Radiative and cloud microphysical effects of the Saharan dust simulated by the WRF-Chem model

Numerical simulations are performed to investigate the radiative and cloud microphysical effects (direct and indirect effects respectively) of the Saharan dust aerosols and the subsequent perturbations of the radiation budget over the tropical East Atlantic Ocean. Simulations are conducted for July 1–31, 2010 over three two-way nested domains covering 0–25°N, 30–60°W using the Weather Research and Forecasting model coupled with Chemistry module (WRF-Chem-V3.6.1). Simulations of the dust direct effect over the outermost domain suggest that the dust-induced lower level heating increases the stability of the lower troposphere and reduces the cloud cover (up to 9%) underneath the maximum dust layer. Once the direct and indirect effects are considered together, dust particles act as cloud condensation nuclei and produce smaller sized cloud droplets. Overall, for the innermost domain, the mean cloud droplet radius becomes ~14% smaller in the presence of dust (12.4 μm and 10.7 μm in dust-free and dust-laden conditions respectively). Contrary to the direct effect, the coupled direct and indirect effects of dust not only reduces (up to 17%) the cloud cover below the maximum dust layer but also enhances (up to 9%) the cloud cover above the dust layers. The direct radiative forcing of dust excluding (including) cloud feedback are −13.66 (−4.83) and −11.25 (−1.22) W/m2 at the surface and at the top of the atmosphere, respectively. Coupled direct and indirect effects of dust result in the net radiative forcing of dust to −8.14 and −5.03 W/m2 at the surface and at the top of the atmosphere, respectively.