a moist layer near the ridge top. The forecast indicates a mountain wave with possible severe turbulence in the lee and a downslope windstorm on the eastern slopes. Your aircraft is unpressurised with a service ceiling of 12,000 ft. The route requires crossing at a pass with a minimum safe altitude of 10,500 ft.
1. What conditions in the setup indicate a mountain wave, and how would you expect the wave to affect the crossing?
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A mountain wave is indicated by a wind nearly perpendicular to the ridge (270 to 290 against a north-south range), wind speed increasing with height (45 kt at 700 hPa to 70 kt at 500 hPa), and a stable layer below the ridge with a moist layer near the ridge top. These are the classic ingredients: strong cross-ridge flow, vertical wind shear, and a stable layer that forces the air to oscillate over the barrier.
Effects on the crossing: expect strong updraughts and downdraughts on the windward and lee sides, severe turbulence in the lee, and a downslope windstorm on the eastern slopes. The downdraughts can exceed the aircraft's climb capability, so you may be unable to maintain the minimum safe altitude of 10,500 ft in the lee. The wave also produces lenticular cloud and rotors, which are hazardous. The unpressurised aircraft is limited to 12,000 ft, so you cannot climb above the wave; the only safe options are to cross at a pass where the wave is weakest, or delay until the wind decreases.
2. Given the service ceiling of 12,000 ft and the minimum safe altitude of 10,500 ft, what is your decision and what mitigations would you apply?
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The margin between the minimum safe altitude of 10,500 ft and the service ceiling of 12,000 ft is only 1,500 ft. In a mountain wave with severe lee turbulence and downdraughts, that margin is inadequate: a downdraught of 1,500 ft/min or more can push you below the minimum safe altitude before you can correct. The downslope windstorm on the eastern slopes makes the lee side especially hazardous.
Decision: do not attempt the crossing under these conditions. Delay until the cross-ridge wind decreases (a common rule of thumb is to avoid crossing when the 700 hPa cross-ridge wind exceeds about 25 to 30 kt, and here it is 45 kt), or route around the range at a lower altitude with terrain clearance, or cancel the VFR flight. If a crossing becomes unavoidable, cross at the pass with maximum terrain clearance, at the highest altitude the aircraft allows, and be prepared to turn back if downdraughts are encountered. Brief the possibility of rotor turbulence and do not rely on visual avoidance, since the wave is invisible except for lenticular cloud.