Fine and coarse dust radiative impact during an intense Saharan dust outbreak over the Iberian Peninsula – short-wave direct radiative effect

Abstract. Mineral dust has a key role in the Earth's radiative balance, and it has become significant over the Iberian Peninsula (IP), where Saharan dust outbreaks have seemed to increase in frequency and intensity. This study quantifies the dust direct radiative effect (DRE) in the short-wave range (SW) during an intense persistent springtime dust episode over the IP. In particular, the vertical distribution of dust optical properties was derived at five lidar stations, and the Global Atmospheric Model (GAME) was used for radiative transfer simulations. Moreover, this study innovates by simulating the SW DRE using two distinct methodologies. The novel approach separates the impacts of fine (Df) and coarse (Dc) dust components and calculates the total DRE as their combined sum. In contrast, the commonly used approach directly simulates the DRE for the total dust. Along the dust pathway crossing the IP, the SW DRE consistently registered a pattern of aerosol-induced cooling at both the surface, i.e the bottom of the atmosphere (BOA), and top of the atmosphere (TOA). Results reveal that the role of Df must be highlighted, as Df particles contributed nearly half of the total SW DRE at BOA and TOA, particularly for this event. DRE simulations based on the separation of dust modes for solar zenith angles < 70° show that nearly 75 % of DRE values are lower (in absolute value) with respect to those obtained by considering the total dust. That is, a less pronounced cooling effect is observed overall when the separation of dust modes is taken into account, although relative differences between approaches are not highly significant in general (−3 % and −6 % on average at BOA and TOA, respectively). However, this behaviour reverses under moderate-to-high-dust conditions when the contributions of the Dc and Df components are considered separately; i.e. dust induces a more pronounced cooling effect. This indicates the relevant role of the fine dust particles in DRE estimations, which is likely underestimated when total dust (no separation) is taken into account in relatively high-dust environments. In addition, the cooling effect is more evident at TOA than at BOA, probably due to the presence of dust at higher levels than those usual in the troposphere. This fact can introduce relevant implications for radiometric measurements from satellite instrumentation.