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Abstract
Strong turbulence was encountered by the German High-Altitude Long-Range Research Aircraft (HALO) at flight level 430 (13.8 km) on 13 October 2016 above Iceland. In this event the turbulence caused altitude changes of the research aircraft of about 50 m within a period of approximately 15 s. Additionally, the automatic thrust control of the HALO could not control the large gradients in the horizontal wind speed and, consequently, the pilot had to switch off this system. Simultaneously, the French Falcon of Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE), flying 2 km below HALO, also encountered turbulence at almost the same location. On that day, mountain-wave (MW) excitation and propagation was favored by the alignment of strong surface winds and the polar front jet. We use a combination of in situ observations, ECMWF and empirical turbulence forecasts, and high-resolution simulations to characterize the observed turbulent event. These show that a pronounced negative vertical shear of the horizontal wind favored overturning and breaking of MWs in the area of the encountered turbulence. The turbulent region was tilted upstream and extended over a distance of about 2 km in the vertical. The analyses suggest that HALO was flying through the center of a breaking MW field while the French Falcon encountered the lower edge of this region. Surprisingly, the pronounced gradients in the horizontal wind speeds leading to the deactivation of the automatic thrust control were located north of the breaking MW field. In this area, our analysis suggests the presence of gravity waves that could have generated the encountered modulation of the horizontal wind field.
Abstract
Strong turbulence was encountered by the German High-Altitude Long-Range Research Aircraft (HALO) at flight level 430 (13.8 km) on 13 October 2016 above Iceland. In this event the turbulence caused altitude changes of the research aircraft of about 50 m within a period of approximately 15 s. Additionally, the automatic thrust control of the HALO could not control the large gradients in the horizontal wind speed and, consequently, the pilot had to switch off this system. Simultaneously, the French Falcon of Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE), flying 2 km below HALO, also encountered turbulence at almost the same location. On that day, mountain-wave (MW) excitation and propagation was favored by the alignment of strong surface winds and the polar front jet. We use a combination of in situ observations, ECMWF and empirical turbulence forecasts, and high-resolution simulations to characterize the observed turbulent event. These show that a pronounced negative vertical shear of the horizontal wind favored overturning and breaking of MWs in the area of the encountered turbulence. The turbulent region was tilted upstream and extended over a distance of about 2 km in the vertical. The analyses suggest that HALO was flying through the center of a breaking MW field while the French Falcon encountered the lower edge of this region. Surprisingly, the pronounced gradients in the horizontal wind speeds leading to the deactivation of the automatic thrust control were located north of the breaking MW field. In this area, our analysis suggests the presence of gravity waves that could have generated the encountered modulation of the horizontal wind field.
Abstract
Stall warnings at flight level 410 (12.5 km) occurred unexpectedly during a research flight of the High Altitude and Long Range Research Aircraft (HALO) over Italy on 12 January 2016. The dangerous flight situation was mitigated by pilot intervention. At the incident location, the stratosphere was characterized by large horizontal variations in the along-track wind speed and temperature. On this particular day, strong northwesterly winds in the lower troposphere in concert with an aligned polar front jet favored the excitation and vertical propagation of large-amplitude mountain waves at and above the Apennines in Italy. These mountain waves carried large vertical energy fluxes of 8 W m−2 and propagated without significant dissipation from the troposphere into the stratosphere. While turbulence is a well-acknowledged hazard to aviation, this case study reveals that nonbreaking, vertically propagating mountain waves also pose a potential hazard, especially to high-flying aircraft. It is the wave-induced modulation of the ambient along-track wind speed that may decrease the aircraft speed toward the minimum needed stall speed.
Abstract
Stall warnings at flight level 410 (12.5 km) occurred unexpectedly during a research flight of the High Altitude and Long Range Research Aircraft (HALO) over Italy on 12 January 2016. The dangerous flight situation was mitigated by pilot intervention. At the incident location, the stratosphere was characterized by large horizontal variations in the along-track wind speed and temperature. On this particular day, strong northwesterly winds in the lower troposphere in concert with an aligned polar front jet favored the excitation and vertical propagation of large-amplitude mountain waves at and above the Apennines in Italy. These mountain waves carried large vertical energy fluxes of 8 W m−2 and propagated without significant dissipation from the troposphere into the stratosphere. While turbulence is a well-acknowledged hazard to aviation, this case study reveals that nonbreaking, vertically propagating mountain waves also pose a potential hazard, especially to high-flying aircraft. It is the wave-induced modulation of the ambient along-track wind speed that may decrease the aircraft speed toward the minimum needed stall speed.
