Modification of the shortwave radiation budget of the Mediterranean Basin during intense dust episodes (2005-2018)

Dust aerosols significantly affect the shortwave (SW) radiation budget from global to regional scales. This effect strengthens during intense dust outbreaks taking place with variable frequency and features over and near to the great world deserts. The greater Mediterranean Basin is such a region, frequently undergoing dust episodes originating from the nearby Sahara Desert. In the present study, a climatological assessment of the direct SW radiative effects (DREs) of intense Mediterranean dust episodes is made for the first time. Specifically, the modification of the top-of-atmosphere (TOA), atmospheric and surface SW radiative fluxes caused by 162 spatially extended intense dust episodes that took place from 2005 to 2018 is estimated using the FORTH spectral radiative transfer model (RTM). Also, the consequent modification of the regional atmospheric thermal structure and dynamics due to these DREs is computed, aiming to shed light on the role of dust aerosols on regional climate. The RTM computations are driven by a synergy of contemporary satellite (ISCCP-H) and reanalysis (MERRA-2) climatological data.

The reliability of the dust DREs (DDREs) is ensured by comparisons of the model outputs with reference fluxes at the region’s surface (BSRN stations) and TOA (CERES). The results are satisfactory indicating a nice correlation with BSRN and CERES (R values equal to 0.95 and 0.98, respectively) and a slight underestimation (5.4%) at surface and overestimation at TOA (2.7%). During the 162 intense dust episodes the surface of the Mediterranean Basin is cooled by up to -72 W/m² on average, while the atmosphere is correspondingly heated by up to 75 W/m². At TOA opposite effects are induced, namely a planetary heating (up to 26 W/m²) over Africa and a cooling (as much as -20 W/m²) over the Mediterranean Sea. These values are larger (up to 100 W/m²) on a seasonal basis and even stronger on a daily or hourly basis. Besides, the DDREs induce an atmospheric heating up to about 0.4 K/3-hours on average, while this heating is as strong as 2.5 K during the time interval 12:00-15:00 of the dust episode days, creating a significant positive buoyancy over the dust affected areas.