SELF-MOBILIZATION OF DUST AEROSOLS IN PRESENT-DAY AND DUSTY CLIMATES

During the Last Glacial Maximum (LGM), the global atmospheric dust load was believed to be 2-5 times larger than the present-day load due to a combination of increased surface wind and reduced vegetation cover. In response to the scattering and absorption of radiation by dust, the surface fluxes and atmospheric circulation are altered which feeds back upon dust mobilization at the surface. It has been hypothesized (Yung et al 1996) that the increased LGM burden was partly due to an increased aerosol lifetime, caused by reduced precipitation and efficiency of wet deposition.

In AGCM simulations of dust in the current climate, the dust load is reduced by the addition of dust radiative forcing. With the reduction of sunlight beneath the dust cloud, the surface sensible heat flux is reduced, which decreases boundary layer mixing of interior momentum toward the surface and thus surface wind speed. This is a negative feedback upon dust emission. However, reduced evaporation and rainfall decrease the wet deposition efficiency. Thus, dust radiative forcing reduces surface emission of dust, but increases the particle lifetime, especially for smaller particles that are removed from the atmosphere primarily by wet deposition. In the current climate, the combination of these two feedbacks is negative.

It is of interest to compute the individual feedbacks during the LGM when the dust load and associated radiative forcing were larger, and the relative strength of the feedbacks were possibly different. As an alternative to the LGM calculation, we increase the dust burden compared to the current climate by arbitrarily increasing the emission for a given wind speed. In this `dusty' climate, the wet deposition lifetime increases by 15 percent, given a four-fold increase in the dust burden. Despite this change, the feedback of dust radiative forcing upon the dust load remains negative. The increase in the wet deposition lifetime is almost exactly offset by an increase in dry deposition. As the surface reduction in sunlight increases with the column burden of dust, boundary layer mixing weakens and dust is increasingly concentrated near the surface, within the reach of dry deposition. This suggests that dust radiative forcing will cause little change to the current particle lifetime even during the dusty LGM.