Feedback upon dust emission by dust radiative forcing through the planetary boundary layer

Large changes to the amount of airborne soil particles (or "mineral dust'' aerosol) inferred from the climate record raise the question of whether radiative forcing by dust particles amplifies or else diminishes these changes. A previous study with an atmospheric general circulation model (AGCM) indicates that dust radiative forcing in the present-day climate reduces emission of dust into the atmosphere. Here, we interpret this reduction as an interaction between dust radiative forcing and the planetary boundary layer (PBL). By reducing sunlight incident upon the surface, dust decreases the turbulent flux of sensible heat into the atmosphere. This reduces turbulent mixing within the PBL, along with the downward transport of momentum to the surface, resulting in a decrease of surface wind speed and dust emission. We illustrate this mechanism by comparing the diurnal cycle of emission simulated by two versions of an AGCM, one containing dust radiative forcing, and the other with this forcing set to zero. At some of the most productive source regions, morning emission is reduced in proportion to the surface radiative forcing. We suggest that this negative feedback is underestimated by the AGCM.