Cloud Influences on the Surface Energy Budget at MOSAiC

Abstract:

Clouds have a substantial impact on the Arctic sea-ice surface energy budget. In particular, cloud phase is a strong determinant of the overall effect of clouds on surface radiation and the response of other surface energy budget terms to radiative forcing. These processes are one key focus of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), wherein the icebreaker Polarstern drifted passively with the central Arctic sea ice for a full year from October 2019 to October 2020. During MOSAiC, a sophisticated suite of measurements was used to identify and quantify cloud properties over this full annual cycle, including cloud phase, the amount of condensed liquid water, and the overall cloud radiative effect on the surface. The bi-modal state of the Arctic atmospheric system was clearly present during MOSAiC, wherein the occurence of liquid clouds leads to an increase of 40-50 W/m2 in the surface longwave radiative balance relative to periods without liquid clouds. The rising sun and evolving surface albedo as summer progresses complicate these radiative states, but liquid clouds remain important drivers of variability. Cloud-driven changes in surface radiation modulate surface temperatures, the surface turbulent heat exchange, the conduction of heat through the sea ice, and phase transitions within the coupled atmosphere-ice-ocean system. The primary focus here is on the winter evolution towards summer, capitalizing on measurements that are exceedingly rare at this time of year in the central Arctic. Results suggest that in winter liquid clouds warm the surface by 10 degrees Celsius or more, and decrease the conduction of heat upward through the ice by 10-20 W/m2 depending on snow depth, with these effects diminishing towards summer. The net effects of clouds on each component of this system are examined and, to the extent possible, quantified, to support development of a conceptual model of cloud impacts on Central Arctic sea-ice.