The vertical distribution of dust in the Martian atmosphere during northern spring and summer: Observations by the Mars Climate Sounder and analysis of zonal average vertical dust profiles

The vertical distribution of dust in Mars's atmosphere is a critical and poorly known input in atmospheric physical and chemical models and a source of insight into the lifting and transport of dust and general vertical mixing in the atmosphere. We investigate vertical profiles of dust opacity retrieved from limb observations by Mars Climate Sounder during the relatively dust-clear Martian northern summer of 2006-2007 (L-s = 111 degrees-177 degrees of Mars year (MY) 28) and Martian northern spring and summer of 2007-2008 (L-s = 0 degrees-180 degrees of MY 29). To represent local maxima in inferred mass mixing ratio in these profiles, we develop an empirical alternative to the classic "Conrath profile" for representing the vertical distribution of dust in the Martian atmosphere. We then assess the magnitude and variability of atmospheric dust loading, the depth of dust penetration during these seasons, and the impact of the observed vertical dust distribution on the radiative forcing of the circulation. During most of northern spring and summer, the dust mass mixing ratio in the tropics has a maximum at 15-25 km above the local surface (the high-altitude tropical dust maximum (HATDM)). The HATDM appears to have increased significantly in magnitude and altitude during middle to late northern summer of MY 29. The HATDM gradually decayed during late summer of MY 28. Interannual variability in the dust distribution during middle to late northern summer may be connected with known interannual variability in tropical dust storm activity.