A 16 month series of lidar measurements in the stratosphere and mesosphere-lower thermosphere (MLT) region over Davis Station (69 ∘ S, 78 ∘ E) in Antarctica is used to study gravity waves. The unprecedentedly large number of observations totaling 2310 h allows us to investigate seasonal variations in gravity wave activity in great detail. In the stratosphere the gravity wave potential energy density (GWPED) is shown to have a large seasonal variation with a double peak in winter and minimum in summer. We find conservative wave propagation to occur between 29 and 41 km altitude in winter as well as in summer, whereas smaller energy growth rates were observed in spring and autumn. These results are consistent with selective critical-level filtering of gravity waves in the lower stratosphere. In the MLT region the GWPED is found to have a semiannual oscillation with maxima in winter and summer. The structure of the winter peak is identical to that in the stratosphere, suggesting that the gravity wave flux reaching the MLT region is controlled by the wind field near the tropopause level. IntroductionAtmospheric gravity waves are important for vertical coupling in the atmosphere. They transport energy and momentum vertically and horizontally over large distances. At high latitudes, dissipation of these waves in the mesosphere-lower thermosphere region (hereafter MLT region) transfers momentum into the background flow, driving a global meridional circulation from the summer pole to the winter pole [Lindzen, 1981;Holton, 1983]. Associated with this circulation is the upwelling of air at the summer pole causing the strong adiabatic cooling of the summer MLT region [Andrews et al., 1987;Becker, 2012]. This gravity wave-induced cooling gives rise to observed temperatures as low as 130 K which are far from radiative equilibrium [Lübken, 1999;Lübken et al., 2014]. For this reason, phenomena like noctilucent clouds and polar mesospheric summer echoes are limited to the summer polar region [Olivero and Thomas, 1986]. Without gravity wave-induced cooling, temperatures in the summer MLT remain above the frost point [Rapp and Thomas, 2006]. The occurrence of noctilucent clouds is thus a result of gravity waves propagating from the troposphere/lower stratosphere into the MLT region.Gravity waves have been extensively studied in models [e.g., Zhang, 2004] as well as through employing observational techniques such as lidars [e.g., Rauthe et al., 2008;Yamashita et al., 2009], radars [e.g., Nicolls et al., 2010 Lue et al., 2013], radiosondes [e.g., Allen andVincent, 1995;Moffat-Griffin et al., 2011], satellite-based radiometers [e.g., Alexander et al., 2008;Wright and Gille, 2013], and Global Positioning System radio occultation [e.g., Wang and Alexander, 2010]. Among all observational techniques, lidars provide the highest temporal and vertical resolutions over a wide altitude range and observation periods up to several days.

We present the first detection of thermal tides in the middle atmosphere at polar latitudes in summer. The IAP iron lidar is in operation in Davis (69°S, 78°E), Antarctica, since December 2011 and measures temperatures in the height range 84–96 km with an accuracy of 1–3 K (after 1 hour integration) with an effective altitude resolution of 1 km. Iron densities are observed from ∼75–140 km. We analyze 171 hours of observations in the period 11–28 January, 2011, and find strong tidal modulations in Fe density and temperatures. Typical amplitudes of thermal tides are 2–4 K for both the diurnal and semidiurnal component. The diurnal tide is larger (smaller) than the semidiurnal component below (above) ∼90 km. The phase of the diurnal tide decreases with altitude by ∼1.3 h/km which corresponds to a vertical wavelength of ∼30 km. A similar phase progression is observed in Fe densities extending below and above the height range of temperature measurements. The overlay of diurnal and semidiurnal components leads to tidal modulations of up to ±6 K for temperatures, and up to ±4000/ccm (=±40%) for Fe number densities. These modulations are much larger compared to most simulations and point to some missing processes in tidal modeling.

Metal layers in the upper mesosphere and lower thermosphere are created through meteoric ablation. They are important for understanding the temperature structure, dynamics, and chemistry of this atmospheric region. Recent lidar observations have shown a regular downward extension of the Fe layer bottomside which correlates with solar radiation. In this study we combine lidar observations, quantum chemical calculations, and model simulations to show that this bottomside extension is primarily caused by photolysis of FeOH. We determine the photolysis rate to be J(FeOH)=()6±30.3em×0.3em10−3 s−1. We also show that the reaction normalFeOH+H0.3em→0.3emnormalFeO+H2 is slower at mesospheric temperatures than previous estimates. With these updated rate coefficients, we are able to significantly improve the modeling of the Fe layer bottomside. The calculations further show the nearly complete depletion of FeOH during sunlit periods. This may have implications for cloud nuclei in the middle atmosphere.

We present quasi‐continuous measurements of temperature profiles in the Southern Hemisphere mesopause region during the transition from winter to summer conditions in 2011/2012. In a period of 120 days around solstice, we have performed iron lidar observations at Davis (69°S), Antarctica, for a total of 736 h. The winter/summer transition is identified by a downward shift of the mesopause which occurs on 8 November 2011. Soon after transition, mesopause heights and temperatures are similar to the Northern Hemisphere (NH) colatitude summer (88 km, 130 K). Around solstice, the mesopause is elevated for several days by 4–5 km and is colder than typical NH temperatures by 10 K. In this period individual profiles show temperatures as low as 100 K. The occurrence of polar mesosphere summer echoes is closely connected to low temperatures. Below 88 to 90 km and in the main summer season of 2011/2012 temperatures at Davis are generally warmer compared to the NH by 5–15 K, whereas temperatures are generally colder above 90 km. The winter/summer transition and the first appearance of polar mesosphere summer echoes are strongly correlated to maximum zonal winds in the stratosphere which constrain gravity waves with eastward momentum reaching the mesosphere. At the breakdown of the stratospheric vortex around solstice, the mesopause is higher and, surprisingly, colder than normal.