Abstract
Abstract. During the last decade, several limb sounding satellites have measured the global sodium (Na) number densities in the mesosphere and lower thermosphere (MLT). Datasets are now available from Global Ozone Monitoring by Occultation of Stars (GOMOS), the SCanning Imaging Absorption spectroMeter for Atmospheric CHartography (SCIAMACHY) (both on Envisat) and the Optical Spectrograph and InfraRed Imager System (OSIRIS) (on Odin). Furthermore, global model simulations of the Na layer in the MLT simulated by the Whole Atmosphere Community Climate Model, including the Na species (WACCM-Na), are available. In this paper, we compare these global datasets.The observed and simulated monthly averages of Na vertical column densities agree reasonably well with each other. They show a clear seasonal cycle with a summer minimum most pronounced at the poles. They also show signs of a semi-annual oscillation in the equatorial region. The vertical column densities vary from 0. 5 × 109 to 7 × 109 cm−2 near the poles and from 3 × 109 to 4 × 109 cm−2 at the Equator. The phase of the seasonal cycle and semi-annual oscillation shows small differences between the Na amounts retrieved from different instruments. The full width at half maximum of the profiles is 10 to 16 km for most latitudes, but significantly smaller in the polar summer. The centroid altitudes of the measured sodium profiles range from 89 to 95 km, whereas the model shows on average 2 to 4 km lower centroid altitudes. This may be explained by the mesopause being 3 km lower in the WACCM simulations than in measurements. Despite this global 2–4 km shift, the model captures well the latitudinal and temporal variations. The variation of the WACCM dataset during the year at different latitudes is similar to the one of the measurements. Furthermore, the differences between the measured profiles with different instruments and therefore different local times (LTs) are also present in the model-simulated profiles. This capturing of latitudinal and temporal variations is also found for the vertical column densities and profile widths.
Highlights
The metal layers in the mesosphere and lower thermosphere (MLT) are formed by ablation from meteoroids entering the Earth’s atmosphere (see, e.g., Plane (2003) and Plane et al (2015) for reviews)
Throughout the whole layer, especially at the bottom, the metals react to form molecular species such as carbonates, hydroxides and oxides. These molecules further are involved in chemical processes and produce condensation nuclei for the formation of particles eventually resulting in meteoric smoke particles
Ensemble mean (M), Global Ozone Monitoring by Occultation of Stars (GOMOS) (G), Optical Spectrograph and InfraRed Imager System (OSIRIS) descending leg (Od), OSIRIS ascending leg (Oa), SCIAMACHY dayglow (S), WACCM colocated with the local times (LTs) of ascending leg of GOMOS (W G), WACCM colocated with the LT of the descending leg of OSIRIS (W Od), WACCM colocated with the LT of the ascending leg of OSIRIS (W Oa), WACCM colocated with the dayglow measurements of SCIAMACHY and the descending leg of GOMOS (W S)
Summary
The metal layers in the mesosphere and lower thermosphere (MLT) are formed by ablation from meteoroids entering the Earth’s atmosphere (see, e.g., Plane (2003) and Plane et al (2015) for reviews). In the last decades have spaceborne spectrometer measurements provided number density profiles or column density datasets with (nearly) global coverage for continuous time periods of several years These spaceborne spectrometers typically were on board satellites with Sun-synchronous and polar orbits and a maximum scanned latitude of up to 82◦ that retrieved densities for Mg and/or Mg+ (see, e.g., Joiner and Aikin, 1996; Correira et al, 2008; Scharringhausen et al, 2008; Langowski et al, 2015), K (see, e.g., Dawkins et al, 2014) and Na (see, e.g., Fussen et al, 2004; Casadio et al, 2007; Fan et al, 2007; Gumbel et al, 2007; Fussen et al, 2010; Hedin and Gumbel, 2011; Langowski et al, 2016). Ensemble mean (M), GOMOS (G), OSIRIS descending leg (Od), OSIRIS ascending leg (Oa), SCIAMACHY dayglow (S), WACCM colocated with the LT of ascending leg of GOMOS (W G), WACCM colocated with the LT of the descending leg of OSIRIS (W Od), WACCM colocated with the LT of the ascending leg of OSIRIS (W Oa), WACCM colocated with the dayglow measurements of SCIAMACHY and the descending leg of GOMOS (W S)
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