Impact of a bimodal dust distribution on the 2018 Martian global dust storm with the NASA Ames Mars global climate model

Dust plays a critical role in the Martian climate, significantly impacting heating and circulation in the atmosphere. Global dust storms (GDS) are planet-encircling extreme dust events that occur every few Mars years with the most recent one happening in 2018, Mars Year (MY) 34. While they do occur on a relatively regular basis (recent ones occurring in 2001/MY25, 2007/MY28, 2018/MY34), much is still to be learned about the processes that lead to various characteristics of dust during these storms. Global climate models (GCMs) have been useful tools to interpret various observations made during the MY34 GDS. Here, we have implemented a bimodal dust lifting scheme in the NASA Ames Mars GCM. This new method for lifting dust splits the mass and number of dust particles lifted into multiple log-normal size modes as prescribed. We ran simulations assuming a small mode of dust with an effective radius of 0.3 μm and a large mode of dust with an effective radius of 3 μm lifted from the surface, with the amount of dust lifted in each mode prescribed as a fraction of the dust to be lifted. The dust is lifted to match a Mars Year 34 climatology map (Montabone et al., 2020), then transported by the general circulation and sediments with gravity. Dust is assumed to coagulate following Bertrand et al. (2022). Coagulation was found to have the greatest effect on the small mode of dust due to the large number of small dust particles lifted in that mode. We find that using a simple bimodal dust lifting scheme transports dust to higher altitude in the model simulations of the MY34 GDS, bringing the model closer to observations of atmospheric temperature, surface temperature, tides, etc.