MARTIAN GLOBAL DUST STORMS - ZONALLY SYMMETRICAL NUMERICAL SIMULATIONS INCLUDING SIZE-DEPENDENT PARTICLE-TRANSPORT

The size-dependent transport of dust particles in the Martian atmosphe re away from a specified surface source is numerically investigated em ploying a coupled system of a zonally symmetric primitive equation gri d point model of the Martian atmosphere and an aerosol transport/micro physical model. The coupled model accounts for diabatic heating due to a radiatively active evolving dust field but neglects feedbacks betwe en atmosphere-surface interactions and surface dust lifting, which is prescribed. The differing suspension lifetimes of various sized dust p articles (radius = 1-80 mum) in conjunction with spatially varying atm ospheric dynamics results in latitudinal differences in several measur ements of the column integrated particle concentration. The latitudina l extent of dust (opacity) transport away from a southern subtropical source is enhanced by the presence in suspension of smaller dust parti cles (r < 1 mum) when compared to the presence of only a single dust p article size (2.5 mum). The input particle size distribution, that inf erred by Toon et al. (1977) from Mariner 9 IRIS measurements, is not p reserved in suspension at subsolar latitudes for spherical particles ( r = 1 - 10 mum), counter to Toon et al.'s interpretation of IRIS spect ra. Visible to 9-mum 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 measurem ents. The invocation of slower falling nonspherical (disk) shaped part icles results in an improved maintenance in suspension of the input pa rticle size distribution at subtropical latitudes and visible to 9-mum opacity ratios in better agreement with inferred values. Calculated v isible opacity values at the Viking lander latitudes increase less abr uptly than was observed at the onsets of the two 1977 dust storms, whi ch has implications for both source locations and magnitudes. This wor k indicates the importance of considering the full range of particle s izes (and shapes) of the suspended dust during Martian global dust sto rms and their impact upon the spatial extent and wavelength-dependent radiative influence of such storms.