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

Abstract

The dust direct radiative effect (DRE) in the long-wave (LW) (DRELW), and the net effect (DRENET), is analysed during an intense and long-lasting Saharan dust intrusion over the Iberian Peninsula, complementing the study on the short-wave (SW) DRE (DRESW) (López-Cayuela et al., 2025). In LW, a warming effect at both the bottom-of-atmosphere (BOA) and the top-of-atmosphere (TOA) levels is induced by the fine (Df) and coarse (Dc) dust particles, while Dc being dominant. The DRELW-to-DRESW ratio for Df ranged 4 %–8 % at BOA (1 %–4 % at TOA), and for Dc it was rather higher (39 %–54 % at BOA and 20 %–50 % at TOA). DRENET was consistently negative (net cooling) at both levels, and the derived atmospheric DRENET was positive (net warming). The Df contribution to DRENET was 12 % (LW) and 30 % (SW). The SW aerosol heating rate (AHR) peaked at higher altitudes, inducing warming within the dust layer, than LW AHR (weaker cooling). Consequently, a net warming inside the dust layer was found, with potential cooling below and above. While SW dominates the net atmospheric warming, LW cooling partially mitigates it. As a novelty of this study, two methodologies for estimating DRE in both LW and net spectral ranges are compared. Differences in DRE between a classical approach considering total dust and an approach separating fine and coarse modes are analysed. DRELW (and DRENET) is underestimated (overestimated) by using the dust-mode separation approach in comparison to the classical one (no separation) when fine radii are lesser (greater) than a particular threshold (e.g. 0.1 µm), revealing the particle size impact in DRELW. The dust-induced net effect is primarily driven by SW and modulated by LW. The classical (no separation) approach overestimates DRENET, with mean (standard deviation) relative differences of -5 % (7 %) at BOA and -9 % (13 %) at TOA. Moreover, under moderate-to-high dust, separating Df and Dc contributions yields a weaker (stronger) net cooling at BOA (TOA).