Modelling the Transport of Meteoric Sodium through the Mesosphere-Lower Thermosphere

Sodium is released by meteors as they ablate in the Earth's atmosphere. Subsequent neutral and ion chemistry results in the formation of an atomic sodium layer between 80-100 km altitude, peaking at around 90 km, which is well observed by both satellite and lidar instruments. The relatively long chemical lifetime of the sodium layer allows its use in dynamical studies of the mesosphere-lower thermosphere (MLT). However, a discrepancy exists between the observed and simulated magnitude of the layer when using best estimates of the rate of sodium injection into the atmosphere. Using the US Whole Atmosphere Community Climate Model (WACCM), Marsh et al. (A global model of meteoric sodium, J. Geophys. Res., 118, 11,442- 11,452, doi:10.1002/jgrd.50870, 2013) required the input of meteoric material to be reduced by a factor of five to replicate observations. Issues in the meridional transport of sodium through the MLT were also identified. There is therefore motivation to investigate this problem using chemistry-climate models with different dynamical core formulations and horizontal resolutions.

We report results from simulations using an extended version UK Met Office Unified Model coupled to the UK Chemistry and Aerosol model (UM-UKCA) including a sodium chemistry scheme. This is an intermediate 100 km altitude lid version of the model (previously reaching 85 km) with higher extensions under development. UM-UKCA performance is evaluated by comparing the concentration and distribution of key sodium species, as well as the seasonal transport of sodium in the MLT, to observations and the WACCM study. Our findings provide: first, an understanding of the balance between the input of sodium and the resulting layer size, and second, an insight into the importance of accurate model representation of MLT transport in replicating the evolution of the sodium layer over seasonal timescales.