Atmos. Chem. Phys., 18, 371–384, 2018 https://doi.org/10.5194/acp-18-371-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Temporal variability of tidal and gravity waves during a record long 10-day continuous lidar sounding Kathrin Baumgarten, Michael Gerding, Gerd Baumgarten, and Franz-Josef Lübken Leibniz-Institute of Atmospheric Physics at the University of Rostock, Kühlungsborn, Germany Correspondence: Kathrin Baumgarten ([email protected]) Received: 8 June 2017 – Discussion started: 15 June 2017 Revised: 22 November 2017 – Accepted: 5 December 2017 – Published: 12 January 2018 Abstract. Gravity waves (GWs) as well as solar tides are a 1 Introduction key driving mechanism for the circulation in the Earth’s at- mosphere. The propagation of gravity waves is strongly af- The knowledge of atmospheric waves is crucial for our un- fected by tidal waves as they modulate the mean background derstanding of the circulation in the Earth’s atmosphere. The wind field and vice versa, which is not yet fully understood propagation of different waves, e.g., gravity and tidal waves, and not adequately implemented in many circulation mod- and their interaction is a vital geophysical process, which els. The daylight-capable Rayleigh–Mie–Raman (RMR) li- couples the different atmospheric layers due to the transport dar at Kühlungsborn (54◦ N, 12◦ E) typically provides tem- of momentum and energy. Gravity waves (GWs) and ther- perature data to investigate both wave phenomena during one mal tides differ in their sources. Gravity waves are mostly full day or several consecutive days in the middle atmosphere generated in the troposphere/lower stratosphere by the flow between 30 and 75 km altitude. Outstanding weather con- above orographic structures, convective instabilities, wind ditions in May 2016 allowed for an unprecedented 10-day shears, jet streams, or wave–wave interactions (e.g., Fritts continuous lidar measurement, which shows a large variabil- and Alexander, 2003). Thermal tides are typically excited by ity of gravity waves and tides on timescales of days. Using solar heating of water vapor in the troposphere, ozone in the a one-dimensional spectral filtering technique, gravity and stratosphere and mesopause region, and oxygen above 90 km tidal waves are separated according to their specific periods altitude but can also be excited by latent-heat release due or vertical wavelengths, and their temporal evolution is stud- to deep convection (Chapman and Lindzen, 1970; Forbes, ied. During the measurement period a strong 24 h wave oc- 1984; Hagan and Forbes, 2002). Due to the excitation pro- curs only between 40 and 60 km and vanishes after a few cess, tides have periods of 1 solar day (24 h) and its har- days. The disappearance is related to an enhancement of monics, like 12 or 8 h. The tidal propagation can be either gravity waves with periods of 4–8 h. Wind data provided by Sun-synchronous or not, and accordingly tides are called mi- ECMWF are used to analyze the meteorological situation at grating or non-migrating tides (Forbes, 1995). They modu- our site. The local wind structure changes during the observa- late the background wind field together with planetary waves tion period, which leads to different propagation conditions and therefore have an impact on the propagation conditions for gravity waves in the last days of the measurement period for gravity waves (e.g., Eckermann and Marks, 1996; Senf and therefore a strong GW activity. The analysis indicates a and Achatz, 2011; Yigit˘ and Medvedev, 2017). Upward- further change in wave–wave interaction resulting in a min- propagating GW transport energy and momentum and de- imum of the 24 h tide. The observed variability of tides and posit them during their breaking and filtering to the mean gravity waves on timescales of a few days clearly demon- background flow (Holton and Alexander, 2000; Fritts and strates the importance of continuous measurements with high Alexander, 2003). Models typically use only simplified lin- temporal and spatial resolution to detect interaction phenom- ear parametrization schemes of gravity wave drag, resulting ena, which can help to improve parametrization schemes of in larger discrepancies between model and measurement data GWs in general circulation models. (e.g., Kim et al., 2003). Therefore, additional data are re- quired for validation and more observational data are nec- Published by Copernicus Publications on behalf of the European Geosciences Union. 372 K. Baumgarten et al.: Variability of tides and gravity waves essary for improving these parametrizations (Geller et al., nighttime conditions (Gardner and Voelz, 1987; Wilson et al., 2013). There are approaches of gravity wave parametriza- 1991). tion schemes, which improve the structure and magnitude This paper presents main features of wave activity at mid- of tides, but a validation with observational data is still rare latitudes for an altitude range from the lower stratosphere (Yigit˘ et al., 2008; Yigit˘ and Medvedev, 2017). to the upper mesosphere on short timescales of 10 days in The middle atmosphere is one of the key regions for the May 2016. To our knowledge, this is the longest continu- interaction of gravity waves and tides. To investigate both ous data set retrieved by a RMR lidar. The daylight capabil- wave phenomena, different satellite, in situ (radiosondes, bal- ity of the Kühlungsborn RMR lidar as well as exceptionally loon and rocket soundings), and ground-based techniques good weather conditions make it possible to investigate wave (lidar, radar, and airglow measurements) have been devel- structures over this time period, which allows the short-term oped in recent decades (e.g., Gille et al., 2008; Hertzog variability of gravity waves and tides to be studied. The li- et al., 2008; Preusse et al., 2008). Satellite data give a global dar data are analyzed in the spatial domain on the one hand overview of GWs and tides. For instance, the climatology of and in the time domain on the other hand to distinguish be- tides in the mesosphere–lower thermosphere (MLT) region tween different waves because of either their vertical wave- has been revealed by temperature/wind observations such as lengths or their periods. Data from the European Centre for the High Resolution Doppler Interferometer (HRDI), Wind Medium-Range Weather Forecasts (ECMWF) are used to Imaging Interferometer (WINDII) and the Microwave Limb characterize the background conditions in the troposphere Sounder (MLS) on board the UARS satellite, or the TIMED and stratosphere based on hourly high-resolution forecasts Doppler Interferometer (TIDI) and the Sounding of the At- (cycle 41r2 TCO1279/O1280). The organization of this pa- mosphere using Broadband Emission Radiometry (SABER) per is as follows. In Sect. 2 we describe our lidar instrument instrument on board the TIMED satellite (e.g., Sakazaki and how the data are treated. Section 3 presents the avail- et al., 2012). However, satellites typically need a large time able temperature data during the 10 days of continuous lidar interval of typically several weeks to cover 24 h of local data in May 2016 and their related temperature deviations. time. Consequently, any short-term variability in the dynamic In Sect. 4 we present the short-term variation of the gravity features gets lost. Nevertheless, there are a few approaches wave activity as well as the tidal activity. In addition to the to extract the short-term variability of non-migrating tidal lidar data, ECMWF data are used in Sect. 5 to characterize modes from satellite data using a deconvolution method the background state of the atmosphere. The results are dis- (Oberheide et al., 2002; Lieberman et al., 2015; Pedatella cussed in Sect. 6. Finally, the findings are summarized and a et al., 2016). But these approaches are limited to lower lat- conclusion is given in Sect. 7. itudes (< 50◦ N) to resolve non-migrating tides (Oberheide et al., 2002). Therefore, this method is not suitable to re- solve a day-to-day variability of tides at our latitudes. Radar 2 Instrumental setup and data measurements of horizontal winds produce nearly continu- ous data sets, from which the short-term variability of grav- The Rayleigh–Mie–Raman lidar at Kühlungsborn was de- ity and tidal waves can be investigated, but only in a limited veloped in 2009/2010 (Gerding et al., 2016). The transmit- altitude range of approximately 70–100 km (Hoffmann et al., ter mainly consists of a flashlamp-pumped, injection-seeded 2010). To cover the entire middle atmosphere, the combina- Nd:YAG laser. We use the second harmonic of the laser tion of different lidars using several scattering mechanisms output at 532 nm as the emission wavelength due to a bet- (e.g., Rayleigh and resonance scattering) is the only mea- ter signal-to-noise ratio than the fundamental laser output surement technique which provides temperature data from at 1064 nm. To measure during daytime, special spatial and the troposphere/lower stratosphere to the mesopause region spectral filtering techniques are used to suppress the solar or even higher with a suitable temporal and vertical resolu- background during the day. As a prerequisite for these tech- tion to resolve the short-term variability. Lidar data provide niques, the seeder is locked to an iodine absorption line for vertical information of the atmospheric parameters over time achieving high-frequency stability, and the laser beam diver- at the particular location. The Rayleigh–Mie–Raman (RMR) gence is reduced to ∼ 50 µrad using a 10× beam-widening lidar located at Kühlungsborn is able to provide this infor- telescope. Afterward, the beam is guided co-axially with mation up to 75 km altitude without an additional resonance the receiving telescope into the atmosphere. The field of lidar. The advantage of the RMR lidar at Kühlungsborn is the view (FOV) of the receiver is limited by a fiber cable with ability to measure under nighttime as well as under daytime a small core diameter of 0.2 mm, resulting in a small field conditions, resulting in a continuous temperature time series of view of only 62 µrad.