A document by Tyler Mixa. Click on the document to view its contents.

Structure of the quasi-2-day wave (Q2DW) in the mesosphere and lower thermosphere (MLT) was compared between the northern and southern hemispheres, employing temperature and geopotential height data obtained from the Microwave Limb Sounder (MLS) onboard NASA’s Earth Observing System (EOS) Aura satellite. The Q2DW in horizontal winds was derived using balance equations with MLS geopotential height data. Amplitudes were maximized at ~40° in summer with larger amplitudes in the meridional wind than the zonal wind in both hemispheres, but with much larger amplitudes in the southern hemisphere and a longer duration of enhancements in the northern hemisphere. Weaker enhancements were exhibited in winter in both hemispheres, but maximized at higher latitudes only in the southern hemisphere meridional component. Responses were moderately enhanced from late April to early May only in the southern hemisphere. The westward propagating zonal wavenumber 3 (W3) was largest in summer in both hemispheres, but the Q2DW comprised superposition with other modes in winter. Eliassen-Palm fluxes were derived for each mode. In the southern hemisphere, W3, W2, and W1 in January exhibited upward fluxes at lower latitudes, poleward fluxes at lower altitudes and equatorward fluxes at higher altitudes. A W3 mode in July in the northern hemisphere, on the other hand, exhibited upward and poleward fluxes in the entire altitude range. The Q2DW balance winds were compared with the radar winds. They agreed reasonably in amplitude and phase in summer in the southern hemisphere and lower latitudes in summer in the northern hemisphere and in winter hemispheres.

Gravity waves (GWs) generated by orographic forcing, also known as mountain waves (MWs) have been studied for decades. First measured in the troposphere, then in the stratosphere, they were only imaged at mesospheric altitude in 2008. Their characteristics have been investigated during several recent observation campaigns, but many questions remain concerning their impacts on the upper atmosphere, and the effects of the background environment on their deep propagation. An Advanced Mesospheric Temperature Mapper (AMTM) and the Southern Argentina Agile MEteor Radar (SAAMER) have been operated simultaneously during the Austral winter 2018 from Rio Grande, Argentina (53.8°S). This site is located near the tip of South America, in the lee of the Andes Mountains, a region considered the largest MW hotspot on Earth. New AMTM image data obtained during a 6-month period show almost 100 occurrences of MW signatures penetrating into the upper mesosphere. They are visible ~30% of time at the height of the winter season (mid-May to mid-July). Their intermittency is highly correlated with the zonal wind controlled by the semi-diurnal tide, revealing the direct effect of the atmospheric background on MW penetration into the Mesosphere Lower Thermosphere (MLT, altitude 80-100 km). Measurements of their momentum fluxes (MF) were determined to reach very large values (average ~250 m/s), providing strong evidence of the importance and impacts of small-scale gravity waves at mesospheric altitudes.

Polar mesospheric cloud (PMC) imaging and lidar profiling performed aboard the 5.9 day PMC Turbo balloon flight from Sweden to northern Canada in July 2018 revealed a wide variety of gravity wave (GW) and instability events occurring nearly continuously at approximately 82 km. We describe one event exhibiting GW breaking and associated vortex rings driven by apparent convective instability. Using PMC Turbo imaging with spatial and temporal resolution of 20 m and 2 s, respectively, we quantify the GW horizontal wavelength, propagation direction, and apparent phase speed, and we identify vortex rings with diameters of 3-5 km and horizontal spacing of ~5 km. Lidar data show GW vertical displacements of ±0.3 km. From the data, we find a GW intrinsic frequency and vertical wavelength of 0.009 ± 0.003 rad s-1 and 9 ± 4 km, respectively. We show that these values are consistent with the predictions of numerical simulations of idealized GW breaking. We estimate the momentum deposition rate per unit mass during this event to be 0.04 ± 0.02 m s-2 and show that this value is consistent with the observed GW. Comparison to simulation gives a mean energy dissipation rate for this event of 0.05-0.4 W kg-1, which is consistent with other reported in-situ measurements at the Arctic summer mesopause.

Spectral model turbulence analysis technique is widely used to derive kinetic energy dissipation rates of turbulent structures (ε) from different in situ measurements in the Earth’s atmosphere. Essence of this method is to fit a model spectrum to measured spectra of velocity or scalar quantity fluctuations and thereby to derive ε only from wavenumber dependence of turbulence spectra. Owing to simplicity of spectral model of Heisenberg (1948) its application dominates in the literature. Making use of direct numerical simulations (DNS) which are able to resolve turbulence spectra down to smallest scales in dissipation range, we advance the spectral model technique by quantifying uncertainties for two spectral models, the Heisenberg (1948) and the Tatarskii (1971) model, depending on 1) resolution of measurements, 2) stage of turbulence evolution, 3) model used. We show that model of Tatarskii 1971 can yield more accurate results and reveals higher sensitivity to lowest ε-values. This study shows that the spectral model technique can reliably derive ε if measured spectra only resolve half decade of power change within viscous (viscous-convective) subrange. In summary we give some practical recommendations how to derive most precise and detailed turbulence dissipation field from in situ measurements depending on their quality. We also supply program code of the spectral models used in this study in Python, IDL, and Matlab.