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Author: Chau, Jorge L. × End date: 2018 × Start date: 2018 × Has files: Yes × Type: Article × WGL Institute: IAP ×

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    Seasonal variability of atmospheric tides in the mesosphere and lower thermosphere: Meteor radar data and simulations
    (München : European Geopyhsical Union, 2018) Pokhotelov, Dimitry; Becker, Erich; Stober, Gunter; Chau, Jorge L.
    Thermal tides play an important role in the global atmospheric dynamics and provide a key mechanism for the forcing of thermosphere–ionosphere dynamics from below. A method for extracting tidal contributions, based on the adaptive filtering, is applied to analyse multi-year observations of mesospheric winds from ground-based meteor radars located in northern Germany and Norway. The observed seasonal variability of tides is compared to simulations with the Kühlungsborn Mechanistic Circulation Model (KMCM). It is demonstrated that the model provides reasonable representation of the tidal amplitudes, though substantial differences from observations are also noticed. The limitations of applying a conventionally coarse-resolution model in combination with parametrisation of gravity waves are discussed. The work is aimed towards the development of an ionospheric model driven by the dynamics of the KMCM.
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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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    On the role of anisotropic MF/HF scattering in mesospheric wind estimation
    (Heidelberg : Springer, 2018-10-1) Renkwitz, Toralf; Tsutsumi, Masaki; Laskar, Fazlul I.; Chau, Jorge L.; Latteck, Ralph
    The Saura radar is designed and used to measure winds and electron densities at polar latitudes (69∘N) within the D region, namely between 50 and 100 km altitude. A relatively narrow radar beam can be generated and steered into distinct pointing directions as a rather large antenna array is used. From the observed radial velocities of the individual pointing directions, the horizontal and vertical wind fields can be obtained using the Doppler beam swinging (DBS) method. With recent upgrades to the radar, the interferometric capabilities are largely improved allowing simultaneous application of different wind estimation techniques now, and also echo localization. In recent studies, Saura DBS winds assuming isotropic scattering were found to be underestimated in comparison with highly reliable winds observed with the MAARSY MST radar in the presence of polar mesospheric summer echoes (PMSE). This underestimation has been investigated by analyzing the scattering positions as well as applying the imaging Doppler interferometry technique. Besides this, Saura winds derived with the classical DBS method seem to be error prone at altitudes above 90 km and even below this altitude for periods of enhanced ionization, e.g., particle precipitations. Various methods taking into account the scattering positions have been used to correct the wind underestimation. These winds are compared to MST radar winds during PMSE, and an optimal combination of these methods for the Saura radar is presented. This combined wind data appears to be reliable; it shows reasonable amplitudes as well as tidal structures for the entire altitude region.