Distribution of mineral dust on the global scale and its impacts on radiation fluxes as simulated by the WRF/Chem model

Mineral dust is produced from both natural and anthropogenic sources. Dust aerosols can be transported over long distances in the atmosphere. They reduce the incident shortwave radiation to the surface by absorbing and scattering the solar radiation; thereby leading to a cooling effect at the surface and lower tropospheric temperature. On the other hand, by absorption and re-emission of longwave radiation, they increase the net surface longwave radiation at the surface. This direct interaction of dust aerosols with shortwave and longwave radiation, known as the direct radiative impact, plays a key role in the radiation budget of the atmosphere. Although mineral dust is one of the most significant aerosols in the atmosphere, according to the Intergovernmental Panel on Climate Change (IPCC, 2007), uncertainty in its spatial distribution and radiative forcing, remains as a great challenge in climate studies. In the present study, the Weather Research and Forecasting with Chemistry (WRF-Chem) regional model is used to simulate distribution of mineral dust and its impacts on radiation fluxes on the global scale. The model was executed using 335 × 168 horizontal grid points with a horizontal spacing of 120 km, and 28 vertical levels for January and July 2011. The National Centers for Environmental Prediction (NCEP) Final Analysis (FNL) re-analysis data were used as meteorological initial conditions. The GOCART (Goddard Global Ozone Chemistry Aerosol Radiation and Transport) simple aerosol scheme was used for the simulation of dust emission and airborne dust distribution. Two experiments were conducted: the control simulation with no dust; and the interactive simulation for which dust aerosols feedback to the atmosphere. Differences between these two simulations indicate the perturbation of radiation by dust. Results indicate that the concentration of dust particles is generally much higher in the Northern Hemisphere than the Southern Hemisphere. The main sources of dust are located over the Sahara and Sahel, the Middle East, and East Asia, especially the Gobi Desert of China and Mongolia. The Eyre Basin in central Australia was identified as the most important source of dust in the Southern Hemisphere. Over the Sahara, dust emission was most intense in January, but substantially decreased in July. In contrast, in response to drier soils and higher wind speeds, sources of dust in the Middle East were more active in July than January. The Gobi Desert was also found to have much more dust activity in January than July, primarily due to stronger wind speeds during this month. On the global scale, monthly-averaged dust optical depth (DOD) was estimated to be 0.046 and 0.069 in January and July, respectively. Globally, perturbation of shortwave and longwave radiation by dust at the top of the atmosphere (TOA) was estimated to be -1.84 and 1.34 W m-2 in January, and -2.38 and 0.68 W m-2 in July, respectively. At the surface, it was estimated that perturbation of shortwave and longwave radiation to be -2.07 and 0.82 W m-2 in January, and -4.14 and 1.02 W m-2 in July, respectively. It was also found that perturbation of radiation is larger closer to the sources of dust. For instance, the perturbation of shortwave radiation exceeds -20 W m-2 over the Sahara. Globally, we identified that dust has a negative effect on the shortwave, but a positive effect on the longwave radiation at the surface. However, in snow covered regions (such as over the Tibetan Plateau, northern parts of the Scandinavia and the United States in January) deposition of dust on the surface increases the net shortwave radiation reaching the surface (due to reduction of surface albedo) and decreases net longwave radiation by increasing outgoing longwave radiation from the surface.