Increasing wintertime cloud opacity increases surface longwave radiation at a long-term Arctic observatory.

Area of Science:

• * Atmospheric Science
• * Arctic Climate Dynamics
• * Cloud Physics

Background:

• * Arctic amplification leads to significant changes in winter cloud properties.
• * Uncertainty in Arctic cloud feedback mechanisms hinders climate projections.
• * Observing Arctic low clouds is challenging, particularly from space.

Purpose of the Study:

• * To constrain and explain longwave (LW) radiative flux changes in the Arctic winter.
• * To investigate the role of cloud radiative effect (CRE) in surface warming.
• * To determine the drivers of increasing cloud opacity.

Main Methods:

• * Utilized two decades (1998-2023) of surface-based observations from Alaska's North Slope.
• * Analyzed longwave flux changes and their correlation with temperature, greenhouse gases, and cloud properties.
• * Quantified the contribution of cloud radiative effect to surface energy balance.

Main Results:

• * Observed a significant increase in downward longwave radiative flux at the surface.
• * Found that temperature and greenhouse gases alone cannot explain the observed flux increase.
• * Demonstrated that increasing cloud radiative effect (0.96 ± 0.64 W/m²/K) is necessary to explain the warming trend, driven by enhanced cloud opacity in both ice-only and mixed-phase clouds.

Conclusions:

• * Increased cloud opacity is a key driver of amplified surface warming in the Arctic winter.
• * Surface-based observations provide crucial constraints on understanding Arctic cloud feedback.
• * The findings suggest a positive cloud feedback loop contributing to Arctic amplification.