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Horizontal Propagation of Large-Amplitude Mountain Waves Journal of Geophysical Research: Atmospheres RESEARCH ARTICLE Horizontal propagation of large-amplitude mountain 10.1002/2016JD025621 waves into the polar night jet Key Points: • Analysis of a large-amplitude Benedikt Ehard1 , Bernd Kaifler1 , Andreas Dörnbrack1 , Peter Preusse2, mountain wave event above Stephen D. Eckermann3 , Martina Bramberger1 , Sonja Gisinger1, Natalie Kaifler1, New Zealand • Vertical propagation of mountain Ben Liley4 ,Johannes Wagner1, and Markus Rapp1,5 waves in the lower stratosphere is attenuated by instabilities 1Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen, Germany, 2Institute • Mountain waves are refracted in the of Energy and Climate Research (IEK-7: Stratosphere), Forschungszentrum Jülich, Jülich, Germany, 3Space Science upper stratosphere and propagate 4 toward the polar night jet Division, U.S. Naval Research Laboratory, Washington, District of Columbia, USA, National Institute of Water and Atmospheric Research, Lauder, New Zealand, 5Meteorological Institute Munich, Ludwig-Maximilians-Universität München, Munich, Germany Correspondence to: B. Ehard, [email protected] Abstract We analyze a large-amplitude mountain wave event, which was observed by a ground-based lidar above New Zealand between 31 July and 1 August 2014. Besides the lidar observations, European Citation: Centre for Medium-Range Weather Forecasts (ECMWF) data, satellite observations, and ray tracing Ehard, B., et al. (2017), Vertical simulations are utilized in this study. It is found that the propagation of mountain waves into the middle propagation of large-amplitude mountain waves in the vicinity atmosphere is influenced by two different processes at different stages of the event. At the beginning of of the polar night jet, J. Geophys. the event, instabilities in a weak wind layer cause wave breaking in the lower stratosphere. During the Res. Atmos., 122, 1423–1436, course of the event the mountain waves propagate to higher altitudes and are refracted southward toward doi:10.1002/2016JD025621. the polar night jet due to the strong meridional shear of the zonal wind. As the waves propagate out of the observational volume, the ground-based lidar observes no mountain waves in the mesosphere. Ray Received 6 JUL 2016 tracing simulations indicate that the mountain waves propagated to mesospheric altitudes south of New Accepted 13 JAN 2017 Accepted article online 16 JAN 2017 Zealand where the polar night jet advected the waves eastward. These results underline the importance of Published online 2 FEB 2017 considering horizontal propagation of gravity waves, e.g., when analyzing locally confined observations of gravity waves. 1. Introduction It is generally acknowledged that gravity waves play an important role for the thermal and dynamical struc- ture of the middle atmosphere [Fritts and Alexander, 2003]. Gravity waves are not only the key driver of the mesospheric residual circulation [HoltonandAlexander, 2000], but they also affect the polar winter stratopause [Hitchman et al., 1989]. A major source of gravity waves is the flow over orography which has been studied by several measurement campaigns over the past decades [see Smith et al., 2016, Appendix A]. A recent field campaign studying orographic gravity waves was the DEEPWAVE campaign (the Deep Propa- gating Gravity Wave Experiment) [Fritts et al., 2016] conducted in New Zealand during austral winter 2014. During DEEPWAVE, a combination of ground-based, airborne, and in situ measurements was utilized to study the propagation of gravity waves from the troposphere into the middle atmosphere. New Zealand was cho- sen because it is a hot spot region for gravity waves in wintertime [e.g., Jiang et al., 2005]. In the southern hemispheric winter, the upper branch of the polar night jet usually extends over New Zealand at altitudes above 30–40 km. This yields stratospheric westerlies, which provide favorable conditions for the propaga- tion of mountain waves deep into the middle atmosphere [Fritts et al., 2016]. Furthermore, there is generally a large vertical shear of the horizontal wind above New Zealand. Additionally, the zonal winds in the lower and middle stratosphere increase poleward toward the core of the polar night jet, resulting in a strong meridional shear. Kaifler et al. [2015] analyzed ground-based Rayleigh lidar measurements obtained during DEEPWAVE to inves- tigate the influence of source and background conditions on mountain wave penetration into the middle atmosphere directly above the lidar station. They found deep propagating mountain waves above the obser- © 2017. American Geophysical Union. vational site on average under two conditions: (1) weak to moderate westerlies in the lower troposphere and All Rights Reserved. (2) moderate to strong westerly winds in the stratosphere. This indicates that large-amplitude mountain waves EHARD ET AL. VERTICAL PROPAGATION OF MOUNTAIN WAVES 1423 Journal of Geophysical Research: Atmospheres 10.1002/2016JD025621 are likely to become unstable and break at stratospheric altitudes, while mountain waves with smaller ampli- tudes can propagate deep into the mesosphere, provided that the background wind conditions permit the vertical propagation. Kaifler et al. [2015] noted in their conclusion that besides instability processes, refraction of gravity waves might occur as well, thus contributing to the absence of large-amplitude mountain waves in the lidar observations at mesospheric altitudes. The importance of gravity wave refraction in the middle atmosphere has been addressed by Dunkerton [1984]. He showed that vertically propagating gravity waves are refracted and focused into the polar night jet [cf. Dunkerton, 1984, Figure 3] due to the meridional shear of the zonal wind. Following Dunkerton [1984], sev- eral studies investigated this wave focusing: Utilizing satellite measurements and ray tracing calculations over the southern Andes, Preusse et al. [2002] illustrated the importance of the wave vector modification for the focusing of orographic waves. By utilizing global satellite observations and ray tracing calculations, Preusse et al. [2009] found peak values of 15∘ for the meridional propagation of gravity waves. Sato et al. [2009, 2012], demonstrated the refraction of orographic gravity waves simulated by a gravity wave resolving general circu- lation model. Moreover, Sato et al. [2012] stated that the wave energy from orographic waves can thereby be advected over large horizontal distances. With the combination of mesoscale simulations and ray tracing cal- culations, Jiang et al. [2013] showed orographic gravity waves over the southern Andes, which were refracted and propagated zonally downstream over a distance of more than 1000 km. During DEEPWAVE the observations were mostly restricted to the South Island of New Zealand, with additional research flights being conducted over the Pacific Ocean toward the southwest or northwest of New Zealand [cf. Smith et al., 2016, Figure 1]. This flight strategy was motivated by satellite measurements which show a stratospheric gravity wave hot spot above New Zealand [e.g., Gongetal., 2012; Hoffmannetal., 2016; Frittsetal., 2016]. Thus, the assumption was made that mountain waves should be observable in the middle atmosphere directly above New Zealand. However, due to the meridional wind shear induced by the polar night jet to the south of New Zealand, it is likely that orographic waves are focused into the polar night jet. Thus, the question arises whether there were cases during DEEPWAVE where deep vertical propagation of gravity waves occurred, which was not observed by the ground-based observations due to the waves being refracted away from the observational volume. To evaluate this question, we investigate the propagation of large-amplitude mountain waves under strong tropospheric forcing between 31 July and 1 August 2014. This particular mountain wave event resulted in the largest stratospheric gravity wave response recorded by the ground-based lidar during DEEPWAVE [Kaifler et al., 2015]. We utilize lidar measurements obtained at Lauder, New Zealand (45.0∘S, 169.7∘E), to investigate the temporal and vertical evolution of the wave event. European Centre for Medium-Range Weather Forecasts (ECMWF) data are used to characterize the ambient conditions in the troposphere and lower stratosphere. Ray tracing simulations are conducted in order to examine the propagational pathways of gravity waves. The results are compared to satellite measurements and ECMWF data. The paper is structured as follows: In section 2 we describe the instruments and data sets used in this study. The meteorological situation is described in section 3. The results are presented in section 4, which are discussed in section 5. Conclusions are drawn in section 6. 2. Instruments and Models 2.1. Temperature Lidar for Middle Atmosphere Research The Temperature Lidar for Middle Atmosphere Research (TELMA) is a ground-based Rayleigh/Raman lidar. It was operated during the DEEPWAVE campaign at Lauder, New Zealand (45.0∘S, 169.7∘E). The laser emits 12 W optical power at a wavelength of 532 nm with a pulse repetition rate of 100 Hz. The backscattered light is collected with a 63 cm diameter telescope. This allows for a retrieval of atmospheric temperature profiles with an effective spatial and temporal resolution of 900 m × 10 min in an altitude range of typically 22 to 85 km. For further details regarding the instrument, the temperature retrieval, and associated uncertainties, see Kaifler
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