[1] The first global model of meteoric iron in the atmosphere (WACCM-Fe) has been developed by combining three components: the Whole Atmosphere Community Climate Model (WACCM), a description of the neutral and ion-molecule chemistry of iron in the mesosphere and lower thermosphere (MLT), and a treatment of the injection of meteoric constituents into the atmosphere. The iron chemistry treats seven neutral and four ionized iron containing species with 30 neutral and ion-molecule reactions. The meteoric input function (MIF), which describes the injection of Fe as a function of height, latitude, and day, is precalculated from an astronomical model coupled to a chemical meteoric ablation model (CABMOD). This newly developed WACCM-Fe model has been evaluated against a number of available ground-based lidar observations and performs well in simulating the mesospheric atomic Fe layer. The model reproduces the strong positive correlation of temperature and Fe density around the Fe layer peak and the large anticorrelation around 100 km. The diurnal tide has a significant effect in the middle of the layer, and the model also captures well the observed seasonal variations. However, the model overestimates the peak Fe + concentration compared with the limited rocket-borne mass spectrometer data available, although good agreement on the ion layer underside can be obtained by adjusting the rate coefficients for dissociative recombination of Fe-molecular ions with electrons. Sensitivity experiments with the same chemistry in a 1-D model are used to highlight significant remaining uncertainties in reaction rate coefficients, and to explore the dependence of the total Fe abundance on the MIF and rate of vertical transport.

It has been known since the 1960s that the layers of Na and K atoms, which occur between 80 and 105 km in the Earth's atmosphere as a result of meteoric ablation, exhibit completely different seasonal behavior. In the extratropics Na varies annually, with a pronounced wintertime maximum and summertime minimum. However, K varies semiannually with a small summertime maximum and minima at the equinoxes. This contrasting behavior has never been satisfactorily explained. Here we use a combination of electronic structure and chemical kinetic rate theory to determine two key differences in the chemistries of K and Na. First, the neutralization of K + ions is only favored at low temperatures during summer. Second, cycling between K and its major neutral reservoir KHCO 3 is essentially temperature independent. A whole atmosphere model incorporating this new chemistry, together with a meteor input function, now correctly predicts the seasonal behavior of the K layer.

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.

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A large number of temperature profiles of the altitude range 80 to 105 km were obtained between 71°S and 54°N latitude from late April to early July 1996. The measurements were performed by a ship‐borne lidar, resolving the Doppler broadening of the K(D1) resonance line. The most notable result of this field campaign is the finding that throughout our observation series the mesopause altitude was located at altitudes of either 100±3 km or 86±3 km. The high “winter” level extended from 71°S to 23°N, the low “summer” level from 24°N until the end of the field observations at 54°N. Our latitudinally distributed observations indicate strongly a worldwide bimodal character of the mesopause altitude. Furthermore, our mesopause temperatures are generally lower than previonsly measured in the northern hemisphere.