Impact of Dust Particle Size on Longwave Forcing in the NASA GEOS-5 Earth System Model

Mineral dust aerosols impart a direct radiative forcing on Earth's climate system that can manifest itself as changes in the local and large-scale circulation. Dust reflects and absorbs incident shortwave radiation, and can have significant longwave forcing owing to the large sizes of dust particles compared to other aerosol species. In a recent study carried out with a size resolving dust module included in the Goddard Earth Observing System (GEOS-5) Earth system model, we found that dust radiative forcing enhanced dust emissions by increasing surface wind speeds. Dust radiative forcing also resulted in dust being transported from the Sahara Desert at higher altitudes and more northern latitudes due to a change in the African Easterly Jet position. These features were a function of the dust shortwave single scattering albedo assumed in our model, with both the Saharan dust plume altitude and emission rate increasing with increasing shortwave absorption. We further found that the best comparisons to observed dust distributions generally resulted from the simulations that included the strongest shortwave dust absorption. This result is at odds with remote sensing observations, which suggest that dust is weakly absorbing in the shortwave. Here we turn our attention to dust longwave forcing as a possible resolution of this apparent conflict. Our previous studies showed that our model was deficient in representing the largest dust particles over the source region. We explore the impact of a better representation of these particles in our model on the dust longwave forcing and feedback on dust emissions and transport.