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 long-wave (DRE_LW), and net effect (DRE_NET), is analysed during an intense and long-lasting Saharan dust intrusion over the Iberian Peninsula, complementing the study on the short-wave DRE (DRE_SW) (López-Cayuela et al., 2025). In LW, a warming effect at both bottom-of-atmosphere (BOA) and the top-of-atmosphere (TOA) levels is induced by the fine (Df) and coarse (Dc) dust particles (Dc dominant). The DRE_LW-to-DRE_SW 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). DRE_NET was consistently negative (net cooling) at both levels, and hence the atmospheric DRE_NET was positive (net warming). The Df contribution to DRE_NET 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. DRE_LW (and DRE_NET) is underestimated (overestimated) by using the dust-mode separation approach when fine radii are lesser (greater) than a particular threshold (e.g., 0.1 μm), revealing the particle size impact in DRE_LW. The dust-induced net effect is primarily driven by SW and modulated by LW. The classical (no separation) approach overestimates DRE_NET, with mean relative differences of -5 %/-9 % at BOA/TOA. Moreover, under moderate-to-high dust, separating Df and Dc contributions yields a weaker (stronger) net cooling at BOA (TOA).