Modeling the B Regional Dust Storm on Mars: Observations and Simulated Dust Plumes

The "B storm" is a southern summer, regional scale dust event that has been observed annually over the south pole of Mars in years lacking a global dust storm (Kass et al., 2016; GRL, 43). Middle atmospheric temperatures and dust retrieved from orbit show that 50 Pa temperatures warm by ~20 K and zonal mean dust mixing ratios increase by 2-3 ppm as dust is lofted above the boundary layer. 50 Pa temperatures maximize over the pole around ~220 K and dust mixing ratios peak around 10 ppm at solstice.

The B storm is reproduced by NASA Ames Mars Global Climate Model running at 1°x1° horizontal resolution and is comprised of a series of dust plumes that form semi-regularly in the eastern hemisphere (~100° E) poleward of 60° S. The plumes are typically 50° longitude wide and 3-4 scale heights (SHs) tall (10-5 Pa) and can be 100° longitude wide and 5 SHs tall (3 Pa) at solstice. Plumes travel southwest for ~1 sol, sometimes producing detached dust layers that maintain altitude (25-3 Pa, 3-5 SHs) throughout their lifetime. At the center of the plumes, dust mixing ratios are ~14 ppm and shortwave heating (SWH) rates, which are highly correlated with the dust, exceed 60 K/day. Simulated vertical velocities are similarly correlated, exceeding 40 cm/s within the dust plumes on the dayside of the planet.

The relationship between dust, SWH rates, and upward vertical velocities suggests that dust radiative heating drives lofting with the plumes. This is consistent with recent studies that suggest dust radiative-dynamic feedbacks are crucial for the solar escalator effect (Daerden et al., 2015; GRL, 42) and rocket dust storms (Spiga et al., 2013; JGR, 118). We show that dust is never lofted above the boundary layer and dust plumes do not form when the radiative effects of the dust are excluded from the simulation.

In addition to our modeling study, we have traced the trajectories of hundreds of individual dust particles in the simulation using an offline trajectory code (Rafkin, personal communication). The trajectories indicate that differential heating of the dust plumes over a diurnal cycle causes periods of enhanced convection that accelerate lofting during the day. We have also found evidence of detached dust layers in MCS observations. At this meeting, we will summarize our observational, modeling, and trajectory study results.