Abstract
Direct radiative forcing by mineral dust is important as it significantly affects the climate system by scattering and absorbing short-wave and long-wave radiation. The multi-angle imaging spectro radiometer (MISR) and cloud–aerosol lidar with orthogonal polarisation (CALIOP) aerosol data are used to observe mineral dust distribution over Australia. In addition, the weather research and forecasting with chemistry (WRF/Chem) model is used to estimate direct radiative forcing by dust. At the surface, the model domain clear-sky short-wave and long-wave direct radiative forcing by dust averaged for a 6-month period (austral spring and summer) was estimated to be −0.67 W m−2 and 0.13 W m−2, respectively. The long-wave warming effect of dust therefore offsets 19.4% of its short-wave cooling effect. However, over Lake Eyre Basin where coarse particles are more abundant, the long-wave warming effect of dust offsets 60.9% of the short-wave cooling effect. At the top of the atmosphere (TOA), clear-sky short-wave and long-wave direct radiative forcing was estimated to be −0.26 W m−2 and −0.01 W m−2, respectively. This leads to a net negative direct radiative forcing of dust at the TOA, indicating cooling of the atmosphere by an increase in outgoing radiation. Short-wave and long-wave direct radiative forcing by dust is shown to have a diurnal variation due to changes in solar zenith angle and in the intensity of infrared radiation. Atmospheric heating due to absorption of short-wave radiation was simulated, while the interaction of dust with long-wave radiation was associated with atmospheric cooling. The net effect was cooling of the atmosphere near the surface (below 0.2 km), with warming of the atmosphere at higher altitudes.
Highlights
The model results are complemented by satellite data to investigate monthly variation of mineral dust distribution
There are some differences between the simulated and satellite retrievals of AOD which mainly arise from using the simplified GOCART dust emission scheme that considers preferential sources based on erodible fraction rather than individual soil particle properties (Cavazos Guerra, 2011)
These transport pathways are consistent with the results of Alizadeh Choobari et al (2012a) using the dust transport (DUSTRAN) module embedded within the WRF/Chem model for a single severe dust event over Australia during 22Á23 September 2009
Summary
Mineral dust aerosols contribute more than half of the total global aerosol burden (Textor et al, 2006) and have a significant influence on the climate system directly through scattering and absorption of solar and infrared radiation (McCormick and Ludwig, 1967; Miller and Tegen, 1998), semi-directly through changes in atmospheric temperature structure and the evaporation rate of cloud droplets (i.e. the cloud burning effect; Hansen et al, 1997; Ackerman et al, 2000; Koren et al, 2004), and indirectly in a complex way through impact on optical properties of clouds (i.e. enhancing cloud reflectance by increasing total droplet cross-sectional area; Gunn and Phillips, 1957; Liou and Ou, 1989) and suppression (Ferek et al, 2000; Rosenfeld, 2000). By conducting a field experiment, Radhi et al (2010) investigated optical, physical and chemical characteristics of Australian dust Their mineralogical analysis indicates that the iron content of Australian dust is higher than other major sources of dust in the Northern Hemisphere, similar to the results of Qin and Mitchell (2009). By investigating short-wave direct radiative forcing of Australian mineral dust, Alizadeh Choobari et al (2012b) found that suspended dust modifies the boundary layer profile and stabilises the lower atmosphere, leading to an overall reduction of wind speed near the surface, and its increase within the upper boundary layer and lower free atmosphere. The atmospheric heating and cooling rates due to the interaction of mineral dust aerosols with short-wave and long-wave radiation are discussed in Section 6, while Section 7 presents a discussion and an overall conclusion
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