Gravity wave instability structures and turbulence from more than 1.5 years of OH* airglow imager observations in Slovenia

Abstract

Abstract. We analysed 286 nights of data from the OH* airglow imager FAIM 3 (Fast Airglow IMager) acquired at Otlica Observatory (45.93∘ N, 13.91∘ E), Slovenia, between 26 October 2017 and 6 June 2019. Measurements have been performed with a spatial resolution of 24 m per pixel and a temporal resolution of 2.8 s. A two-dimensional fast Fourier transform is applied to the image data to derive horizontal wavelengths between 48 m and 4.5 km in the upper mesosphere/lower thermosphere (UMLT) region. In contrast to the statistics of larger-scale gravity waves (horizontal wavelength up to ca. 50 km; Hannawald et al., 2019), we find a more isotropic distribution of directions of propagation, pointing to the presence of wave structures created above the stratospheric wind fields. A weak seasonal tendency of a majority of waves propagating eastward during winter may be due to instability features from breaking secondary gravity waves that were created in the stratosphere. We also observe an increased southward propagation during summer, which we interpret as an enhanced contribution of secondary gravity waves created as a consequence of primary wave filtering by the meridional mesospheric circulation. We present multiple observations of turbulence episodes captured by our high-resolution airglow imager and estimated the energy dissipation rate in the UMLT from image sequences in 25 cases. Values range around 0.08 and 9.03 W kg−1 and are on average higher than those in recent literature. The values found here would lead to an approximated localized maximum heating of 0.03–3.02 K per turbulence event. These are in the same range as the daily chemical heating rates for the entire atmosphere reported by Marsh (2011), which apparently stresses the importance of dynamical energy conversion in the UMLT.

Analysis of 2D airglow imager data with respect to dynamics using machine learning

“…As FAIM 3 exhibits a high spatial (23 m pixel -1 ) and temporal (2.8 s per image) resolution, turbulence parameters can be derived to estimate the energy diffusion rate. Similar to the results the authors found for another FAIM station (Sedlak et al, 2021), the values of energy dissipation rate range from 0.03 to 3.18 W kg -1 .…”

“…This includes instability features of gravity waves, such as 'ripples' (Peterson, 1979;Taylor and Hapgood, 1990;Li et al, 2017), but also turbulence (Hecht et al, 2021;Sedlak et al, 2016Sedlak et al, , 2021. Former studies at Oberpfaffenhofen (Sedlak et al, 2016) and Otlica, Slovenia (Sedlak et al, 2021) using the high-resolution airglow imager FAIM 3 have shown that the observation of turbulent episodes in the OH* layer is possible with this kind of instrument.…”

“…Within this inertial subrange of turbulence energy is cascaded to smaller and smaller structures until it is dissipated via viscous damping, causing a heating effect on the atmosphere. Observing turbulence episodes with high-resolution airglow imagers, the respective energy dissipation rate can be derived from the image series by reading out the typical length scale 𝐿 of the eddies and the root-meansquare velocity 𝑈, i.e., the velocity of single eddy patches relative to the background motion (Hecht et al, 2021;Sedlak et al, 2021).…”

Hydroxyl airglow observations for investigating atmospheric dynamics: results and challenges

“…Another possibility to analyse turbulence is the observation of vortices as Sedlak et al (2016) and Hecht et al (2021) showed based on case studies of an imager system with a very high spatial resolution of 17 and 25 m per pixel, respectively. In their more recent work, Sedlak et al (2021) extended their study to 1.5 years of data which include some 20 to 30 case studies. With an instrument originally set up for astronomical observations (NOTCam spectrograph mounted on the Nordic Optical Telescope (NOT) in La Palma), Franzen et al (2018) found quasi-periodic structures with minimal horizontal wavelengths of 4.5 m following a Kolmogorov-type energy cascade during breaking.…”

A statistical study of convective and dynamic instabilities in the polar upper mesosphere above Tromsø

“…Possible other instabilities from inertia and rotation, horizontal wind shear (cf. Sedlak et al 2021) or non-local turbulence could not be evaluated.…”