High-resolution observations of small-scale gravity waves and turbulence features in the OH airglow layer

Abstract

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 −1 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 subkilometre scale in the airglow layer supports the findings of theoretical simulations and modellings.

Derivation of horizontal and vertical wavelengths using a scanning OH(3-1) airglow spectrometer

“…Tang et al (2014) and Wachter et al 10 (2015), e.g., find horizontal phase speeds of up to 160-180 m/s. The horizontal wavelengths cannot easily be compared since many authors focus on smaller horizontal scales (see, for example, Tang et al, 2014;Taylor et al, 2009;Hannawald et al, 2016;Sedlak et al, 2016). However, Reid (1986) presents in his fig.…”

“…Wachter et al (2015) show that the combination of three airglow spectrometers measuring under different azimuth angles allows the additional derivation of horizontal wavelengths. Due to the setup of the three instruments, their fields of view (FoV) and the data analysis technique, the retrieved wavelengths lie mostly in the range of some 100 km, the addressed wave periods range from 1 to 14 h with a maximum between 2 and 4 h. Small-scale horizontal features in the order of some 10 km or even 15 turbulent structures like they are observed with OH* cameras as shown by Sedlak et al (2016) and Hannawald et al (2016) cannot be investigated based on this approach. Schmidt et al (2017) introduce a method to additionally derive vertical wavelengths from OH* spectrometer measurements by observing two vibrational transitions, OH(3-1) and OH(4-2).…”

Retrieval of intrinsic mesospheric gravity wave parameters using lidar and airglow temperature and meteor radar wind data

“…In order to retrieve hor-izontal wavelengths larger than the field of view (FOV) of the imager, Takahashi et al (2009) and Fritts et al (2014) analysed keogram representations of airglow imager data. In addition to process studies, also large-period observations of OH layer emissions with imaging instruments are employed to derive statistics and seasonal variations in GW parameters at different locations (Walterscheid et al, 1999;Nakamura et al, 1999;Suzuki et al, 2004;Li et al, 2016Li et al, , 2018Shiokawa et al, 2009).…”

Observation of Quasiperiodic Structures in the Hydroxyl Airglow on Scales Below 100 m

“…The hydroxyl (OH) airglow near 90 km (Meinel, , ) is one of the more intense emissions, and observations of the OH airglow are often employed to measure gravity waves and breaking features in the MLT region (Diettrich et al, ; Fritts et al, ; Li et al, ). Responding to a wide range of vertical wavelengths (Liu & Swenson, ; Swenson & Gardner, ), measurements of the horizontal wavelength of gravity waves have been made with OH airglow imagers (Sedlak et al, ; Taylor et al, ). Studies such as these typically detected atmospheric gravity waves in the OH airglow layer on scales of the order ~1 to hundreds of kilometers.…”