Annual-cycle of aerosol direct radiative effect over south-east Italy and sensitivity studies

Aerosol direct radiative effect (DRE) calculations are presented to illustrate the annual cycle of the aerosol impact on the radiative energy balance of the Earth-atmosphere system over southeast Italy. Meteorological parameters from radiosondes, aerosol vertical profiles by lidar, aerosol optical and microphysical properties by ground-based Sun/sky photometry and satellite (MODIS) derived data of solar surface albedo, all referring to the 2003–2004 years, constitute the necessary input to radiative transfer simulations. The monthly evolution of both the solar and infrared aerosol direct radiative effect is examined at the top of the atmosphere (ToA), within the atmosphere and at the Earth's surface. Aside from cloud-free conditions (clear-sky), all-sky conditions are also addressed by adopting ISCCP monthly products. Model results reveal a strong seasonality of the solar aerosol DRE for southeast Italy, which is mainly driven by the available sunlight, but it is also modulated by variations in aerosol properties. Under cloud-free conditions, winter aerosol DREs are within −(6 ± 2) W/m2 at the ToA and −(7 ± 2) W/m2 at the surface, while summer aerosol DREs are within −(9 ± 1) W/m2 at the ToA and −(15 ± 3) W/m2 at the surface. The larger difference in summer indicates that the heating of the aerosol layer (atmospheric forcing) is much stronger in summer and actually largest in early fall. Under all-sky conditions the solar aerosol direct radiative effect decreases to −(4 ± 1) W/m2 and −(3 ± 1) W/m2 in winter and to −(12 ± 3) W/m2 and −(7 ± 1) W/m2 in summer, at surface and ToA, respectively. Model results also reveal that solar aerosol DREs dominates infrared aerosol DREs. The performed sensitivity studies demonstrate the relatively weak impact of meteorological parameters and aerosol vertical profiles and the strong impact of solar surface albedo values. Finally, it is shown that the simulated all-sky aerosol DRE at the surface is consistent with radiometer measurements.