Aerial observations of the evolution of ice surface conditions during summer

Abstract

During spring and summer, the Arctic pack ice cover undergoes a dramatic change in surface conditions, evolving from a uniform, reflective surface to a heterogeneous mixture of bare ice, melt ponds, and leads. This transformation is accompanied by a significant decrease in areally averaged, integrated albedo. The key factors contributing to this reduction in albedo are the melting of the snow cover, the formation and growth of the melt ponds, and the increase in the open water fraction. To document these changes and enable quantification of the evolution of the ponds throughout the melt season, a program of aerial photography was carried out at the main site of the Surface Heat Budget of the Arctic Ocean (SHEBA) program. A modified square pattern, 50 km on a side, surrounding the SHEBA site was flown at altitudes ranging from 1220 to 1830 m. Twelve of these aerial survey photography flights were completed between 20 May and 4 October 1998. The flights took place at approximately weekly intervals at the height of the melt season, with occasional gaps as long as 3 weeks during August and September due to persistent low clouds and fog. In addition, flights on 17 May and 25 July were flown in a closely spaced pattern designed to provide complete photo coverage of a 10‐km square centered on the SHEBA main site. Images from all flights were scanned at high resolution and archived on CD‐ROMs. Using personal computer image processing software, we have measured ice concentration, melt pond coverage, statistics on size and shape of melt ponds, lead fraction, and lead perimeter for the summer melt season. The ponds began forming in early June, and by the height of the melt season in early August the pond fraction exceeded 0.20. The temporal evolution of pond fraction displayed a rapid increase in mid‐June, followed by a sharp decline 1 week later. After the decline, the pond fraction gradually increased until mid‐August when the ponds began to freeze. By mid‐September the surface of virtually all of the ponds had frozen. The open water fraction varied between 0.02 and 0.05 from May through the end of July. In early August the open water fraction jumped to 0.20 in just a few days owing to ice divergence. Melt ponds were ubiquitous during summer, with number densities increasing from 1000 to 5000 ponds per square kilometer between June and August.