High-resolution observations of small-scale gravity waves and turbulence features in the OH airglow layer
Article, Other literature type
(issn: 1867-8548, eissn: 1867-8548)
A new version of the Fast Airglow Imager (FAIM) for the detection
of atmospheric waves in the OH airglow layer has been set up at the German
Remote Sensing Data Center (DFD) of the German Aerospace Center (DLR) at
Oberpfaffenhofen (48.09° N, 11.28° E), Germany. The spatial
resolution of the instrument is 17 m pixel<sup>−1</sup> in zenith direction with
a field of view (FOV) of 11.1 km × 9.0 km at the OH layer height
of ca. 87 km. Since November 2015, the system has been in operation in two
different setups (zenith angles 46 and 0°) with a temporal resolution
of 2.5 to 2.8 s.
In a first case study we present observations of two small wave-like
features that might be attributed to gravity wave instabilities. In order to
spectrally analyse harmonic structures even on small spatial scales down to
550 m horizontal wavelength, we made use of the maximum entropy method (MEM)
since this method exhibits an excellent wavelength resolution. MEM further
allows analysing relatively short data series, which considerably helps to
reduce problems such as stationarity of the underlying data series from a
statistical point of view. We present an observation of the subsequent decay
of well-organized wave fronts into eddies, which we tentatively interpret in
terms of an indication for the onset of turbulence.
Another remarkable event which demonstrates the technical capabilities of
the instrument was observed during the night of 4–5 April 2016. It
reveals the disintegration of a rather homogenous brightness variation into
several filaments moving in different directions and with different speeds.
It resembles the formation of a vortex with a horizontal axis of rotation
likely related to a vertical wind shear. This case shows a notable
similarity to what is expected from theoretical modelling of
Kelvin–Helmholtz instabilities (KHIs).
The comparatively high spatial resolution of the presented new version of the
FAIM provides new insights into the structure of atmospheric
wave instability and turbulent processes. Infrared imaging of wave dynamics
on the sub-kilometre scale in the airglow layer supports the findings of
theoretical simulations and modellings.