Reassessment of the common concept to derive the surface cloud radiative forcing in the Arctic: Consideration of surface albedo n cloud interactions

Abstract. The concept of cloud radiative forcing (CRF) is commonly used to quantify the warming or cooling effect due to clouds on the radiative energy budget (REB). In the Arctic, radiative interactions between micro- and macrophysical properties of clouds and the surface influence the CRF and complicate its estimate obtained from observations or models. In this study the individual components and processes related to the surface CRF are analysed separately using simulations and measurement from low-level airborne observations of the REB in the heterogeneous springtime marginal sea ice zone (MIZ). The measurements were obtained during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign. The effect of changing surface albedo, due to the presence of clouds, and its dependence on cloud optical thickness was found to be relevant for the estimation of the solar CRF. A method to correct this albedo effect by retrieving the cloud-free surface albedo from observations under cloudy conditions is proposed. The application of this new concept to ACLOUD data shows, that the estimated average solar cooling effect by clouds almost doubles over snow and ice covered surfaces (−63 W m −2 instead of −33 W m −2 ), if surface albedo-cloud interactions are considered. Concerning the seasonal cycle of the surface albedo, this effect would potentially enhance solar cooling in periods where cold snow and ice dominate the surface and weaken the cooling by optical thin clouds and surface albedos commonly found during the summertime Arctic melting season. These findings suggest, that the surface albedo-cloud interaction needs to be represented in global climate models and in long-term observations to obtain a realistic estimate of the solar CRF and a reasonable representation of cloud radiative feedback mechanisms in the Arctic and to quantify the role of clouds in Arctic amplification.