Multi-scale Modelling of Mesospheric Metals (4M)

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Roughly 50 tonnes of interplanetary dust enters the earth's atmosphere every day. The dust particles collide with air molecules at speeds between 11 and 72 km/s, causing most of the particles to flash heat, melt and evaporate. This produces metal atoms which then appear in layers between about 75 and 110 km. The Na, Fe, Ca and Ca+ layers have been observed since the 1970s using the ground-based lidar technique. Recently it has become possible to observe the metal layers, including Mg and Mg+, on a global basis using optical instruments on satellites. The need to explain these atmospheric observations has stimulated laboratory studies of the reactions which these metals and their ions undergo in the atmosphere, and the consequent development of local scale atmospheric models. The mesospheric metal layers are extremely useful probes of the chemistry and dynamics of the upper atmosphere. This is because the layers, whose widths of just a few km are controlled by fast photochemical processes, are very responsive to dynamical processes such as gravity waves and tides, and changes in atmospheric constituents such as O, H and O3. Noctilucent clouds, which form around 83 km when the temperature falls below 150 K during summer at high latitudes, cause substantial depletion of the metal layers because of rapid uptake of the metals on the ice surfaces. During winter at high latitudes, convergence of the meridional circulation over the polar vortex appears to cause a substantial increase in the metal concentrations. Solar proton events also cause significant perturbations to the metal layers. Lastly, the chemistry which controls the heights of the layers is largely driven by atmospheric pressure, and so the layer heights are sensitive to global cooling caused by increasing greenhouse gases such as CO2 and CH4 in the middle atmosphere. The objective of this proposal is to produce the first global model of four metals - sodium, iron, calcium and magnesium. These metals all behave quite differently in the mesosphere. We will insert the chemistry of these metals into a state-of-the-art general circulation model, the Whole Atmosphere Chemistry Climate Model (WACCM), which has been developed at the US National Center for Atmospheric Research over the past decade. This general circulation model extends from the earth's surface to 140 km, and includes all the neutral and ionized constituents with which the metals interact. In preparation for this project, we have recently installed and run WACCM on the UK's front-line national supercomputing service HECToR (High-End Computing Terascale Resource). Modelling the metal layers also requires as input the rates at which each metal is injected into the atmosphere from ablating interplanetary dust, as a function of height, season, latitude and time-of-day. These injection rates will be calculated using our new Chemical Ablation Model, combined with an astronomical model of the meteor input function. A project student will retrieve, for the first time, a global data set of Fe and Fe+ observations, using the SCIAMACHY instrument on ENVISAT. This will supplement the ground-based lidar measurements of Fe. The WACCM predictions of the metal layer densities, peak heights, and diurnal and seasonal variability will be compared with the observational data base. The WACCM mesospheric winds and temperatures will also be compared with measurements. These comparisons will enable the WACCM mesosphere to be optimised e.g. through higher vertical resolution and better treatment of gravity waves. We will then investigate the likely impact of the solar cycle and climate change on the distributions of all four metals, for instance as a guide to future observations. Finally, the improved model will be used to study the impact of the mesosphere on stratospheric ozone and climate.