Martian global dust storms: Zonally symmetric numerical simulations including size‐dependent particle transport

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Abstract
The size‐dependent transport of dust particles in the Martian atmosphere away from a specified surface source is numerically investigated employing a coupled system of a zonally symmetric primitive equation grid point model of the Martian atmosphere and an aerosol transport/microphysical model. The coupled model accounts for diabatic heating due to a radiatively active evolving dust field but neglects feedbacks between atmosphere‐surface interactions and surface dust lifting, which is prescribed. The differing suspension lifetimes of various sized dust particles (radius = 1–80 μm) in conjunction with spatially varying atmospheric dynamics results in latitudinal differences in several measurements of the column integrated particle concentration. The latitudinal extent of dust (opacity) transport away from a southern subtropical source is enhanced by the presence in suspension of smaller dust particles (r < 1 μm) when compared to the presence of only a single dust particle size (2.5 μm). The input particle size distribution, that inferred by Toon et al. (1977) from Mariner 9 IRIS measurements, is not preserved in suspension at subsolar latitudes for spherical particles (r = 1–10 μm), counter to Toon et al.'s interpretation of IRIS spectra. Visible to 9‐μm dust opacity ratios, which provide an indication of the relative number of large particles, are generally smaller (<1.7) than values (2–2.5) inferred from both Mariner 9 and Viking measurements. The invocation of slower falling nonspherical (disk) shaped particles results in an improved maintenance in suspension of the input particle size distribution at subtropical latitudes and visible to‐9 μm opacity ratios in better agreement with inferred values. Calculated visible opacity values at the Viking lander latitudes increase less abruptly than was observed at the onsets of the two 1977 dust storms, which has implications for both source locations and magnitudes. This work indicates the importance of considering the full range of particle sizes (and shapes) of the suspended dust during Martian global dust storms and their impact upon the spatial extent and wavelength‐dependent radiative influence of such storms.