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 (DRELW), and 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 DRE (DRESW) (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 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 hence the 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. DRELW (and DRENET) 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 DRELW. The dust-induced net effect is primarily driven by SW and modulated by LW. The classical (no separation) approach overestimates DRENET, 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).