Gravity waves excited during a minor sudden stratospheric warming

An exceptionally deep upper-air sounding launched from Kiruna airport (67.82∘ N, 20.33∘ E) on 30 January 2016 stimulated the current investigation of internal gravity waves excited during a minor sudden stratospheric warming (SSW) in the Arctic winter 2015/16. The analysis of the radiosonde profile revealed large kinetic and potential energies in the upper stratosphere without any simultaneous enhancement of upper tropospheric and lower stratospheric values. Upward-propagating inertia-gravity waves in the upper stratosphere and downward-propagating modes in the lower stratosphere indicated a region of gravity wave generation in the stratosphere. Two-dimensional wavelet analysis was applied to vertical time series of temperature fluctuations in order to determine the vertical propagation direction of the stratospheric gravity waves in 1-hourly high-resolution meteorological analyses and short-term forecasts. The separation of upward- and downward-propagating waves provided further evidence for a stratospheric source of gravity waves. The scale-dependent decomposition of the flow into a balanced component and inertia-gravity waves showed that coherent wave packets preferentially occurred at the inner edge of the Arctic polar vortex where a sub-vortex formed during the minor SSW

Does Strong Tropospheric Forcing Cause Large-Amplitude Mesospheric Gravity Waves? A DEEPWAVE Case Study

On 4 July 2014, during the Deep Propagating Gravity Wave Experiment (DEEPWAVE), strong low-level horizontal winds of up to 35 m s−1 over the Southern Alps, New Zealand, caused the excitation of gravity waves having the largest vertical energy fluxes of the whole campaign (38 W m−2). At the same time, large-amplitude mesospheric gravity waves were detected by the Temperature Lidar for Middle Atmospheric Research (TELMA) located at Lauder (45.0°S, 169.7°E), New Zealand. The coincidence of these two events leads to the question of whether the mesospheric gravity waves were generated by the strong tropospheric forcing. To answer this, an extensive data set is analyzed, comprising TELMA, in situ aircraft measurements, radiosondes, wind lidar measurements aboard the DLR Falcon as well as Rayleigh lidar and advanced mesospheric temperature mapper measurements aboard the National Science Foundation/National Center for Atmospheric Research Gulfstream V. These measurements are further complemented by limited area simulations using a numerical weather prediction model. This unique data set confirms that strong tropospheric forcing can cause large-amplitude gravity waves in the mesosphere, and that three essential ingredients are required to achieve this: first, nearly linear propagation across the tropopause; second, leakage through the stratospheric wind minimum; and third, amplification in the polar night jet. Stationary gravity waves were detected in all atmospheric layers up to the mesosphere with horizontal wavelengths between 20 and 100 km. The complete coverage of our data set from troposphere to mesosphere proved to be valuable to identify the processes involved in deep gravity wave propagation

Gravity-wave-induced cross-isentropic mixing: a DEEPWAVE case study

Orographic gravity waves (i.e., mountain waves) can potentially lead to cross-isentropic fluxes of trace gases via the generation of turbulence. During the DEEPWAVE (Deep Propagating Gravity Wave Experiment) campaign in July 2014, we performed tracer measurements of carbon monoxide (CO) and nitrous oxide (N2O) above the Southern Alps during periods of gravity wave activity. The measurements were taken along two stacked levels at 7.9 km in the troposphere and 10.9 km in the stratosphere. A detailed analysis of the observed wind components shows that both flight legs were affected by vertically propagating gravity waves with momentum deposition and energy dissipation between the two legs. Corresponding tracer measurements indicate turbulent mixing in the region of gravity wave occurrence. For the stratospheric data, we identified mixing leading to a change of the cross-isentropic tracer gradient of N2O from the upstream to the downstream region of the Southern Alps. Based on the quasi-inert tracer N2O, we identified two distinct layers in the stratosphere with different chemical composition on different isentropes as given by constant potential temperature 2. The CO–N2O relationship clearly indicates that irreversible mixing between these two layers occurred. Further, we found a significant change of the vertical profiles of N2O with respect to 2 from the upstream to the downstream side above the Southern Alps just above the tropopause. A scale-dependent gradient analysis reveals that this cross-isentropic gradient change of N2O is triggered in the region of gravity wave occurrence

Mesoscale fine structure of a tropopause fold over mountains

We report airborne remote-sensing observations of a tropopause fold during two crossings of the polar front jet over northern Italy on 12 January 2016. The GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) observations allowed for a simultaneous mapping of temperature, water vapour, and ozone. They revealed deep, dry, and ozone-rich intrusions into the troposphere. The mesoscale fine structures of dry filaments at the cyclonic shear side north of the jet and tongues of moist air entraining tropospheric air into the stratosphere along the anticyclonic shear side south of the jet were clearly resolved by GLORIA observations. Vertically propagating mountain waves with recorded temperature residuals exceeding ±3 K were detected above the Apennines. Their presence enhanced gradients of all variables locally in the vicinity of the tropopause. The combination of H2O−O3 correlations with potential temperature reveals an active mixing region and shows clear evidence of troposphere-to-stratosphere and stratosphere-to-troposphere exchange. High-resolution short-term deterministic forecasts of ECMWF\u27s integrated forecast system (IFS) applying GLORIA\u27s observational filter reproduce location, shape, and depth of the tropopause fold very well. The fine structure of the mixing region, however, cannot be reproduced even with the 9 km horizontal resolution of the IFS, used here. This case study demonstrates convincingly the capabilities of linear limb-imaging observations to resolve mesoscale fine structures in the upper troposphere and lower stratosphere, validates the high quality of the IFS data, and suggests that mountain wave perturbations have the potential to modulate exchange processes in the vicinity of tropopause folds

The North Atlantic Waveguide and Downstream Impact Experiment

The North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) explored the impact of diabatic processes on disturbances of the jet stream and their influence on downstream high-impact weather through the deployment of four research aircraft, each with a sophisticated set of remote sensing and in situ instruments, and coordinated with a suite of ground-based measurements. A total of 49 research flights were performed, including, for the first time, coordinated flights of the four aircraft: the German High Altitude and Long Range Research Aircraft (HALO), the Deutsches Zentrum für Luft- und Raumfahrt (DLR) Dassault Falcon 20, the French Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE) Falcon 20, and the British Facility for Airborne Atmospheric Measurements (FAAM) BAe 146. The observation period from 17 September to 22 October 2016 with frequently occurring extratropical and tropical cyclones was ideal for investigating midlatitude weather over the North Atlantic. NAWDEX featured three sequences of upstream triggers of waveguide disturbances, as well as their dynamic interaction with the jet stream, subsequent development, and eventual downstream weather impact on Europe. Examples are presented to highlight the wealth of phenomena that were sampled, the comprehensive coverage, and the multifaceted nature of the measurements. This unique dataset forms the basis for future case studies and detailed evaluations of weather and climate predictions to improve our understanding of diabatic influences on Rossby waves and the downstream impacts of weather systems affecting Europe

Mountain wave impact on flight conditions of high-flying aircraft

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.Seitdem Segelflieger Gebirgswellen entdeckt haben, sind diese ein wohl-bekanntes Phänomen der Atmosphäre, weil sie die atmosphärische Strömung auf relativ kurzen horizontalen Skalen (λh ≈ 20 km) maßgeblich beeinflussen. Das Ziel dieser Arbeit ist es, den Einfluss von Gebirgswellen auf hochfliegende Flugzeuge zu untersuchen(Fluglevel (FL) > 20000 ft (= FL200)). Deshalb werden zwei Fälle untersucht, in welchen Gebirgswellen den Flugzustand des Forschungsflugzeuges HALO (High Altitude LOng Range Research Aircraft) auf verschiedene Art beeinflussten. In der ersten Fallstudie werden unerwartete Warnungen vor Strömungsabriss (stall) untersucht, welche während eines Forschungsfluges von HALO am 12. Januar 2016 in 12.5km Höhe (FL410) über Italien auftraten. Am Ort des Zwischenfalls war die Stratosphäre geprägt von großen horizontalen Variationen in der Temperatur und der Komponente des Horizontalwindes entlang des Flugzeuges. An diesem Tag begünstigte die atmosphärische Grundströmung die Anregung und Ausbreitung von Gebirgswellen an und über den Apenninen, Italien. Diese Gebirgswellen hatten große vertikale Energieflüsse von 8 W m−2 und breiteten sich ohne nenneswerte Dissipation von der Troposphäre bis in die Stratosphäre aus. In der zweiten Fallstudie trat starke Turbulenz bei einem Forschungsflug von HALO am 13. Oktober 2016 über Island auf. Bei diesem Ereignis erfuhr das Forschungsflugzeug Höhenänderungen von ca 50 m innerhalb von ca 15 s. Zusätzlich konnte die automatische Schubkontrolle von HALO die großen Gradienten im Horizontalwind nicht ausregeln, weshalb der Pilot dieses System abschalten musste. An diesem Tag breiteten sich die angeregten Gebirgswellen vertikal über Island aus. Im Höhenbereich des Turbulenzereignisses war die Atmosphäre durch eine starke negative Vertikalscherung des Horizontalwindes geprägt, welche das Brechen von Wellen begünstigt. Messungen und Simulationen von EULAG (Eulerian semi-Lagrangian fluid solver) legen nahe, dass HALO durch das Zentrum eines Wellenbrechungsgebietes flog. Durch die Analyse von hoch aufgelösten in situ Messungen und Aufzeichnungen des ’Quick Access Recorder’ (”Blackbox”) von HALO konnte der Horizontalwind als maßgeblicher atmosphärischer Einfluss auf die Geschwindigkeit von hochfliegenden Flugzeugen für diesen Fall identifiziert werden. Desweiteren wurde herausgefunden, dass vertikal propagierende Gebirgswellen den Flugzustand eines hoch fliegenden Flugzeuges beeinflussen. Während Turbulenz eine anerkannte Gefahr für den Luftverkehr ist, zeigen die Fallstudien, dass nicht brechende, sich vertikal ausbreitende Gebirgswellen auch eine Gefahr darstellen, indem sie das Horizontalwindfeld auf Skalen modulieren, die durch das Avioniksystem nicht schnell genug ausgeregelt werden können. Dies kann auf der einen Seite zu einer Reduktion der Flugzeuggeschwindigkeit zu den minimal nötigen Geschwindigkeiten führen, um Strömungsabriss zu vermeiden oder auf der anderen Seite zu Variationen in der Flugzeuggeschwindigkeit, welche durch die automatische Schubkontrolle nicht ausgeregelt werden können. Desweiteren werden in situ Messungen zu operationellen Analysen und Vorhersagen des integrierten Vorhersagesystems (IFS) des europäischen Zentrums für mittelfristige Wettervorhersage (EZMW) verglichen. Dieser Vergleich zeigt, dass großskalige Strukturen sehr gut vorhergesagt wurden. Allerdings wurden die beobachteten Amplituden von Strukturen auf Skalen < 5 km in allen meteorologischen Parametern unterschätzt. Die Anwendung des graphischen Turbulenz Guiding System (GTG) stellt eine Bereicherung dar, weil der Ort und die Stärke der maximal beobachteten Turbulenz korrekt vorhergesagt wurde. Allerdings konnte die beobachtete Intermittenz nicht reproduziert werden und es wurde eine klare Tendenz zur Überschätzung der Turbulenzstärke gefunden

Mountain wave impact on flight conditions of high-flying aircraft

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

Realistic Simulation of Tropical Atmospheric Gravity Waves Using Radar‐Observed Precipitation Rate and Echo Top Height

Abstract Gravity waves (GWs) generated by tropical convection are important for the simulation of large‐scale atmospheric circulations, for example, the quasi‐biennial oscillation (QBO), and small‐scale phenomena like clear‐air turbulence. However, the simulation of these waves still poses a challenge due to the inaccurate representation of convection, and the high computational costs of global, cloud‐resolving models. Methods combining models with observations are needed to gain the necessary knowledge on GW generation, propagation, and dissipation so that we may encode this knowledge into fast parameterized physics for global weather and climate simulation or turbulence forecasting. We present a new method suitable for rapid simulation of realistic convective GWs. Here, we associate the profile of latent heating with two parameters: precipitation rate and cloud top height. Full‐physics cloud‐resolving WRF simulations are used to develop a lookup table for converting instantaneous radar precipitation rates and echo top measurements into a high‐resolution, time‐dependent latent heating field. The heating field from these simulations is then used to force an idealized dry version of the WRF model. We validate the method by comparing simulated precipitation rates and cloud tops with scanning radar observations and by comparing the GW field in the idealized simulations to satellite measurements. Our results suggest that including variable cloud top height in the derivation of the latent heating profiles leads to better representation of the GWs compared to using only the precipitation rate. The improvement is especially noticeable with respect to wave amplitudes. This improved representation also affects the forcing of GWs on large‐scale circulation

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

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