Mountain-Wave Turbulence Encounter of the Research Aircraft HALO above Iceland

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.

Observation and simulation of mountain wave turbulence above Iceland: Turbulence intensification due to wave interference

“…As seen from the meteorological overview in Bramberger et al . (2020), the position of the jet core is slightly to the east of Iceland. The associated strong zonal gradient is thus not accounted for in the simulations.…”

“…A detailed analysis of the in situ data and the meteorological situation is presented in Bramberger et al . (2020). Here, we only briefly summarize the aspects which are important for the following analysis.…”

“…The simulations build upon the results presented in Bramberger et al . (2020). We advanced the model set‐up by extending the simulation domain to three dimensions and using a finer resolved orography.…”

“…">The orography data used in the study of Bramberger et al . (2020) has a rather coarse resolution (∼ 8 km). With this resolution, smaller mountains in the vicinity of Hofsjökull and Langjökull are not resolved, which could have lead to the mismatch between the observed and simulated turbulence position.…”

“…This turbulence encounter was already analyzed by Bramberger et al . (2020). They combined the HALO in situ observations and in situ observations from the coordinated flight of the French Falcon from SAFIRE (Service des Avions Français Instrumentés pour la Recherche en Environnement) with 2D numerical simulations along the flight track.…”