Recorded presentation9a.1
Is every 50 years enough? What -- and how -- have we learned about the Arctic from the IPY
Michael Tjernstrom, Stockholm University, Stockholm, Sweden
While the unexpected and substantial ice loss of the summer of 2007 has been attributed partly to an unusual weather pattern that summer, many awaited the summer of 2008 to see if there would be a repetition of similar conditions during. Although for a limited area and for a limited time, this presentation provides an overview of weather conditions in the high Arctic (near 87°N and 5°W) from observations taken during the Arctic Summer Cloud-Ocean Study (ASCOS) during August and early September, 2008. The presentations serves as a background for several other related presentations on ASCOS submitted to this conference. The data that will be presented include surface analyses and satellite images, weather station data, some results from the surface-based remote sensing instruments and vertical profiles from the soundings, in some instances attempting a comparison to results from earlier extensive measurement campaigns, such as the AOE-2001 in August of 2001 and SHEBA during the same time of the year.
The synoptic-scale weather during ASCOS was quite vigorous with a series of weather systems moving westward from the Russian Arctic south of Svalbard and across the Atlantic sector to northern Greenland. These systems caused several events of high winds, causing the boundary-layer over the ice to be deeper than experienced during both AOE-2001 and SHEBA. Some were also associated with intrusions of warm and moist air on top of the boundary layer; those events were several times more common during ASCOS than during both AOE-2001 and SHEBA. Cloudy conditions prevailed with extended periods of fog or low clouds. The near-surface temperatures were generally confined to between near zero and -2 °C, as dictated by the melting/freezing of snow and ice at the surface and relative humidity was high. The exception is towards the end of the ice drift, when temperatures dropped to -10 °C and below, as the ice surface froze over and the freeze up apparently began during the last week of August. Of particular interest is a two-day period in late August when the temperature fell to around -6 °C and many melt ponds froze, but the surface energy flux at the ice surface indicate an upward flux of sensible heat; thus this cooling was thus not a consequence of local radiative cooling of the surface. A similar episode appeared during AOE-2001.
The lower troposphere was moist and near-neutrally stratified; in the upper parts of the boundary-layer often moist neutral indicating the presence of shallow convection. The low-level moist layer, with RH ~100%, extended higher into the lower troposphere than the near-neutral conditions, and specific humidity often increased over the boundary-layer capping inversion. Multi-layered structures, with clouds at several layers in the lower troposphere were common. The wind speed aloft on average increase significantly with height, much more so that for either AOE-2001 or SHEBA, indicating a more baroclinic atmosphere, consistent with the many weather systems occurring, especially during the first half of August.
Session 9a, Conference Luncheon
Tuesday, 19 May 2009, 12:00 PM-1:30 PM, Madison Ballroom