Impacts of the Three-dimensional Radiative Effects on Cloud Droplet Number Concentration Retrieval and Aerosol Cloud Interaction Analysis

Abstract. Cloud droplet number concentration (𝑁𝑑) in warm liquid clouds play a crucial role in understanding cloud microphysical processes and the influence of aerosol–cloud interactions (ACI) on Earth’s climate. 𝑁𝑑 from satellite-retrieved cloud properties such as the cloud optical thickness (τ) and cloud droplet effective radius (𝑟𝑒) can be biased due to the three-dimensional (3D) radiative transfer (RT) effects. Using Large-Eddy Simulation (LES) cloud fields and RT simulations, this study investigates how biases in cloud property retrievals caused by 3D-RT effects impact the derived 𝑁𝑑 and subsequent ACI analyses. Our sensitivity studies confirm that the bi-spectral retrievals using the 3.7 𝜇𝑚 channel—whose 𝑟𝑒 retrieval is closest to cloud top— shows better agreement with 𝑁𝑑 from our LES models, compared to results based on the 1.6 and 2.1 𝜇𝑚 retrievals. At native LES resolution, 𝑁𝑑 across all absorbing channels is strongly impacted by the 3D-effects, with the magnitude depending on the solar zenith angles (SZAs); on average, for high/low sun conditions 𝑁𝑑 under 3D-RT underestimates/overestimates its 1D-RT counterpart, which indicates dominant darkening/brightening effects. At coarser satellite-like resolutions, average statistics between 1D and 3D retrievals agree better, indicating compensation between 3D and plane-parallel effects. Furthermore, the impact of 3D-effects on ACI analyses produced similar results across all spectral band pairings, with minimal disagreement between 1D and 3D at coarse spatial resolution. Together, these results indicate that 3D retrieval artifacts in bi-spectral 𝑁𝑑 retrievals do not seem to drive uncertainties associated with radiative impact applications, resulting in reliable ACI and flux-related analyses.