Radiative forcing by mineral dust aerosols: Sensitivity to key variables

Abstract

We examine diurnally averaged radiative forcing by mineral dust aerosols in shortwave and longwave spectral regions using a one‐dimensional column radiation model. At the top of the atmosphere (TOA), net (shortwave plus longwave) dust radiative forcing can be positive (heating) or negative (cooling) depending on values of key variables. We derive an analytical expression for the critical single‐scattering albedo at which forcing changes sign for an atmosphere containing both cloud and aerosol layers. At the surface, net dust forcing can be positive or negative under clear‐sky conditions, whereas it is always cooling in the presence of a low‐level stratus cloud. Longwave radiative forcing is essentially zero when clouds are present. We also study the sensitivity of dust diurnally averaged forcing to the imaginary part of refractive index (k), height of the dust layer, dust particle size, and dust optical depth. These variables play different roles as follows: (1) under both clear‐ and cloudy sky conditions, net TOA forcing is more sensitive to k than net surface forcing; (2) clear‐sky longwave forcing and cloudy‐sky TOA shortwave forcing are very sensitive to the altitude of the dust layer; although clear‐sky shortwave forcing is not sensitive to it; (3) clear‐sky shortwave forcing is much more sensitive to particle size than cloudy‐sky shortwave forcing; longwave forcing is not sensitive to particle size; and (4) all forcings are sensitive to optical depth except cloudy‐sky longwave forcing.