Quantifying the impact of solar zenith angle, cloud optical thickness, and surface albedo on the solar radiative effect of Arctic low-level clouds over open ocean and sea ice

Abstract. Due to their complex interactions with surface albedo, aerosol particles, and water vapour, clouds play one of the most uncertain roles in the Arctic climate system. Consequently, the cloud radiative effect (CRE), which is a quantitative measure of the impact of clouds on the radiative energy budget (REB), is subject to considerable uncertainty. To reduce this uncertainty and better understand the importance of the driving processes, it is crucial to quantitatively disentangle the various cloud and non-cloud factors that non-linearly affect the CRE. Therefore, this study uses a combination of a CRE parameterization and low-level airborne REB observations to quantify the impact of concurrently observed cloud optical thickness, solar zenith angle, and surface albedo on the solar CRE at the surface. Based on a case study characterized by inhomogeneous cloud and surface conditions in the marginal sea ice zone, the contributions of surface albedo and cloud optical thickness to the solar CRE are derived similar to the approximated partial radiative perturbation technique applied in climate dynamics. It is shown that the surface albedo contributed more than 95 % to the solar CRE difference between open ocean and sea ice. Using the same approach, the analysis was extended to observations from a series of aircraft campaigns, indicating that the non-cloud conditions frequently dominate seasonal and surface-type differences of the solar CRE.