Abstract
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.
Highlights
[2] The mesosphere lower thermosphere (MLT) region ( 60–120 km) connects the atmosphere below with space above and is a region of increasing scientific and practical interest, because this region is affected by solar variability and climate change
[13] One of the important loss processes for the metal layers is the loss of iron-containing molecules on meteoric smoke particles (MSPs) [Plane, 2004], which have been observed through a number of independent techniques including the first global measurements using optical extinction with the SOFIE instrument on the AIM satellite [Hervig et al, 2009a, 2009b]
In the Whole Atmosphere Community Climate Model (WACCM)-Fe run presented in this study, the change in the Fe column abundance from solar minimum (2005 to 2007) to solar maximum (2009 to 2011) ranges from –3% at high southern hemisphere (SH) latitudes to +3% at NH middle to high latitudes
Summary
[2] The mesosphere lower thermosphere (MLT) region ( 60–120 km) connects the atmosphere below with space above and is a region of increasing scientific and practical interest, because this region is affected by solar variability and climate change. [3] Previously, modeling studies of the mesospheric metal layers have employed 1-D models to investigate the chemistry controlling the metal layers [e.g., Plane, 2003; Plane and Whalley, 2012]. These models are useful for optimizing detailed neutral and ion-molecule chemistry schemes, where not all the relevant rate coefficients have been measured in the laboratory under mesospheric conditions. The model output is typically compared to lidar observations at a single observing location. This approach has been reasonably successful, since the removal lifetime of a metal atom
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