Dust emission size distribution impact on aerosol budget and radiative forcing over the Mediterranean region: a regional climate model approach

The present study investigates the dust emission and load over the Mediterranean basin using the coupled chemistry-aerosol-regional climate model RegCM-4. The first step of this work focuses on dust particle emission size distribution modeling. We compare a parameterization in which the emission is based on the individual kinetic energy of the aggregates striking the surface to a recent parameterization based on an analogy with the fragmentation of brittle materials. The main difference between the two dust schemes concerns the mass proportion of fine aerosol that is reduced in the case of the new dust parameterization, with consequences for optical properties. At the episodic scale, comparisons between RegCM-4 simulations, satellite and ground-based data show a clear improvement using the new dust distribution in terms of aerosol optical depth (AOD) values and geographic gradients. These results are confirmed at the seasonal scale for the investigated year 2008. This change of dust distribution has sensitive impacts on the simulated regional dust budget, notably dry dust deposition and the regional direct aerosol radiative forcing over the Mediterranean basin. In particular, we find that the new size distribution produces a higher dust deposition flux, and smaller top of atmosphere (TOA) dust radiative cooling. A multi-annual simulation is finally carried out using the new dust distribution over the period 2000-2009. The average SW radiative forcing over the Mediterranean Sea reaches -13.6W m(-2) at the surface, and -5.5W m(-2) at TOA. The LW radiative forcing is positive over the basin: 1.7W m(-2) on average over the Mediterranean Sea at the surface, and 0.6W m(-2) at TOA.