Airborne observations of the surface cloud radiative effect during different seasons over sea ice and open ocean in the Fram Strait

• Published in: Atmospheric Chemistry and Physics
• Main Authors: S. Becker, A. Ehrlich, M. Schäfer, M. Wendisch
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2023
• Subjects: Physics; QC1-999; Chemistry; QD1-999; Arctic; Svalbard; albedo; Fram Strait; Sea ice
• Online Access: https://doi.org/10.5194/acp-23-7015-2023; https://doaj.org/article/da24aefad92247898f7511aa888f377e

This study analyses the cloud radiative effect (CRE) obtained from near-surface observations of three airborne campaigns in the Arctic north-west of Svalbard: Airborne measurements of radiative and turbulent FLUXes of energy and momentum in the Arctic boundary layer (AFLUX, March/April 2019), Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD, May/June 2017), and Multidisciplinary drifting Observatory for the Study of Arctic Climate – Airborne observations in the Central Arctic (MOSAiC-ACA, August/September 2020). The surface CRE quantifies the potential of clouds to modify the radiative energy budget at the surface and is calculated by combining broadband radiation measurements during low-level flight sections in mostly cloudy conditions with radiative transfer simulations of cloud-free conditions. The significance of surface albedo changes due to the presence of clouds is demonstrated, and this effect is considered in the cloud-free simulations. The observations are discussed with respect to differences of the CRE between sea ice and open-ocean surfaces and between the seasonally different campaigns. The results indicate that the CRE depends on cloud, illumination, surface, and thermodynamic properties. The solar and thermal-infrared (TIR) components of the CRE, CRE sol and CRE TIR , are analysed separately, as well as combined for the study of the total CRE ( CRE tot ). The inter-campaign differences of CRE sol are dominated by the seasonal cycle of the solar zenith angle, with the strongest cooling effect in summer. The lower surface albedo causes a stronger solar cooling effect over open ocean than over sea ice, which amounts to − 259 W m −2 ( − 108 W m −2 ) and − 65 W m −2 ( − 17 W m −2 ), respectively, during summer (spring). Independent of campaign and surface type, CRE TIR is only weakly variable and shows values around 75 W m −2 . In total, clouds show a negative CRE tot over open ocean during all campaigns. In contrast, over sea ice, the positive CRE tot suggests a warming ...