Modelling the impact of noctilucent cloud formation on atomic oxygen and other minor constituents of the summer mesosphere

International audienceThe formation, evolution and eventual sublimation of noctilucent clouds (NLC) may have a significant effect on the odd oxygen and hydrogen chemistry of the high latitude summer mesosphere. Three mechanisms are considered here: the direct uptake of atomic oxygen on the surface of the ice particles; the redistribution of water vapour, which changes the photochemical source of odd hydrogen species; and the direct photolysis of the ice particles themselves to produce odd hydrogen species in the gas phase. A 1-D photochemical model is employed to investigate the potential importance of these mechanisms. This shows, using the recently measured uptake coefficients of O on ice, that the heterogeneous removal of O on the surface of the cloud particles is too slow by at least a factor of 5x103 to compete with gas-phase O chemistry. The second and third mechanisms involve the solar Lyman-? photolysis of H2O in the gas and solid phase, respectively. During twilight, Lyman-? radiation is severely attenuated and these mechanisms are insignificant. In contrast, when the upper mesosphere is fully illuminated there is a dramatic impact on the O profile, with depletion of O at the base of the cloud layer of close to an order of magnitude. A correspondingly large depletion in O3 is also predicted, while H, OH, HO2 and H2O2 are found to be enhanced by factors of 3-5. In fact, rocket-borne mass spectrometer measurements during summer have revealed local H2O2 enhancements in the region of the clouds. Rocket-borne measurements of atomic O and O3 profiles in the presence of mesospheric clouds in the daytime are highly desirable to test the predictions of this model and our understanding of the genesis of mesospheric clouds

Diurnal variation of the potassium layer in the upper atmosphere

Measurements of the diurnal cycle of potassium (K) atoms between 80 and 110km have been made during October (for the years 2004–2011) using a Doppler lidar at Kühlungsborn, Germany (54.1°N,11.7°E). A pronounced diurnal variation is observed in the K number density, which is explored by using a detailed description of the neutral and ionized chemistry of K in a three-dimensional chemistry climate model. The model captures both the amplitude and phase of the diurnal and semidiurnal variability of the layer, although the peak diurnal amplitude around 90 kmis overestimated. Themodel shows that the total potassium density (≈K+K++KHCO3) exhibits little diurnal variation at each altitude, and the diurnal variations are largely driven by photochemical conversion between these reservoir species. In contrast, tidally driven vertical transport has a small effect at this midlatitude location, and diurnal fluctuations in temperature are of little significance because they are small and the chemistry of K is relatively temperature independent

Numerical simulations of metallic ion density perturbations in sporadic E layers caused by gravity waves

Tidal signatures in sporadic E (Es) layer have been confirmed by observations and simulations. However, the effect of gravity waves (GWs) on the Es layer formation process has not yet been fully understood. In this paper, the modulation of Es layers by GWs is examined through numerical simulations, in which a physics-based model of Es layer is forced by neutral winds from the High Altitude Mechanistic General Circulation Model that can resolve GWs with horizontal wavelengths longer than 156 km (λh > 156 km). Comparison of the simulation results with and without the GWs (1,350 km > λh > 156 km) forcing reveals that the inclusion of GWs leads to short-period (1.2–3 hr) density perturbations in Es layers, which are also seen in ground-based ionosonde observations. At a given time, the metallic ion density at altitudes between 120 and 150 km can either increase (by up to ∼+600%) or reduce (by up to −90%) in response to GW forcing. The relative density perturbations are smaller (by up to 60%) between 90 and 120 km altitude. It is also found that the GW effect on the metallic ion density relates to the longitude, which is mostly explained by the geographical distribution of GWs activity in the mesosphere and lower thermosphere region. The longitudinal variation of the background geomagnetic field plays only a secondary role

The photolysis of FeOH and its effect on the bottomside of the mesospheric Fe layer

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±3)×10−3s−1. We also show that the reaction FeOH+H→FeO+H2 is slower at mesospheric temperatures than previous estimates. With these updated rate coefficients, we are able to significantly improve the modelling 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

On the size and velocity distribution of cosmic dust particles entering the atmosphere

The size and velocity distribution of cosmic dust particles entering the Earth's atmosphere is uncertain. Here we show that the relative concentrations of metal atoms in the upper mesosphere, and the surface accretion rate of cosmic spherules, provide sensitive probes of this distribution. Three cosmic dust models are selected as case studies: two are astronomical models, the first constrained by infrared observations of the Zodiacal Dust Cloud and the second by radar observations of meteor head echoes; the third model is based on measurements made with a spaceborne dust detector. For each model, a Monte Carlo sampling method combined with a chemical ablation model is used to predict the ablation rates of Na, K, Fe, Mg, and Ca above 60 km and cosmic spherule production rate. It appears that a significant fraction of the cosmic dust consists of small (<5 µg) and slow (<15 km s−1) particles

Summer time Fe depletion in the Antarctic mesopause region

We report common volume measurements of Fe densities, temperatures and ice particle occurrence in the mesopause region at Davis Station, Antarctica (69°S) in the years 2011–2012. Our observations show a strong correlation of the Fe-layer summer time depletion with temperature, but no clear causal relation with the onset or occurrence of ice particles measured as noctilucent clouds (NLC) or polar mesosphere summer echoes (PMSE). The combination of these measurements indicates that the strong summer depletion can be explained by gas-phase chemistry alone and does not require heterogeneous removal of Fe and its compounds on ice particles

The 27-day solar rotational cycle response in the mesospheric metal layers at low latitudes

To investigate the response of the meteoric metal layers in the mesosphere and lower thermosphere region to the 27-day solar rotational cycle, a long-term simulation of the Whole Atmosphere Climate Community Model (WACCM) with the chemistry of three metals (Na, K, and Fe) was analysed. The correlation between variability in the metal layers and solar 27-day forcing during different phases of the solar 11-year cycle reveals that the response in the metal layers is much stronger during solar maximum. The altitude dependent correlation and sensitivity of the metal layers to the solar spectral irradiance demonstrates that there is a significant increase in sensitivity to solar rotational cycle with increasing altitude. Above 100 km, the sensitivity of the metals to changes of 10% in the SSI at Lyman-alpha is estimated to be -5%. A similar response is seen in Na layer measurements made by the OSIRIS instrument on the Odin satellite

First global observations of the mesospheric potassium layer

Metal species, produced by meteoric ablation, act as useful tracers of upper atmosphere dynamics and chemistry. Of these meteoric metals, K is an enigma: at extratropical latitudes, limited available lidar data show that the K layer displays a semiannual seasonal variability, rather than the annual pattern seen in other metals such as Na and Fe. Here we present the first near-global K retrieval, where K atom number density profiles are derived from dayglow measurements made by the Optical Spectrograph and Infrared Imager System spectrometer on board the Odin satellite. This robust retrieval produces density profiles with typical layer peak errors of ±15% and a 2km vertical grid resolution. We demonstrate that these retrieved profiles compare well with available lidar data and show for the first time that the unusual semiannual behavior is near-global in extent. This new data set has wider applications for improving understanding of the K chemistry and of related upper atmosphere processes. Key Points First quantitative retrieval of the terrestrial K layer from space The unusual semiannual behavior of K is near global in extent