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Now showing 1 - 10 of 55
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    Observation of Kelvin–Helmholtz instabilities and gravity waves in the summer mesopause above Andenes in Northern Norway
    (München : European Geopyhsical Union, 2018) Stober, Gunter; Sommer, Svenja; Schult, Carsten; Latteck, Ralph; Chau, Jorge L.
    We present observations obtained with the Middle Atmosphere Alomar Radar System (MAARSY) to investigate short-period wave-like features using polar mesospheric summer echoes (PMSEs) as a tracer for the neutral dynamics. We conducted a multibeam experiment including 67 different beam directions during a 9-day campaign in June 2013. We identified two Kelvin–Helmholtz instability (KHI) events from the signal morphology of PMSE. The MAARSY observations are complemented by collocated meteor radar wind data to determine the mesoscale gravity wave activity and the vertical structure of the wind field above the PMSE. The KHIs occurred in a strong shear flow with Richardson numbers Ri <0.25. In addition, we observed 15 wave-like events in our MAARSY multibeam observations applying a sophisticated decomposition of the radial velocity measurements using volume velocity processing. We retrieved the horizontal wavelength, intrinsic frequency, propagation direction, and phase speed from the horizontally resolved wind variability for 15 events. These events showed horizontal wavelengths between 20 and 40km, vertical wavelengths between 5 and 10km, and rather high intrinsic phase speeds between 45 and 85ms−1 with intrinsic periods of 5–10min.
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    Long‐Term Changes in the Northern Midwinter Middle Atmosphere in Relation to the Quasi‐Biennial Oscillation
    (Hoboken, NJ : Wiley, 2019) Gabriel, A.
    Long-term changes in the middle atmosphere due to anthropogenic greenhouse gas emissions are examined in relation to the effect of the equatorial Quasi-Biennial Oscillation (QBO) on the northern midwinter circulation. The examinations are based on the Coupled Model Intercomparison Project Phase 5 simulations for 1979–2100 with the Earth-System-Model MPI-ESM-MR that generates the QBO internally. In particular, the three-dimensional residual circulation is used as proxy for the Brewer-Dobson circulation, revealing an increasing downwelling in the center of the polar low over Northern Europe/Siberia (~5% per decade). The changes in northern midwinter temperature, zonal wind, and residual circulation are much stronger during westerly (QBO-W) than easterly (QBO-E) phase of QBO (e.g., for a moderate increase in greenhouse gases, we find maximum decreases in the zonal mean westerly jet at 60°N and 3 hPa of about −14.8 ± 5.4 m/s for QBO-W but only −4.7 ± 5.2 m/s for QBO-E). This is due to a change of the extratropical QBO-W signature toward QBO-E signature while the equatorial QBO remains nearly unchanged (i.e., a change toward disappearance of the so-called Holton-Tan relationship). Similar to the current change from QBO-W to QBO-E signature, the changes during QBO-W include an increase in amplitude and eastward shift in phase of stratospheric stationary Wave 1 at the cost of Wave 2, with decreasing westerlies over North America and increasing downwelling over Siberia. The eastward shift in phase of stationary Wave 1 is related to the associated increase in meridional transport of planetary vorticity. © 2019. The Authors.
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    Enhancing the spatiotemporal features of polar mesosphere summer echoes using coherent MIMO and radar imaging at MAARSY
    (Göttingen : Copernicus GmbH, 2019) Urco, J.M.; Chau, J.L.; Weber, T.; Latteck, R.
    Polar mesospheric summer echoes (PMSEs) are very strong radar echoes caused by the presence of ice particles, turbulence, and free electrons in the mesosphere over polar regions. For more than three decades, PMSEs have been used as natural tracers of the complicated atmospheric dynamics of this region. Neutral winds and turbulence parameters have been obtained assuming PMSE horizontal homogeneity on scales of tens of kilometers. Recent radar imaging studies have shown that PMSEs are not homogeneous on these scales and instead they are composed of kilometer-scale structures. In this paper, we present a technique that allows PMSE observations with unprecedented angular resolution (∼0.6). The technique combines the concept of coherent MIMO (Multiple Input Multiple Output) and two high-resolution imaging techniques, i.e., Capon and maximum entropy (MaxEnt). The resulting resolution is evaluated by imaging specular meteor echoes. The gain in angular resolution compared to previous approaches using SIMO (Single Input Multiple Output) and Capon is at least a factor of 2; i.e., at 85 km, we obtain a horizontal resolution of ∼900 m. The advantage of the new technique is evaluated with two events of 3-D PMSE structures showing: (1) horizontal wavelengths of 8-10 km and periods of 4-7 min, drifting with the background wind, and (2) horizontal wavelengths of 12-16 km and periods of 15-20 min, not drifting with the background wind. Besides the advantages of the implemented technique, we discuss its current challenges, like the use of reduced power aperture and processing time, as well as the future opportunities for improving the understanding of the complex small-scale atmospheric dynamics behind PMSEs. © 2019 Author(s).
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    Comparative study between ground-based observations and NAVGEM-HA analysis data in the mesosphere and lower thermosphere region
    (Katlenburg-Lindau : EGU, 2020) Stober, Gunter; Baumgarten, Kathrin; McCormack, John P.; Brown, Peter; Czarnecki, Jerry
    Recent studies have shown that day-to-day variability of the migrating semidiurnal solar (SW2) tide within the mesosphere and lower thermosphere (MLT) is a key driver of anomalies in the thermosphere-ionosphere system. Here, we study the variability in both the amplitude and phase of SW2 using meteor radar wind and lidar temperature observations at altitudes of 75-110 km as well as wind and temperature output from the Navy Global Environmental Model-High Altitude (NAVGEM-HA), a high-altitude meteorological analysis system. Application of a new adaptive spectral filter technique to both local radar wind observations and global NAVGEM-HA analyses offers an important cross-validation of both data sets and makes it possible to distinguish between migrating and non-migrating tidal components, which is difficult using local measurements alone. Comparisons of NAVGEM-HA, meteor radar and lidar observations over a 12-month period show that the meteorological analyses consistently reproduce the seasonal as well as day-to-day variability in mean winds, mean temperatures and SW2 features from the ground-based observations. This study also examines in detail the day-to-day variability in SW2 during two sudden stratospheric warming, events that have been implicated in producing ionospheric anomalies. During this period, both meteor radar and NAVGEM-HA winds show a significant phase shift and amplitude modulation, but no signs of coupling to the lunar tide as previous studies have suggested. Overall, these findings demonstrate the benefit of combining global high-altitude meteorological analyses with ground-based observations of the MLT region to better understand the tidal variability in the atmosphere. © 2020 Author(s).
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    Mesospheric semidiurnal tides and near-12 h waves through jointly analyzing observations of five specular meteor radars from three longitudinal sectors at boreal midlatitudes
    (Göttingen : Copernicus GmbH, 2019) He, M.; Chau, J.L.

    In the last decades, mesospheric tides have been intensively investigated with observations from both ground-based radars and satellites. Single-site radar observations provide continuous measurements at fixed locations without horizontal information, whereas single-spacecraft missions typically provide global coverage with limited temporal coverage at a given location. In this work, by combining 8 years (2009-2016) of mesospheric winds collected by five specular meteor radars from three different longitudinal sectors at boreal midlatitudes (49±8.5ĝ N), we develop an approach to investigate the most intense global-scale oscillation, namely at the period TCombining double low line12±0.5 h. Six waves are resolved: The semidiurnal westward-Traveling tidal modes with zonal wave numbers 1, 2, and 3 (SW1, SW2, SW3), the lunar semidiurnal tide M2, and the upper and lower sidebands (USB and LSB) of the 16 d wave nonlinear modulation on SW2. The temporal variations of the waves are studied statistically with a special focus on their responses to sudden stratospheric warming events (SSWs) and on their climatological seasonal variations. In response to SSWs, USB, LSB, and M2 enhance, while SW2 decreases. However, SW1 and SW3 do not respond noticeably to SSWs, contrary to the broadly reported enhancements in the literature. The USB, LSB, and SW2 responses could be explained in terms of energy exchange through the nonlinear modulation, while LSB and USB might previously have been misinterpreted as SW1 and SW3, respectively. Besides, we find that LSB and M2 enhancements depend on the SSW classification with respect to the associated split or displacement of the polar vortex. In the case of seasonal variations, our results are qualitatively consistent with previous studies and show a moderate correlation with an empirical tidal model derived from satellite observations.

    © Author(s) 2019.
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    Four-Dimensional Quantification of Kelvin-Helmholtz Instabilities in the Polar SummerMesosphere Using Volumetric Radar Imaging
    (Hoboken, NJ : Wiley, 2020) Chau, J.L.; Urco, J.M.; Avsarkisov, V.; Vierinen, J.P.; Latteck, R.; Hall, C.M.; Tsutsumi, M.
    We present and characterize in time and three spatial dimensions a Kelvin-Helmholtz Instability (KHI) event from polar mesospheric summer echoes (PMSE) observed with the Middle Atmosphere Alomar Radar System. We use a newly developed radar imaging mode, which observed PMSE intensity and line of sight velocity with high temporal and angular resolution. The identified KHI event occurs in a narrow layer of 2.4 km thickness centered at 85 km altitude, is elongated along north-south direction, presents separation between billows of ~ 8 km in the east-west direction, and its billow width is ~ 3 km. The accompanying vertical gradients of the horizontal wind are between 35 and 45 m/s/km and vertical velocities inside the billows are ± 12 m/s. Based on the estimated Richardson (< 0.25), horizontal Froude ( ~ 0.8), and buoyancy Reynolds ( ~ 2.5 × 10 4) numbers, the observed event is a KHI that occurs under weak stratification and generates strong turbulence. © 2019. The Authors.
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    Statistical climatology of mid-latitude mesospheric summer echoes characterised by OSWIN (Ostsee-Wind) radar observations
    (Göttingen : Copernicus GmbH, 2019) Pokhotelov, D.; Stober, G.; Chau, J.L.
    Mid-latitude mesospheric summer echoes (MSEs) appear in radar observations during summer months. The geophysical factors controlling the formation of MSEs include solar and energetic particle ionisation, neutral temperature, turbulence, and meridional transport. A total of 12 years of summer months observations with the OSWIN (Ostsee-Wind) radar in Kühlungsborn, Germany, have been analysed to detect MSE events and to analyse statistical connections to these controlling factors. A more sensitive and consistent method for deriving signal-to-noise ratio has been utilised. Daily and monthly composite analysis demonstrates strong daytime preference and early summer seasonal preference for MSEs. The statistical results are not entirely conclusive due to the low-occurrence rates of MSEs. Nevertheless, it is demonstrated that the meridional transport from colder high-latitude summer mesosphere is the important controlling factor, while no clear connection to geomagnetic and solar activity is found. © 2019 Author(s).
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    A comparison of different solutions for the dynamic smagorinsky model applied in a GCM
    (Stuttgart : Bornträger, 2018) Schaefer-Rolffs, U.
    A discussion of different approaches and solutions of the basic tensor equation within the Dynamic Smagorinsky Model (DSM) suitable for General Circulation Models (GCM) is presented. Particular interest is dedicated to the relationship between various approaches (i.e., the specific formulation of the tensor equation), namely a least-square approach, a time lag approach, and a simple tensor contraction approach, and the impact of the specific solution (i.e., how to solve the equation) on the Smagorinsky parameter c2S . In addition to the standard solutions, clipped solutions, absolute solutions, and tensor norm solutions are examined. The numerical results are based on calculations from a general circulation model, where the different approaches are applied to provide the turbulent horizontal momentum diffusion. Here, they are examined with focus on two issues: 1) At the beginning of the simulations, the different choices for the tensor equation result in different values for the locally distributed and zonally averaged values of the Smagorinsky parameter. These values show that for the standard solutions almost half of the values of c2S are negative, in accordance with known results from isotropic turbulence and leads to unstable simulations. In addition, the tensor norm is related to the absolute solution via the Cauchy-Schwarz inequality. 2) As the simulations proceed, the differences of the Smagorinsky parameter values diminish except for the tensor norm solutions while evolving to a stationary state in a process of self-organization such that they form a group with values comparable to isotropic three-dimensional simulations. In summary, the least-squares and time lag approaches provide reasonable results, while the simple contraction approach fluctuates more. For the solutions, it is discussed whether the clipped or the tensor norm solution is more reasonable. © 2018 The authors.
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    Climatologies and long-term changes in mesospheric wind and wave measurements based on radar observations at high and mid latitudes
    (Göttingen : Copernicus GmbH, 2019) Wilhelm, S.; Stober, G.; Brown, P.
    We report on long-term observations of atmospheric parameters in the mesosphere and lower thermosphere (MLT) made over the last 2 decades. Within this study, we show, based on meteor wind measurement, the long-term variability of winds, tides, and kinetic energy of planetary and gravity waves. These measurements were done between the years 2002 and 2018 for the high-latitude location of Andenes (69.3°N, 16°E) and the mid-latitude locations of Juliusruh (54.6°N, 13.4°E) and Tavistock (43.3°N, 80.8°W). While the climatologies for each location show a similar pattern, the locations differ strongly with respect to the altitude and season of several parameters. Our results show annual wind tendencies for Andenes which are toward the south and to the west, with changes of up to 3°m s-1 per decade, while the mid-latitude locations show smaller opposite tendencies to negligible changes. The diurnal tides show nearly no significant long-term changes, while changes for the semidiurnal tides differ regarding altitude. Andenes shows only during winter a tidal weakening above 90°km, while for the Canadian Meteor Orbit Radar (CMOR) an enhancement of the semidiurnal tides during the winter and a weakening during fall occur. Furthermore, the kinetic energy for planetary waves showed strong peak values during winters which also featured the occurrence of sudden stratospheric warming. The influence of the 11-year solar cycle on the winds and tides is presented. The amplitudes of the mean winds exhibit a significant amplitude response for the zonal component below 82°km during summer and from November to December between 84 and 95°km at Andenes and CMOR. The semidiurnal tides (SDTs) show a clear 11-year response at all locations, from October to November. © 2019 by ASME.
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    Retrieving horizontally resolved wind fields using multi-static meteor radar observations
    (Göttingen : Copernicus GmbH, 2018) Stober, G.; Chau, J.L.; Vierinen, J.; Jacobi, C.; Wilhelm, S.
    Recently, the MMARIA (Multi-static, Multi-frequency Agile Radar for Investigations of the Atmosphere) concept of a multi-static VHF meteor radar network to derive horizontally resolved wind fields in the mesosphere-lower thermosphere was introduced. Here we present preliminary results of the MMARIA network above Eastern Germany using two transmitters located at Juliusruh and Collm, and five receiving links: two monostatic and three multi-static. The observations are complemented during a one-week campaign, with a couple of addition continuous-wave coded transmitters, making a total of seven multi-static links. In order to access the kinematic properties of non-homogenous wind fields, we developed a wind retrieval algorithm that applies regularization to determine the non-linear wind field in the altitude range of 82-98 km. The potential of such observations and the new retrieval to investigate gravity waves with horizontal scales between 50-200 km is presented and discussed. In particular, it is demonstrated that horizonal wavelength spectra of gravity waves can be obtained from the new data set. © Author(s) 2018.