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

## TL;DR

Winter clouds in the Arctic are becoming more opaque, increasing the amount of heat they trap and contributing to surface warming.

## Contribution

This study provides direct observational evidence that increasing cloud opacity drives rising wintertime surface radiation in the Arctic.

## Key findings

- Longwave flux into the surface is increasing during winter.
- Cloud opacity increases equally from ice-only and mixed-phase clouds.
- Cloud radiative effect explains the unaccounted increase in longwave flux.

## Abstract

As the Arctic warms, winter clouds are known and expected to change. Yet the extent to which these cloud changes amplify or dampen warming (cloud feedback) remains uncertain. This uncertainty results from systemic difficulties in modeling and observing Arctic low clouds. Surface-based observations avoid many of these difficulties. Here, we use two decades of surface-based observations (1998–2023) to constrain and explain longwave flux change during winter. We find that longwave flux into the surface is increasing and that this increase cannot be explained by direct impacts of temperature and greenhouse gases alone. Only when increasing cloud radiative effect (0.96 ± 0.64 W/m2/K) is considered can increasing longwave flux be explained. Cloud radiative effect increases due to increasing cloud opacity, which is driven equally by ice-only and mixed-phase clouds. The direct observational constraint from this work suggests that increasing cloud opacity drives increasing net surface radiation on Alaska’s North Slope during winter.

Two decades of measurements at an Arctic research station reveal increasing wintertime radiative flux into the surface due to increasing cloud opacity, which is due to both ice-only and liquid-containing clouds.