From: Renfrew, I. A. and Houghton, N., 2001: Weather Image: Convection in flow off an ice sheet, Weather, 56, 152.

Shown are two infra-red satellite images of the Ross Sea, Antarctica, taken at 19:30 UTC on 17th September 2000. The left and right images are from the 12 and 1.6 micron channels respectively. The images are processed so that brighter shades are colder brightness temperatures; they are roughly 500 km wide and oriented north-south. They show the Ross Ice Shelf to the south, with the largely ice-covered Ross Sea to the north. A number of icebergs are loose in the Ross Sea; these are the result of a major iceberg break out that started with the calving of iceberg B-15 in March 2000.

Streaming away from the icebergs and the Ice Shelf front are 'cloud streets' - linear formations of shallow convective cloud bands oriented approximately in the direction of the low-level wind. Cloud streets are generated by longitudinal 'roll vortices' in the boundary layer. The roll vortices force patterns of convergence and divergence, which can lead to cloudy and clear bands. The system is driven by convective heating of the lower part of the boundary layer by the warm ocean. One can see that there is open water to the lee of the icebergs and ice shelf - the open water is much warmer (darker) than the ice. Such areas of persistent open water are known as polynyas (a Russian word). Along coastlines they are opened by offshore winds literally blowing the sea ice away. Cloud streets are commonly seen in satellite imagery; what is unusual about this image is their formation over polynyas rather than open ocean. Furthermore the cloud streets have an extremely well-defined start point, at the ice front, but then tend to whither away as the air moves further from the moisture source. For cloud to form so soon after the ice shelf edge is unusual. It suggests that the air was near saturation when it encountered the polynya.

The image was taken by the Along Track Scanning Radiometer, which has 7 IR and visible channels. Further details about the instrument, and a gallery of images, can be found at http://www.atsr.rl.ac.uk. © CLRC/NERC/ESA/BNSC.