Vertical distribution of dust in the Martian atmosphere during northern spring and summer: High-altitude tropical dust maximum at northern summer solstice

The vertical distribution of dust in Mars' atmosphere is a critical unknown in the simulation of its general circulation and a source of insight into the lifting and transport of dust. Zonal average vertical profiles of dust opacity retrieved by Mars Climate Sounder show that the vertical dust distribution is mostly consistent with Mars general circulation model (GCM) simulations in southern spring and summer but not in northern spring and summer. Unlike the GCM simulations, the mass mixing ratio of dust has a maximum at 1525 km over the tropics during much of northern spring and summer: the high-altitude tropical dust maximum (HATDM). The HATDM has significant and characteristic longitudinal variability, which it maintains for time scales on the order of or greater than those on which advection, sedimentation, and vertical eddy diffusion would act to eliminate both the longitudinal and vertical inhomogeneity of the distribution. While outflow from dust storms is able to produce enriched layers of dust at altitudes much greater than 25 km, tropical dust storm activity during the period in which the HATDM occurs is likely too rare to support the HATDM. Instead, the lifting of dust by mesoscale circulations over topography, pseudomoist convection due to the solar heating of dust, and scavenging of dust by water ice are all possible drivers of the HATDM.