Impacts of the three-dimensional radiative effects on cloud droplet number concentration retrieval and regression-based albedo susceptibility estimates

Abstract

Abstract. Cloud droplet number concentration (Nd) in warm liquid clouds plays 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 regression-based albedo-susceptibility estimates. 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 1D-RT overestimates/underestimate its 3D-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 averaging assumptions. Furthermore, our regression-based albedo-susceptibility results evaluated at the top of cloud domain show that 3D RT greatly modifies the local α–Nd relationship at native LES resolution, especially under more oblique solar geometry, but the differences between 1D- and 3D-regression-based susceptibility estimates are substantially reduced at coarser resolution and low-to-moderate LWP/τ regimes. These results indicate that although 3D RT can strongly affect domain-top regression-based susceptibility at the LES resolution, its impact is reduced at satellite-like spatial resolution for low to moderate LWP/τ regimes.