Mountain wave impact on flight conditions of high-flying aircraft

Kurzfassung

Ever since their discovery by glider pilots, mountain waves (MWs) are a well known atmospheric process to affect aviation as they can significantly modulate the atmospheric flow field on relatively short scales (λh ≈ 20 km). The goal of this thesis is to study the impact of such a flow field on high-flying aircraft (i.e. flight level (FL) > 20.000 ft = FL200). For that reason, two cases were studied exemplarily where MWs affected flying conditions of the High Altitude LOng Range Research Aircraft, HALO in different ways. In the first case stall warnings at FL 410 (12.5 km) occurred unexpectedly during a research flight of HALO over Italy on 12 January 2016. At the incident location, the stratosphere was characterized by large horizontal variations in the along-track wind speed and temperature. On this day, the general atmospheric circulation favored the excitation and vertical propagation of large-amplitude mountain waves at and above the Apennines, Italy. These mountain waves had achieved large vertical energy fluxes of 8 W m−2 and propagated without significant dissipation from the troposphere into the stratosphere. Strong turbulence was encountered by HALO at FL 430 (13.8 km) on 13 October 2016 above Iceland which constitutes the second case study. In this event the turbulence caused altitude changes of about 50m within about 15s of the research aircraft. Additionally, the automatic thrust control of HALO could not control the large gradients in the horizontal wind speed and, consequently, the pilot had to deactivate this system. On that day, MWs were excited and propagated vertically above Iceland. In the altitude region of the turbulence encounter the atmosphere was characterized by a pronounced negative vertical shear of the horizontal wind. Here, in situ observations together with simulations of the Eulerian semi-Lagrangian fluid solver (EULAG) suggest that HALO was flying through the center of a breaking MW field. First, the question whether aircraft speed is dominantly influenced by the temperature or the horizontal wind could be answered. Analysis of high-resolution in situ observations and recordings of HALO’s Quick Access Recorder (’blackbox’) suggests that it is the horizontal wind speed which dominantly impacts aircraft speed of high flying aircraft. Second, it was found that vertically propagating MWs can affect flight conditions of high-flying aircraft. While turbulence is a well-acknowledged hazard to aviation, the case studies reveal that non-breaking, vertically propagating mountain waves also pose a potential hazard by modulating the ambient along-track wind speed on scales for which the response time of the avionic system is too slow. This may lead on the one hand to a decrease of the aircraft speed towards the minimum needed stall speed or on the other hand to variations in the aircraft speed that cannot be controlled by the automatic thrust control. Furthermore, in situ observations are compared to European Centre for Medium- Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) forecasts and operational analyses. This comparison revealed that large-scale structures are predicted very well. However, on scales smaller than 5km observed amplitudes of all meteorological parameters are underestimated. Here, the application of the Graphical Turbulence Guidance Tool (GTG) proved to be valuable for predicting the correct magnitude and location of the maximum encountered turbulence above Iceland. However, the observed intermittency could not be reproduced and a tendency to overpredict turbulence was found.