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- ItemQuantification of waves in lidar observations of noctilucent clouds at scales from seconds to minutes(Göttingen : Copernicus, 2013) Kaifler, N.; Baumgarten, G.; Fiedler, J.We present small-scale structures and waves observed in noctilucent clouds (NLC) by lidar at an unprecedented temporal resolution of 30 s or less. The measurements were taken with the Rayleigh/Mie/Raman lidar at the ALOMAR observatory in northern Norway (69 N) in the years 2008-2011. We find multiple layer NLC in 7.9% of the time for a brightness threshold of δ β 12 × 10-10 m-1 sr-1. In comparison to 10 min averaged data, the 30 s dataset shows considerably more structure. For limited periods, quasi-monochromatic waves in NLC altitude variations are common, in accord with ground-based NLC imagery. For the combined dataset, on the other hand, we do not find preferred periods but rather significant periods at all timescales observed (1 min to 1 h). Typical wave amplitudes in the layer vertical displacements are 0.2 km with maximum amplitudes up to 2.3 km. Average spectral slopes of temporal altitude and brightness variations are-2.01 ± 0.25 for centroid altitude,-1.41 ± 0.24 for peak brightness and-1.73 ± 0.25 for integrated brightness. Evaluating a new single-pulse detection system, we observe altitude variations of 70 s period and spectral slopes down to a scale of 10 s. We evaluate the suitability of NLC parameters as tracers for gravity waves.
- ItemThe geodynamic and limnological evolution of Balkan Lake Ohrid, possibly the oldest extant lake in Europe(Oxford : Wiley-Blackwell, 2022) Wagner, Bernd; Tauber, Paul; Francke, Alexander; Leicher, Niklas; Binnie, Steven A.; Cvetkoska, Aleksandra; Jovanovska, Elena; Just, Janna; Lacey, Jack H.; Levkov, Zlatko; Lindhorst, Katja; Kouli, Katerina; Krastel, Sebastian; Panagiotopoulos, Konstantinos; Ulfers, Arne; Zaova, Dušica; Donders, Timme H.; Grazhdani, Andon; Koutsodendris, Andreas; Leng, Melanie J.; Sadori, Laura; Scheinert, Mirko; Vogel, Hendrik; Wonik, Thomas; Zanchetta, Giovanni; Wilke, ThomasStudies of the upper 447 m of the DEEP site sediment succession from central Lake Ohrid, Balkan Peninsula, North Macedonia and Albania provided important insights into the regional climate history and evolutionary dynamics since permanent lacustrine conditions established at 1.36 million years ago (Ma). This paper focuses on the entire 584-m-long DEEP sediment succession and a comparison to a 197-m-long sediment succession from the Pestani site ~5 km to the east in the lake, where drilling ended close to the bedrock, to unravel the earliest history of Lake Ohrid and its basin development. 26Al/10Be dating of clasts from the base of the DEEP sediment succession implies that the sedimentation in the modern basin started at c. 2 Ma. Geophysical, sedimentological and micropalaeontological data allow for chronological information to be transposed from the DEEP to the Pestani succession. Fluvial conditions, slack water conditions, peat formation and/or complete desiccation prevailed at the DEEP and Pestani sites until 1.36 and 1.21 Ma, respectively, before a larger lake extended over both sites. Activation of karst aquifers to the east probably by tectonic activity and a potential existence of neighbouring Lake Prespa supported filling of Lake Ohrid. The lake deepened gradually, with a relatively constant vertical displacement rate of ~0.2 mm a−1 between the central and the eastern lateral basin and with greater water depth presumably during interglacial periods. Although the dynamic environment characterized by local processes and the fragmentary chronology of the basal sediment successions from both sites hamper palaeoclimatic significance prior to the existence of a larger lake, the new data provide an unprecedented and detailed picture of the geodynamic evolution of the basin and lake that is Europe’s presumed oldest extant freshwater lake.
- ItemThe impact of planetary waves on the latitudinal displacement of sudden stratospheric warmings(Göttingen : Copernicus, 2013) Matthias, V.; Hoffmann, P.; Manson, A.; Meek, C.; Stober, G.; Brown, P.; Rapp, M.The Northern Hemispheric winter is disturbed by large scale variability mainly caused by Planetary Waves (PWs), which interact with the mean flow and thus result in Sudden Stratospheric Warmings (SSWs). The effects of a SSW on the middle atmosphere are an increase of stratospheric and a simultaneous decrease of mesospheric temperature as well as a wind reversal to westward wind from the mesosphere to the stratosphere. In most cases these disturbances are strongest at polar latitudes, get weaker toward the south and vanish at mid-latitudes around 50° to 60° N as for example during the winter 2005/06. However, other events like in 2009, 2010 and 2012 show a similar or even stronger westward wind at mid-than at polar latitudes either in the mesosphere or in the stratosphere during the SSW. This study uses local meteor and MF-radar measurements, global satellite observations from the Microwave Limb Sounder (MLS) and assimilated model data from MERRA (Modern-ERA Retrospective analysis for research and Applications). We compare differences in the latitudinal structure of the zonal wind, temperature and PW activity between a "normal" event, where the event in 2006 was chosen representatively, and the latitudinal displaced events in 2009, 2010 and 2012. A continuous westward wind band between the pole and 20° N is observed during the displaced events. Furthermore, distinctive temperature differences at mid-latitudes occur before the displaced warmings compared to 2006 as well as a southward extended stratospheric warming afterwards. These differences between the normal SSW in 2006 and the displaced events in 2009, 2010 and 2012 are linked to an increased PWactivity between 30° N and 50° N and the changed stationary wave flux in the stratosphere around the displaced events compared to 2006.
- ItemVery low ozone episodes due to polar vortex displacement(Milton Park : Taylor & Francis, 2000) James, P.M.; Peters, D.; Waugh, D.W.The large-scale ozone distribution over the northern hemisphere undergoes strong fluctuationseach winter on timescales of up to a few weeks. This is closely linked to changes in the stratosphericpolar vortex, whose shape, intensity and location vary with time. Elliptical diagnosticparameters provide an empirical description of the daily character of the polar vortex. Theseparameters are used as an objective measure to define two characteristic wintertime vortexdisplacements, towards northern Europe and Canada, respectively. The large-scale structuresin both the stratosphere and troposphere and the 3D ozone structures are determined for bothvortex displacement scenarios. A linear ozone transport model shows that the contribution ofhorizontal ozone advection dominates locally in the middle stratosphere. Nevertheless, thelargest contribution is due to vertical advection around the ozone layer maximum. The findingsare in agreement with an EOF analysis which reveals significant general modes of ozone variabilitylinked to polar vortex displacement and to phase-shifted large-scale tropospheric waves.When baroclinic waves travel through the regions of vortex-related ozone reduction, the combinedeffect is to produce transient synoptic-scale areas of exceptionally low ozone; namelydynamically induced strong ozone mini-holes.