Reassessment of shortwave surface cloud radiative forcing in the Arctic: consideration of surface-albedo–cloud interactions

• Main Authors: Stapf, Johannes, Ehrlich, André, Jäkel, Evelyn, Lüpkes, Christof, Wendisch, Manfred
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2020
• Subjects: Arctic; albedo; Sea ice
• Online Access: https://epic.awi.de/id/eprint/52920/; https://hdl.handle.net/10013/epic.d5e719a9-9c09-4b2e-b780-d6f42ac7ae40

The concept of cloud radiative forcing (CRF)is commonly applied to quantify the impact of clouds onthe surface radiative energy budget (REB). In the Arctic,specific radiative interactions between microphysical andmacrophysical properties of clouds and the surface stronglymodify the warming or cooling effect of clouds, complicat-ing the estimate of CRF obtained from observations or mod-els. Clouds tend to increase the broadband surface albedoover snow or sea ice surfaces compared to cloud-free con-ditions. However, this effect is not adequately consideredin the derivation of CRF in the Arctic so far. Therefore,we have quantified the effects caused by surface-albedo–cloud interactions over highly reflective snow or sea ice sur-faces on the CRF using radiative transfer simulations andbelow-cloud airborne observations above the heterogeneousspringtime marginal sea ice zone (MIZ) during the ArcticCLoud Observations Using airborne measurements duringpolar Day (ACLOUD) campaign. The impact of a modi-fied surface albedo in the presence of clouds, as comparedto cloud-free conditions, and its dependence on cloud opti-cal thickness is found to be relevant for the estimation of theshortwave CRF. A method is proposed to consider this sur-face albedo effect on CRF estimates by continuously retriev-ing the cloud-free surface albedo from observations undercloudy conditions, using an available snow and ice albedoparameterization. Using ACLOUD data reveals that the esti-mated average shortwave cooling by clouds almost doublesover snow- and ice-covered surfaces (−62 W m−2instead of−32 W m−2), if surface-albedo–cloud interactions are con-sidered. As a result, the observed total (shortwave plus long-wave) CRF shifted from a warming effect to an almost neu-tral one. Concerning the seasonal cycle of the surface albedo, it is demonstrated that this effect enhances shortwave cool-ing in periods when snow dominates the surface and poten-tially weakens the cooling by optically thin clouds duringthe summertime melting season. These findings suggest thatthe surface-albedo–cloud interaction should be considered inglobal climate models and in long-term studies to obtain arealistic estimate of the shortwave CRF to quantify the roleof clouds in Arctic amplification.