Mountain waves arise from the oscillations of wind in the lee side of the highlands due to disturbance in airflow caused by the mountain. The amplitude and wavelength of the oscillations rely on several factors like wind speed, the height of the mountain relative to the nearby environment, and the atmosphere's instability. The following actions influence the formation of lee winds: the stable atmosphere above the peak of the highland with less steady air above and a firm layer below the highland. Besides, high wind speed at the top of the ridge increases with height, influencing the downwind. Lastly, the current direction is within 30 ° to the perpendicular of the highland, with no alternations in direction within the height of the ridge (Skybrary, 2020). Vertical winds within the airflow can go up to 2,000 ft. /minute. The integration of the surface friction with strong vertical oscillations may form rotors beneath the waves leading to severe turbulence. Lee winds are correlated with icing, acute turbulence, and powerful vertical oscillations. The vertical oscillations in the waves can cause level bursts in aircraft due to difficulties maintaining the air routes. Moreover, the currents can lead to loss of control due to fluctuations in wind speeds. Aircraft can lose control after taking off or near the surface before landing, increasing the risks of hard landing or ground conduct. Aircraft can experience structural damage after encountering severe turbulence. The aircraft can break up in extreme cases, and damage to fittings can occur in moderate turbulences. Furthermore, crew and passengers sustain injuries if caught unaware.
On December 20, 2008, the Boeing 737 experienced strong crosswinds during take-off at DIA. Winds encountered during the aircraft crash were from the airport's west side with significant temporal and spatial speed fluctuation. According to NTSB, the strong gust of westerly winds initiated the accident that led to the aircraft crash (Keller et al., 2015). The crew and passengers managed to evacuate quickly with no fatalities.
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References
Keller, T. L., Trier, S. B., Hall, W. D., Sharman, R. D., Xu, M., & Liu, Y. (2015). Lee waves associated with a commercial jetliner accident at Denver International Airport. Journal of Applied Meteorology and Climatology , 54 (7), 1373–1392. https://doi.org/10.1175/jamc-d-14-0270.1
Skybrary. (2020, September 1). Mountain Waves . https://www.skybrary.aero/index.php/Mountain_Waves.
