Fine and coarse dust radiative impact during an intense Saharan dust outbreak over the Iberian Peninsula. Part I: 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 seem 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. A long-term analysis (14 days) was performed over five lidar stations. The vertical distribution of dust optical properties was derived, finding a wide dynamic range of aerosol concentration that allowed a suitable statistical study. The Global Atmospheric Model (GAME) was used for radiative transfer simulations. This study innovates by simulating the SW DRE using two distinct methodologies, finding differences between both approaches. The novel approach separates the impact of both fine (Df) and coarse (Dc) dust components and calculates the total DRE as their combined sum. In contrast, the approach commonly used directly simulates the DRE for the total dust as a whole. Across the dust pathway along the IP, the SW DRE consistently registered a pattern of aerosol-induced cooling at both surface (BOA) and top-of-the-atmosphere (TOA). Results agree with the fact that the Df role cannot be disregarded, that is primarily responsible for SW radiative modulation. In particular, Df contributed nearly half of the total DRE at BOA and TOA in this event. In addition, the Df-to-total ratio influences the differences in DRE obtained by comparing both methodologies, being higher when the differences of the asymmetry factor at 440 nm between the fine and total dust component are greater than a value of 0.1.