Uncertainties in assessing radiative forcing by mineral dust

The assessment of the climatic effects of an aerosol with a large variabili ty like mineral dust requires some approximations whose validity is investi gated in this paper. Calculations of direct radiative forcing by mineral du st (short wave, long-wave and net) are performed with a single-column radia tion model for two standard cases in clear sky condition: a desert case and an oceanic case. Surface forcing result from a large diminution of the sho rt-wave fluxes and of the increase in down-welling long-wave fluxes. Top of the atmosphere (TOA) forcing is negative when short-wave backscattering do minates, for instance above the ocean, and positive when short-wave or long -wave absorption dominates, which occurs above deserts. We study here the s ensitivity of these mineral forcings to different treatments of the aerosol complex refractive index and size distribution. We also describe the impor tance of the dust vertical profile, ground temperature, emissivity and albe do. Among these parameters, the aerosol complex refractive index has been i dentified as a critical parameter given the paucity and the incertitude ass ociated with it. Furthermore, the imaginary part of the refractive index is inadequate if spectrally averaged. Its natural variability (linked to mine ralogical characteristics) lead to variations of up to +/-40% in aerosol fo rcing calculations. A proper representation of the size distribution when m odelling mineral aerosols is required since dust optical properties are ver y sensitive to the presence of small particles. In addition we demonstrate that LW forcing imply a non-negligible sensitivity to the vertical profiles of temperature and dust, the latter being an important constraint for dust effect calculations.