Large eddy simulation (LES) of the Martian convective boundary layer (CBL) with a Mars-adapted version of the Weather Research and Forecasting model (MarsWRF) is used to examine aerosol dust radiative-dynamical feedback upon turbulent mixing. The LES is validated against spacecraft observations and prior modeling. To study dust redistribution by coherent dynamical structures within the CBL, two radiatively-active dust distribution scenarios are used—one in which the dust distribution remains fixed and another in which dust is freely transported by CBL motions. In the fixed dust scenario, increasing atmospheric dust loading shades the surface from sunlight and weakens convection. However, a competing effect emerges in the free dust scenario, resulting from the lateral concentration of dust in updrafts. The resulting enhancement of dust radiative heating in upwelling plumes both generates horizontal thermal contrasts in the CBL and also increases buoyancy production, jointly enhancing CBL convection. We define a dust inhomogeneity index (DII) to quantify how much dust is concentrated in upwelling plumes. If the DII is large enough, the destabilizing effect of lateral heating contrasts can exceed the stabilizing effect of surface shading such that the CBL depth increases with increasing dust optical depth. Thus, under certain combinations of total dust optical depth and the lateral inhomogeneity of dust, a positive feedback may exist among dust optical depth, the vigor and depth of CBL mixing, and—to the extent that dust lifting is controlled by the depth and vigor of CBL mixing—the further lifting of dust from the surface.

Mars planet-encircling or global dust storms are an iconic and enigmatic feature of the Red Planet. Occurring every few Mars Years, on average, they are a stochastic process in the otherwise largely repeatable annual cycle of martian weather. In 2018 (Mars Year 34 in the calendar of Clancy et al. [2000]), an international fleet of spacecraft, 6 orbiters and 2 rovers, observed the most recent global dust storm. This introduction and the articles of this special collection describe the evolution and impacts of the storm from the surface to the exosphere, compare this storm to previous global dust storms, identify new phenomena never-before seen in such storms, and attempt to determine how and when global dust storms develop.

Polar vortices are a prominent feature in Titan's stratosphere. The Cassini mission has provided a detailed view of the breakdown of the northern polar vortex and formation of the southern vortex, but the mission did not observe the full annual cycle of the evolution of the vortices. Here we use a TitanWRF general circulation model simulation of an entire Titan year to examine the full annual cycle of the polar vortices. The simulation reveals a winter weakening of the vortices, with a clear minimum in polar potential vorticity and mid-latitude zonal winds between winter solstice and spring equinox. The simulation also produces the observed post-autumn equinox cooling followed by rapid warming in the upper stratosphere. This warming is due to strong descent and adiabatic heating, which also leads to the formation of an annular potential vorticity structure. The seasonal evolution of the polar vortices is very similar in the two hemispheres, with only small quantitative differences that are much smaller than the seasonal variations, which can be related to Titan's orbital eccentricity. This suggests that any differences between observations of the northern hemisphere vortex in late northern winter and the southern hemisphere vortex in early winter are likely due to the different observation times with respect to solstice, rather than fundamental differences in the polar vortices.