Gravity wave instability structures and turbulence from more than one and a half 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/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) 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 (westward) during winter (summer) may be due to secondary gravity waves originating from breaking primary waves 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. Furthermore, observations of turbulent vortices allowed the estimation of eddy diffusion coefficients in the UMLT from image sequences in 45 cases. Values range around 103–104 m2s-1 and mostly agree with literature. Turbulently dissipated energy is derived taking into account values of the Brunt-Väisälä frequency based on TIMED-SABER (Thermosphere Ionosphere Mesosphere Energetics Dynamics, Sounding of the Atmosphere using Broadband Emission Radiometry) measurements. Energy dissipation rates range between 0.63 W/kg and 14.21 W/kg leading to an approximated maximum heating of 0.2–6.3 K per turbulence event. These are in the same range as the daily chemical heating rates, which apparently stresses the importance of dynamical energy conversion in the UMLT.

Gravity Wave Breaking Associated with Mesospheric Inversion Layers as Measured by the Ship-Borne BEM Monge Lidar and ICON-MIGHTI

“…At 90 km, the extreme feature disappeared, and the wave periods returned to the pattern seen below 80 km. The transient nature of this phenomenon lends further credence to the idea that this could be an instability of the type reported in OH camera data by Sedlak et al [23]. In Figure 11, we see nine frames that represent the continuous wavelet transform scalogram taken along the vertical grid lines shown in Figure 9.…”

“…We also see an increase in the occurrence and intensity of these events between 80 and 90 km-most notably in the statically unstable bottom side of MILs. The observations reported by OH airglow imagers, at 87 km, which show small-scale GW instabilities and turbulence [23], on the same temporal scales, suggest that the high-frequency "hot spots" in our scalograms may be the same phenomena. The scalogram analysis as a function of vertical wavelength shows that, for vertical wavelengths between 2 and 22 km, most energy was present above 80 km, which is consistent with the theoretical work for wind-wave interactions given by McLandress and Ward [36].…”

“…All sky CCD cameras measuring infrared emissions from the OH layer have previously shown small-scale GW "ripples" such as Isler et al [9], Taylor et al [22] and, more recently, Sedlak et al [23]. These events have horizontal scales between 5 and 15 km, and periods approaching the local Brunt-Väisälä frequency (approx.…”