Dust semi-direct effects: low-level cloud response to free-tropospheric dust-induced longwave radiation over the North Atlantic Ocean

Overview

Quantitative assessment of the low-level cloud cover (LLCC) response to variations in a free-tropospheric Saharan dust layer over the North Atlantic during boreal summer (May–Aug, 2007–2017). The study tests the hypothesis that dust semi-direct effects include a significant longwave component arising from dust absorption and emission, distinct from canonical shortwave-driven aerosol semi-direct mechanisms established for biomass-burning aerosol. Observational and radiative-transfer-based diagnostics examine how LLCC anomalies correlate with dust optical depth, geometric thickness, and layer base height, and how dust-induced longwave cloud-top heating modulates the net cloud response.

Methods and approach

Composite analysis of multi-year summer-season collocated aerosol and cloud retrievals over the North Atlantic, stratified by free-tropospheric dust characteristics: dust optical depth (DOD), dust geometric thickness (DGT), and dust-layer base height (DBH). Radiative-transfer calculations quantify dust-induced perturbations in shortwave and longwave fluxes at cloud top, and diagnostic heating-rate profiles are derived to isolate cloud-top warming associated with dust longwave absorption. Sensitivity experiments perturb dust size distribution and refractive index, and evaluate alternative contributions from cloud microphysical and thermodynamic variability to the observed LLCC response.

Results

LLCC exhibits a positive mean response to an overlying dust layer, consistent with enhanced atmospheric stability from shortwave absorption. The magnitude of LLCC enhancement declines with increasing DOD and DGT: a one-standard-deviation increase in DOD reduces the LLCC response by 4.3 ± 1.04%, and a one-standard-deviation increase in DGT reduces it by 1.6 ± 0.65%. DBH exerts a smaller influence (0.19 ± 0.45% change per standard deviation). The attenuation of the cloud enhancement is primarily attributable to amplified dust-induced longwave cloud-top warming, which offsets mean cloud-top radiative cooling by up to 19% (mean offset ~9%). Sensitivity diagnostics indicate that variability in dust optical properties (size distribution, refractive index) predominantly controls the magnitude of dust-induced cloud-top longwave warming, whereas variations in cloud microphysics and background thermodynamic profiles play secondary roles.

Implications

The findings extend the aerosol semi-direct effect framework by demonstrating that dust longwave absorption can substantially modulate low-level cloud responses, reducing the cloud-enhancing tendency associated with shortwave-driven stabilization. Dust-layer optical and microphysical properties critically determine the balance between shortwave-driven stabilization and longwave-driven cloud-top warming, implying that regional and temporal heterogeneity in dust characteristics will produce heterogeneous cloud responses and radiative effects. Incorporation of dust longwave processes and realistic dust property variability into climate and regional models is necessary to reduce uncertainty in aerosol–cloud–radiation interactions and their climatic impacts over dust-affected ocean basins.