Direct radiative effects of an intense dust episode over the Mediterranean Basin (16-18 June 2016)

Perturbation of the Earth’s radiation budget is a key factor for climate change. Such perturbations are caused either from changes in the incoming solar radiation at the top of atmosphere (TOA), i.e. astronomical changes, or from modifications in the absorbed and scattered solar radiation within the Earth-atmosphere system. It is known that the current climate change is mainly attributed to greenhouse gases and aerosols. However, opposite to the achieved significant improvement of our knowledge of the role of greenhouse gases, there is still high uncertainty in the estimations of the aerosol radiative effect, due to their high spatial and temporal variability and complex and changing physical, chemical and optical properties.

Dust Aerosols (DA) is a major contributor of the global aerosol burden, while they modify the Earth’s radiation budget through the absorption and scattering of solar radiation and the absorption and re-emission of terrestrial radiation. Such dust-radiation interactions are known as Direct Radiation Effect (DRE) and generally result in a shortwave cooling effect and a smaller longwave heating effect both at the Top of Atmosphere (TOA) and the Earth’s surface. However, these radiative effects vary significantly in space and time, depending on the DA physical and optical properties, as well as on the underlying surface reflectivity or their vertical position relative to clouds, resulting in changes of the magnitude or even the sign of DREs. These dust-radiation interactions are expected to be maximized when the DA loads and the available solar radiation amounts are high. Therefore, the study of DREs under episodic dust conditions over areas such as the climatically sensitive and threatened Mediterranean Basin (MB), especially on a three-dimensional basis, is of primary importance. This becomes even more challenging when the study involves spectral detailed radiative transfer models (RTMs) and three-dimensionally resolved aerosol optical and atmospheric properties.

Here, all-sky DRE of DA is estimated during a spatially and temporally extended Dust Aerosol Episode Case (DAEC) took place from 16 to 18 June 2016 over the MB. The studied DAEC is identified using a satellite algorithm, which uses aerosol optical properties. The dust DREs are computed using 3-D dust optical properties, namely dust optical depth, single scattering albedo and asymmetry parameter from the MERRA-2 reanalysis, and cloud (i.e., cloud amount, optical depth and top pressure) and other atmospheric properties from the International Satellite Cloud Climatology Project (ISCCP) as input data to the FORTH (Foundation for Research and Technology-Hellas) spectral radiative transfer model. The model runs, with and without DA, on a 3-hourly temporal and 0.5˚×0.625˚ horizontal spatial resolution for the 4-day period from 15 to 18 June 2016. The RTM output includes upwelling and downwelling solar fluxes, as well as DREs, at TOA, at the surface, and at 50 levels in the atmosphere. The vertical and horizontal variation of DA DREs are computed by producing and examining the respective DRE cross-sections, and finally the heating rates caused by the evolving dust episode are estimated in order to yield the radiative effect of dust on the dynamics of the Mediterranean atmosphere.